Welcome to our knowledge base of totally random and probably obscure stuff relating to GM/Chevrolet industrial 366T/427T (366 cubic inch/427 cubic inch tall deck) big block engines!

We've recorded these notes and observations not only for ourselves (because we forget stuff over time), but also in case anyone else can find it useful. If you find something incorrect or wish to contribute something, just let us know here.
 366T/427T engines 366/427 Water pumps
 366T/427T engines 366/427 Cylinder heads
 366T/427T engines 366/427 Brackets and mounting points
 366T/427T engines 366/427 Intake manifolds
 366T/427T engines 366/427 Overall engine differences
 366T/427T engines 366/427 Exhaust manifolds
 366T/427T engines 366/427 HEI conversion
 366T/427T engines 366/427 Governor system
 366T/427T engines 366/427 Oil pans
 366T/427T engines 366/427 Specifications
 366T/427T engines Carburetor versus TBI
 366T/427T engines Carburetor conversion to Edelbrock/Carter
 366T/427T engines Crate engines
 366T/427T engines Factory applications
 366T/427T engines Generation differences
 366T/427T engines Why bother with a 366?
 366T/427T engines 366/427 versus 454
 366T/427T engines Resources


366/427 Water pumps

There are actually three different GM big block water pump nose depths, not just two as common knowledge would have us believe. The factory pump for a tall deck industrial engine (366T/427T or 366 cubic inch tall deck/427 cubic inch tall deck) is approximately 6-3/8" from block to pulley mount surface. Long nose is approximately 7-5/8" from block to pulley mount surface. Short nose is 5-7/8" from block to pulley mount surface. The tall deck industrial pump has a unique bypass housing on top of the pump using a 1-3/4" hose from the intake manifold front. It also has a 2-3/4" intake hose from radiator. The upper radiator hose (output) of the tall deck pump to the radiator is 2". There's plenty of common knowledge out there about the standard long and short nose pumps, so we're mainly concerned with documenting the lesser known tall deck industrial engine pump.

Any pump can be used with the tall deck engine, as long as the bypass from the intake manifold is adapted to fit. The standard long and short nose pumps will usually not work with stock tall deck crankshaft pulleys or fans out of the box, as the tall deck pump has a 2.5" pulley bolt circle and a larger hub. The pump bodies themselves are not too much larger other than the enlarged bypass adapter on top of the industrial pump. The factory tall deck pump is made to flow a higher volume of coolant than the factory short or long nose pumps, thus the much larger intake size.

Below are physical comparisons of the three pumps. In the lefthand picture we've placed a bar across the noses for height comparison. On the left is a short nose pump, the middle one is a stock industrial tall deck pump, and the long nose pump is on the right. Note: some industrial engines have a pump that is offset up higher to place the fan in the middle of a taller radiator, such as those used on heavy trucks. The pump shown below is for a standard height radiator.

Big block chevrolet water pump comparison Big block chevrolet water pump comparison

An example of a tall style industrial pump:

Big block chevrolet water pump comparison

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366/427 Cylinder heads

Since the industrial tall deck engines are sort of ignored by most chevy builders, there's not much out there in the way of solid information about them. The cylinder heads have a lot of disinformation in general.

The cylinder heads are not all actually specific to the tall deck engines. They mostly utilize the same factory castings as other big blocks, obviously with some variation over time. The valves ARE different though. The tall deck engines use sodium filled valve stems for better cooling. There's a lot of good information about how that works out there, so we won't go into it here. The valve springs and rocker arms are not anything special, as these engines were not made to turn high revolutions per minute, instead specializing in low end grunt power.

As can be seen below, this set of factory heads off of a 1972 366T engine is a standard oval port casting.

Cylinder head

Visual indication of standard head use: as can be seen below, the head gaskets on this factory 366 cubic inch engine used standard size gaskets. This means the distance between cylinders on the gasket surface is not different between tall deck engines and standard engines, regardless of displacement. Tall deck engines are no less likely to incur headgasket failure due to this spacing than a standard engine. In the photo can be seen how much smaller the 366 cylinder bore is than the gasket which sits on it (and yes, this is the first disassembly of a factory engine). Not visible in the photo is the slight notch beveled out of the upper cylinder wall side for intake valve clearance on this small 366 cu. in. bore.

366 engine block

Another difference in cylinder heads has to to with the generation change of the blocks in 1991. The water jacket passages in the head can be different, as illustrated by the picture below.
Tall deck cylinder head comparison

Below is some reference data for cylinder head casting numbers with valve sizes, years used, etc.

Casting numbers used on factory 366ci tall deck engines (some used on multiple engines):
330864: 1968 through 1984 oval port, open chamber, 2.06 in. intake and 1.72 in. exhaust valves.
330865: 1968 through 1984 oval port, open chamber, 2.06 in. intake and 1.72 in. exhaust valves.
330866: 1968 through 1978 oval port, open chamber, 2.06 in. intake and 1.72 in. exhaust valves.
343771: 1968 through 1986 oval port, open chamber, 2.06 in. intake and 1.72 in. exhaust valves.
343772: 1968 through 1986 oval port, open chamber, 2.06 in. intake and 1.72 in. exhaust valves.
473328: 1968 through 1969 oval port, open chamber, 2.06 in. intake and 1.72 in. exhaust valves.
3856213: 1966 through 1982 oval port, closed chamber, 2.19 in. intake and 1.88 in. exhaust valves.
3904393: 1968 through 1976 oval port, open chamber, 2.19 in. intake and 1.88 in. exhaust valves.
3917219: 1968 through 1985 oval port, open chamber, 2.06 in. intake and 1.72 in. exhaust valves.
3933148: 1969 through 1984 oval port, open chamber, 2.06 in. intake and 1.72 in. exhaust valves.
3975950: 1968 through 1970 oval port, open chamber, 2.06 in. intake and 1.72 in. exhaust valves.
3986133: 1968 through 1985 oval port, open chamber, 2.06 in. intake and 1.72 in. exhaust valves.
3986135: 1969 through 1973 oval port, open chamber, 2.06 in. intake and 1.72 in. exhaust valves.
3986136: 1968 through 1984 oval port, open chamber, 2.06 in. intake and 1.72 in. exhaust valves.
10101140: 1991 and later oval port, closed chamber, 2.06 in. intake and 1.72 in. exhaust valves.
10487052: 1977 through 1990 oval port, open chamber, 2.06 in. intake and 1.72 in. exhaust valves.
12562932: 1991 through 1995 oval port, open chamber, 2.06 in. intake and 1.72 in. exhaust valves. Used on Gen V truck crate.
12562933: 1991 through 1995 oval port, open chamber, 2.06 in. intake and 1.72 in. exhaust valves. Used on Gen V.
14081052: 1985 through 1987 oval port, open chamber, 2.06 in. intake and 1.72 in. exhaust valves. May be stamped “Hi Perf”.
14092359: 1986 through 1990 oval port, open chamber, 2.06 in. intake and 1.72 in. exhaust valves. May be stamped “Hi Perf”.

Casting numbers used on factory 427ci tall deck engines (some used on multiple engines):
330864: 1968 through 1984 oval port, open chamber, 2.06 in. intake and 1.72 in. exhaust valves.
330865: 1968 through 1984 oval port, open chamber, 2.06 in. intake and 1.72 in. exhaust valves.
330866: 1968 through 1978 oval port, open chamber, 2.06 in. intake and 1.72 in. exhaust valves.
330867: 1968 through 1973 oval port, open chamber, 2.06 in. intake and 1.72 in. exhaust valves.
336765: 1968 through 1978 oval port, open chamber, 2.06 in. intake and 1.72 in. exhaust valves.
336768: 1973 through 1976 oval port, open chamber, 2.06 in. intake and 1.72 in. exhaust valves.
343771: 1968 through 1986 oval port, open chamber, 2.06 in. intake and 1.72 in. exhaust valves.
343772: 1968 through 1986 oval port, open chamber, 2.06 in. intake and 1.72 in. exhaust valves.
366765: 1974 through 1978 oval port, open chamber, 2.06 in. intake and 1.72 in. exhaust valves.
3856213: 1966 through 1982 oval port, closed chamber, 2.19 in. intake and 1.88 in. exhaust valves.
3876875: 1977 through 1985 oval port, open chamber, 2.19 in. intake and 1.88 in. exhaust valves.
3908952: 1967 oval port, open chamber, 2.19 in. intake and 1.88 in. exhaust valves. Truck and marine.
3917219: 1968 through 1985 oval port, open chamber, 2.06 in. intake and 1.72 in. exhaust valves.
3933148: 1969 through 1984 oval port, open chamber, 2.06 in. intake and 1.72 in. exhaust valves.
3933149: 1968 through 1976 oval port, open chamber, 2.06 in. intake and 1.72 in. exhaust valves.
3935401: 1968 through 1976 oval port, open chamber, 2.06 in. intake and 1.72 in. exhaust valves.
3975950: 1968 through 1970 oval port, open chamber, 2.06 in. intake and 1.72 in. exhaust valves.
3986133: 1968 through 1985 oval port, open chamber, 2.06 in. intake and 1.72 in. exhaust valves.
3986135: 1969 through 1973 oval port, open chamber, 2.06 in. intake and 1.72 in. exhaust valves.
3986136: 1968 through 1984 oval port, open chamber, 2.06 in. intake and 1.72 in. exhaust valves.
10052902: Oval port, open chamber, 2.06 in. intake and 1.72 in. exhaust valves. Mark IV crate and truck.
10101140: 1991 and later oval port, closed chamber, 2.06 in. intake and 1.72 in. exhaust valves.
12562932: 1991 through 1995 oval port, open chamber, 2.06 in. intake and 1.72 in. exhaust valves. Used on Gen V truck crate.
12562933: 1991 through 1995 oval port, open chamber, 2.06 in. intake and 1.72 in. exhaust valves. Used on Gen V.
14081052: 1985 through 1987 oval port, open chamber, 2.06 in. intake and 1.72 in. exhaust valves. May be stamped “Hi Perf”.
14092359: 1986 through 1990 oval port, open chamber, 2.06 in. intake and 1.72 in. exhaust valves. May be stamped “Hi Perf”.

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366/427 Brackets and mounting points

Most (if not all) of the tall deck engines were installed with one 'U' shaped engine mount bolted to the front of the engine, with the rear support being on either side of the bellhousing. The blocks were still drilled and tapped for side 'clamshell' style mounts however. So engine mounting is an easy conversion to standard big block mounts. The starter motor was usually mounted in the bellhousing, so not all engine blocks have drilled and tapped starter motor holes. Early blocks are drilled and tapped, not all later blocks are. At some point GM figured out that they weren't installing automatic transmissions on these and that they could save some money by not making or preparing the holes.

Accessory mounting to the front of the engine will be slightly different than standard big blocks due to the fact that the cylinder heads are slightly farther apart. The easiest way to overcome this if not using the complete factory bracket setup is to obtain bracket sets that mount only to the block or heads respectively for each accessory. This eliminates the problem of bolt hole spacing being slightly different between the block and head.

If using a stock cast crankshaft pulley with four or more grooves, note that the grooves begin immediately after the damper. This places the innermost belt very close to the water pump housing on a regular big block pump, and the short nose version must be used in this case. It will be tight, but it will clear most castings. The pump housing may have to be filed slightly or deburred if the belt rubs on installation. Note on the pump comparison pictures above in the water pumps topic, the industrial pump in the middle actually has more housing clearance although it is bigger in other ways.

Here is a short-nose standard big block pump on a tall deck engine with the two innermost belts driving the alternator mounted on the passenger side.

Belt routing

Shown here is a typical industrial/commercial engine setup used in larger trucks. Evident is how different the commercial accessory setup is from a typical big block used in small trucks and cars. This is from a C65 Chevrolet circa 1969, adapted to a non-governed carburetor and HEI ignition, but still typical of the era: Stock accessories

Note the v-belt idler pulleys, used for adjusting belt tension. These were only used on the commercial engines and are hard to locate individually.

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366/427 Intake manifolds

The tall deck intake manifolds are unique to the application. They are a two-plane, cast-iron design, with each side of the carburetor feeding four cylinders. This design encourages low end power, which is what these engines are designed for. The carburetor mounting pad is a square bore, designed to accept a specific Holley carburetor with a vacuum governor to limit RPM.

The distributor mounting pad is standard height, which means that a longer distributor is NOT necessary when using the factory manifold. All standard GM HEI and points distributors will fit these engines (although camshaft gear or oil pump drive shaft modifications may be necessary).

These manifolds have a unique two-thermostat design for a higher coolant flow. Both thermostats are identical, fully open at 180 degrees. When fully opened, a restrictor plate on the bottom of the thermostat descends to plug the bypass opening and force coolant through the radiator only. If these special thermostats are not used, the bypass will remain open at operating temperature and not all coolant will pass through the radiator. Because of the dual thermostat design, these intakes obviously use a unique thermostat housing as well.

366T intake manifold 366T intake manifold

Probably the hardest part of using the stock intake manifold with a standard water pump is adapting the huge 1.75in. radiator bypass down to fit the 3/4in. inlet in the pump. Although this reduces the bypass flow, it does allow use of a non-industial pump and its easier to find (and cheaper) associated parts like standard radiator hoses, etc. One way to solve this issue is to use a 1.75in. elbow from the intake, then adapt down the other end to fit the pump with standard NPT threaded fittings to create a 1.75in. to 3/4in. adapter. Some adapters can by purchased, but they are generally too long to fit the tight space of the elbow.

Radiator bypass hose

Note that the intake carburetor ports are all the same size, and intended for use with the specific Holley carburetor that was used from the factory. If a carburetor with larger secondary plates is used, an adapter plate must be used between the carburetor and the intake manifold to allow the throttle plates to open completely. Make sure to measure and check these carefully before installing your specific carburetor.

There are two different thermostat housings available; one at an approximate 45 degree angle forward, the other nearly straight up. The former was used on smaller trucks with lower radiators, while the straight model was mostly used on trucks with the high mounted water pump and radiator (although this is not always the case). Both use the same gasket and hose size. For water pump differences see here: 366/427 Water pumps

tall deck thermostat housings

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366/427 Overall engine differences

Tall deck engines and standard deck engines are not actually that different externally. The deck height is slightly taller, which allows a taller piston with an extra ring. The crankshaft stroke is unchanged. The taller deck means the cylinder heads are set slightly further apart, but spacer kits are made to use aftermarket intake manifolds. Accessory bracket problems due to this distance change can be overcome by using brackets for each accessory that only mount to the head or block respectively. When retrofitting a tall deck in place of a standard deck, the exhaust manifolds will also be slightly further apart. Almost all tall decks came with a factory cast-iron one piece crankshaft pulley/damper assembly. This can also be replaced with standard parts if desired, but the factory pulley is a deep, four groove and very useful, although heavy. This pulley does necessitate the use of either a short nose water pump or the factory pump.

366 engine block

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366/427 Exhaust manifolds

The factory tall deck manifolds are made for use with the front engine mount, not the 'clamshell' mounts. While the driver's side manifold will work with either, the passenger side manifold will be in the way of the 'clamshell' style mount. If using 'clamshell' style mounts the passenger exhaust manifold must be replaced.

Tall deck exhaust manifolds

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366/427 HEI conversion

Early points style distributers can be converted to later HEI (High Energy Ignition) style distributors by switching the oil pump drive shaft. Removing the factory distributor will remove governor functionality, so be aware of this. But probably if you're reading this, then the factory governed carburetor is long gone anyway. Early distributors use a hex style distributor drive, while standard HEI is a slot drive. In order to convert, either the distributor gear/shaft (as appropriate) must be swapped to the new distributor, or the oil pump must be removed and the drive shaft changed to the newer slot drive style. The outer cases/castings of the two distributors are fully interchangeable, and use the same dimensions from the intake manifold on down (obviously the tops from the intake manifold on up are different). If the original factory engine lifting points are still in place, the rear one may interfere with the vacuum advance canister of later HEI style distributors as the factory points distributor is a smaller diameter overall. This can be easily bent slightly out of the way to allow fitment. Other than some wiring harness changes to adapt to the HEI ignition, this is all that is required for the conversion.

Big block distributor comparison

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366/427 Governer system

Functionality: In operation, when the engine reaches its governed speed of 4000 RPM, the centrifugally-operated spinner valve in the distributor will close, thus applying a vacuum on the carburetor actuator diaphragm which in turn acts to close the throttle valves. The overspeed warning switch in the spinner governer vacuum line also senses this vacuum and lights a warning to alert the driver. The distributer is not specific to the governor system, but it does have some add-ons. There is a diaphragm on the carburetor which uses vacuum to keep the throttle shaft engaged with the throttle linkage. If the engine overspeeds, there is a valve inside the distributor that shuts and pulls the throttle shut. Basically, the distributor will allow air to pass through it until the engine hits the governed speed. Once vacuum builds up in the line to the carburetor, it will pull the throttle plates closed; otherwise the plates are spring loaded against the throttle linkage.

Holley vacuum governer

The governed speed is 4000 RPM no-load, or 3800 RPM full-load. Initial overrun allows for 150-250 RPM before settling to governed speed.

The vacuum determination is made at the distributor based on the rotation speed of the shaft and flyweights, with only the one vacuum line linking distributor and carburetor. The carburetor is a special Holley unit with the large assembly on the passenger side limiting the throttle based on this vacuum input. The rest of the carburetor is mostly standard Holley, but obviously the throttle linkage is different to allow the governor functionality.

There is only a mechanical advance for timing; no vacuum advance. This system was used on industrial engines only, so a vacuum advance offered little benefit. There is usually a vacuum operated electrical switch in the circuit to turn on an overspeed warning light on the dashboard. Most of these distributers have a 'T' connection off the bottom of the carburetor with one side to the carburetor governor, and the other to the electrical switch. There will also be a breather canistor off the side of the distributer.

Mechanical advance governer distributor

To check the mechanical advance: Remove the distributor cap and grab the top of the distributor where the rotor attaches and turn it clockwise. It should turn fairly easily with the only resistance being the pressure from the springs on the flyweights. If it doesn't turn easily or return with the spring pressure, the mechanical advance is hanging up. This can cause low power. It's fairly easy to fix; just remove the distributor, remove the roll pin and drive gear, slide the shaft out of the distributor, take the weights and springs off, slide the lower piece with the point cam off the shaft (will take some lubrication and working back and forth, especially if completely stuck). Clean up both pieces with some emery cloth, lube and reassemble everything in reverse order.

If you wish to switch to a non-governed carburetor but do not want the hassle of changing the distributor, the vacuum 'T' at the distributor base may be removed and the port plugged. Since the distributor is mechanical advance with points, it requires no connections other than the standard electrical.

Other than the more complicated carburetor mechanisms, the governor system is fairly simple and straightforward. Usually problems associated with standard distributor, ignition, or carburetor problems will be erroneously blamed on the governer. Before suspecting the governor system, make sure the other systems have been eleminated.

Holley 4150G carburetor Holley 4150G carburetor specifications

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366/427 Oil pans

The commercial/industrial big block engines came with a larger pan. One reason is the increased clearance for it in the larger engine compartments of bigger trucks, or in stationary engine applications like generators. Another reason is the better cooling and oil life given by an increased oil capacity. Any pan compatible with the proper generation of engine block will fit in its place. The larger pan may not clear some crossmembers in standard cars or trucks.

Big block oil pan comparison

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366/427 Specifications

366/427 tall deck specifications:
     Cylinder block deck height: 10.2 inches
     366 standard bore: 3.935 inches
     427 standard bore: 4.25 inches
     crankshaft stroke: 3.76 inches
     Bore spacing, center-to-center: 4.84 inches
     Piston compression rings: 3
     Connecting rod length: 6.135 inches
     RPM limit: 4,500 RPM
     RPM maximum sustained operation (governed speed): 4,000 RPM
     Except for some 1991+, all have forged crankshafts and forged connecting rods. All castings have 4-bolt crankshaft main bearing caps.
Note that the 427 block is not an overbored 366 casting. They are different castings, with the difference being in the cores used for the cylinder bores. A 366 block cannot be bored out to 427. The cylinder walls are roughly equivalent in thickness for the same heat dissipation. This gives the 366 an ever so slight coolant capacity increase also.

Engine block factory tall deck casting numbers:
 340220 --427 cubic inch, 1968-1976
 364776 --427 cubic inch, 1974-1976
 364779 --366 cubic inch, 1968-1990
 473478 --427 cubic inch, 1977-1990
 3937724 --366 cubic inch, 1968-1976
 3955274 --366 cubic inch, 1968-1976
 3955276 --427 cubic inch, 1968-1973
 3969852 --366 cubic inch, 1968-1976
 3969858 --427 cubic inch, 1968-1973
 3999293 --366 cubic inch, 1968-1976
 3999294 --427 cubic inch, 1968-1973
 10114183 --366 cubic inch, 1991+ (note: these engines may have cast, not forged crankshafts)
 10114184 --427 cubic inch, 1991+ (note: these engines may have cast, not forged crankshafts)

366 and 427 tune-up chart

366 and 427 fastener torque chart

366 and 427 mounting fastener torque chart

366 and 427 thermostat specification chart

366 and 427 fuel pump chart

366 and 427 tune-up chart

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Carburetor versus TBI

The TBI (Throttle Body Injection) versions of the tall deck engines have a different intake manifold, with two intake ports on top rather than the four for the Holley 4150G governed carburetor. There will also not be a mechanical fuel pump mounted on the block, and some later blocks may not have a provision for one. The rocker arm covers also switched to a more modern-looking finned design for these later engines.

The carbureted versions are almost all painted orange or blue from the factory, while the later TBI units are all factory black / gray.

Early tall deck engines (carbureted): early tall deck engines

Later tall deck inch engines (TBI): later tall deck engines

There have been some reports of carburetor use on tall deck engines past the casting updates, as well as TBI. These seem to be mostly isolated to use in school bus chassis. Our best guess on this is that GM was using up old parts before switching to the new systems, or selling chassis that had been manufactured earlier but not gone through upfitting until later and thus being designated as later model years.

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Carburetor conversion to Edelbrock/Carter

One primary thing to note is that most Edelbrock or Carter will not work out-of-the-box (OOTB). Adjustments will most likely be required, including jets and metering rods. Most of these are tuned for small blocks or common engines OOTB, and the tall deck is not a common engine but has specific requirements.

Some Edelbrock/Carter carburetors will bolt up directly to the square bore manifold, but any carburetor with even slightly larger secondaries should be double-checked to ensure all throttle butterflies open completely inside the bores. If not, a spacer or adapter may have to be used.

Removing the factory Holley carburetor will remove governor functionality, so consider this in your swap.

Some Edelbrock/Carter carburetors may be more finicky on fuel supply volume and pressure than the original Holley. These carburetors do not have a tremendously large fuel bowl, so they require an even and constant fuel supply at good pressure. Any fuel irregularities, especially under heavy throttle, may cause poor running characteristics.

Also note that most Edelbrock/Carter carburetors do not have all mechanical adjustments complete OOTB. The secondaries may not open fully until the tangs and rods are adjusted, and the accelerator pump rod may not be in the correct hole, among other things. The carburetor should be carefully gone over before and during installation to ensure everything is adjusted within specification.

The Carter AFB is very similar to the Edelbrock in almost every way. Jets and rods are almost all interchangable, along with almost all other parts. Edelbrocks are newer, while the Carter AFB was the original iteration of this design.

Choose the carburetor carefully, as things like CFM and throttle butterfly size will affect the performance. While metering rod and jet tuning can be performed to account for this a little, the initial CFM selection is very important. From our experience, these engines do like a lot of air and fuel under heavy throttle, so the secondary adjustments may need to account for that. Every carburetor will perform a little differently, especially as height above sea level varies, etc. Take all conditions into consideration when tuning.

Following is an example for a specific tune, tuned with an AFR (air/fuel ratio) guage under all throttle conditions, for use in south-eastern Pennsylvania. As can be seen from the specifications, the jet and rod requirements differ very far from out-of-the-box specs.
Engine: 366 tall deck, all factory specifications, HEI conversion.
Carburetor: New out of the box Carter AFB competition 9000 series carburetor, 9755S 2565.
Note: Carburetor requires adapter plate for secondaries to open fully on standard tall deck intake manifold.
Accelerator pump rod: middle hole.
Primary metering rods: 0.070 x 0.037 primary rods with orange spring.
Jets: 0.116 jets main, 0.113 secondary jets.
Electric choke: Edelbrock 1478 electric choke conversion kit.

Following is a tuning history for this engine/carburetor as these specifications and progressions may help others as they tune to see the variations in performance associated with various changes.
--Initial tune: 120-404 .104 primary jets, .104 secondary jets. Carb hesitates with accelerator pump rod on uppermost hole, less hesitation on middle hole, even less hesitation on lowest hole but still hesitates. AFR far out of spec., not very driveable.
--Tune 1: 16-691 .076 x .055 + orange spring, accel pump in lowest hole. accel hesitation, low power, backfires through exhaust during mid throttle and wide open throttle.
--Tune 2: 16-485 .065 x .052 + silver spring, accel pump in lowest hole. no hesitation, no backfires, better low to mid throttle power, low on wide open throttle power.
--Tune 3: 16-241 .0705 x .0475 + pink spring, accel pump in middle hole. No hesitation, no backfires, runs hot, exhaust manifolds too hot, AFR too lean.
--Tune 4: .113 main jets, .107 secondary jets. Rods .068x .047 #1447. Orange spring. Accelerator pump in middle hole. Runs ok but backfires and low power under heavy throttle.
--Tune 5: .116 jets main, .113 secondary jets. Rods .0705 x .0475 16-241, organge spring, accelerator pump in middle hole. Runs good, slightly rich AFR except when secondaries open and stay open at heavier throttle. Could use some more fuel on accelerator pump according to AFR.
--Tune 6: accelerator pump in top hole, silver metering springs. Still goes lean, but stumbles under accelerator stab.
--Tune 7: accelerator pump in middle hole, silver springs. Ok throttle response, still lean spike under heavy throttle or load, i.e. Going up hills, etc. stumbles.
--Tune 8: .070x.037 primary metering rods, orange spring, adjusted idle for AFR, screws about .75 turns out approximately. Good all around performance, leans out under load sometimes.
--Tune 9: readjusted idle for richer running, runs better when rich. Added second booster fuel pump, no more lean conditions under heavy throttle.

For a quick look at Edelbrock and Carter manuals, see here or here. For a quick look at older Carter series comparisons, see chart below.

All Edelbrock and Carter carburetors have a standard 5-1/8" airhorn for mounting air cleaner.

Carter AFB measurements by series/model (incomplete list):
Carb# | CFM | Venturi Diameter(in.) | Bore Diameter(in.)
4121 | | 1-1/16 | 1-1/4 | 1-7/16 | 1-9/16
4122 | | 1-1/16 | 1-1/4 | 1-7/16 | 1-9/16
4132 | | 1-1/16 | 1-9/16 | 1-7/16 | 1-9/16
4133 | | 1-1/16 | 1-9/16 | 1-7/16 | 1-9/16
4136 | | 1-1/16 | 1-9/16 | 1-7/16 | 1-9/16
4137 | | 1-1/16 | 1-9/16 | 1-7/16 | 1-9/16
4147 | | 1-3/16 | 1-9/16 | 1-9/16 | 1-11/16
4148 | | 1-3/16 | 1-9/16 | 1-9/16 | 1-11/16
4204 | | 1-3/16 | 1-9/16 | 1-9/16 | 1-11/16
4205 | | 1-3/16 | 1-9/16 | 1-9/16 | 1-11/16
4758 | 500 | 1-3/16 | 1-1/4 1-7/16 | 1-11/16
4759 | 625 | 1-3/16 | 1-9/16 | 1-7/16 | 1-11/16
4760 | 750 | 1-7/16 | 1-9/16 | 1-11/16 | 1-11/16
4761 | 500 | 1-3/16 | 1-1/4 | 1-7/16 | 1-11/16
4762 | 750 | 1-7/16 | 1-9/16 | 1-11/16 | 1-11/16
9400 | 400 | 1-1/8 | 1-1/4 | 1-7/16 | 1-7/16
9500 | 500 | 1-3/16 | 1-1/4 | 1-7/16 | 1-11/16
9501 | 500 | 1-3/16 | 1-1/4 | 1-7/16 | 1-11/16
9510 | 500 | 1-3/16 | 1-1/4 | 1-7/16 | 1-11/16
9511 | 500 | 1-3/16 | 1-1/4 | 1-7/16 | 1-11/16
9625 | 625 | 1-3/16 | 1-9/16 | 1-7/16 | 1-11/16
9626 | 625 | 1-3/16 | 1-9/16 | 1-7/16 | 1-11/16
9627 | 625 | 1-3/16 | 1-9/16 | 1-7/16 | 1-11/16
9635 | 625 | 1-3/16 | 1-9/16 | 1-7/16 | 1-11/16
9636 | 625 | 1-3/16 | 1-9/16 | 1-7/16 | 1-11/16
9637 | 625 | 1-3/16 | 1-9/16 | 1-7/16 | 1-11/16
9755 | 750 | | | |

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Crate engines

These engines, especially the 366 cubic inch, were very popular in school bus, Topkick, and Kodiak chassis (or C40, C50, C60 series) commercial chassis trucks. Due to the huge commercial market, they are still available for order in remanufactured long or short block form.

GM used to offer a 366 crate engine through their aftermarket Target/Targetmaster/Goodwrench supply chain, although supplies of these appear to be exhausted and no more are being made.

366 crate engine

Most crate engines still offered are only recommended for a drop-in fit for 1980-1996 trucks, B, C, and P chassis. Since so many of these engines were used in fleet applications like school buses or large companies like ASPLUNDH, there was a huge supply chain for replacement drop-in units that could place a truck back on the road quickly and easily in the event of a failure. Some of these engines are still sitting in warehouses ready to go, and can be purchased cheaply if located.

Due to the relative anonymity of these engines, and their primary use in commercial applications, not many suppliers will be aware of them or their history.

One current supplier seems to be newgmengines.com (this link will take you away from our site. We are not affliliated with this company in any way).

ATK Engines offers several remanufactured units, including options for CNG (compressed natural gas). These are also sold through the Jegs website.

The availability of these engines on the aftermarket after so many years speaks to their longevity and durability, as well as mostly unnoticed (in the mainstream) commercial demand.

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Factory applications

Below is a list of factory applications for the 366 and 427 tall deck engines, based on research to the best of our ability, so errors may occur. The 366 and 427 tall decks first appeared in 1968, and the last tall decks were cast in the 1990's. Later units seem to be cast for certain up until 1992 at GM (and possibly later by aftermarket suppliers like Goodwrench?), although we cannot confirm any final ending casting dates for sure.

If GM installed these engines after the supply-chain end dates, either a large overstock existed as GM phased out these engines, or GM obtained these later engines from its suppliers like Goodwrench or used remanufactured sources in their new installs.

We also have reports that 427 tall decks were installed from factory well after the supposed supply-chain end dates given by GM (thanks to reader Chris for updating us on this), most likely for the reasons given above. Below are photos of a 1992 7.0L emissions label 427 tall deck with TBI, which Chris reports was in a 1997 factory vehicle.

427 TBI engine

Following is a list of factory availability listed for these engines in GM documentation. We guess however, that engines were supplied as options even after some of these dates due to GM having a large backstock of inventory.

RPO codes:
-366 cubic inch tall deck (200 hp): L86 or LS0 (LS0 is unconfirmed, but suppliers say it was used).
-427 cubic inch tall deck (230 hp): L43.
Note that both the L86 and L43 codes were later replaced by different GM engines, so newer charts will have them listed incorrectly.

B chassis bus:
-1968-1992: optional/standard 366, optional 427.
-1992-1998: standard 366.

C chassis medium duty GM truck:
-1968-1973 C40, C50, C60 medium duty trucks: optional 366, 427 optional.
-1973-1989 C40, C50, C60 medium duty trucks: standard 366, 427 optional. Gas engines were not factory available in the C70 trucks.
-1990-1996 GMT530 chassis Kodiak/Topkick: standard 366, 427 optional until 1991.

P chassis van:
-1983 P6000: standard 366.
-1984-1986 P6000: optional 366.
-1987 P6000: optional 366, 427 optional.
-1988 P6000: optional 366.
-1989-1993 P6000: optional 366, 427 optional until 1991.
-1994 P6000: 427 optional.
-1995-1996 P6000: standard 366, 427 optional until 1991.
-1992-1996 P6500: standard 366, 427 optional until 1991.

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Generation differences

There are two generations of factory tall deck big block engines. Those cast up to 1990 follow the same general format as other Mark IV big blocks. Those cast after 1990 were modeled along with the changes to the other big blocks in Generation V.

Below is an example of a Mark IV 366 cubic inch tall deck engine in long block form. These are just a Mark IV Chevy big block casting modified for the taller deck. They were used up until 1991.

Mark IV 366 crate engine

Below is an example of a Generation V (GM changed from 'Mark' to 'Generation' due to Lincoln trademark on the 'Mark') 366 cubic inch tall deck engine in long block form.

Generation V 366 crate engine

Used in post-1991 castings, these have a one-piece rear main seal, no provision for mechanical fuel pump, and slightly different internal oil galleries. The internal casting cores were different, as can be seen in the freeze plug locations, especially on the rear of the engine. The side freeze plugs are also differently located. The drilled and tapped holes on the front are identical, but the casting webbing is very different.

The block was once again a modified version, for the taller deck, of the standard Generation V big block design. The cylinder heads vary slightly, but overall functional design is the same except for the water jackets. For this reason heads are generally not interchangable between generations. The valves are also supposed to be non-adjustable on this generation (according to factory literature). The top of the engine is also identical, and intakes, etc., are completely interchangable. Overall the two generation long blocks can be swapped, EXCEPT for the fuel pump provision on earlier blocks. Of note is the fact that Generation V tall deck blocks DO NOT have the external oil cooler ports above the oil filter like other Generation V blocks (like the 454 casting).

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Why bother with a 366?

This is more of an opinion piece than dealing with direct specifications, but we include this as a summary of why we prefer this engine.

The 366 cubic inch 'big block' is an industrial-specific engine designed from the outset for a set of specific parameters. They are shunned by most hotrodders, engine builders, and modifiers mainly because of a lack of knowledge about them. The general consensus is they are a 'boat anchor', but in reality most people seem to dismiss them just because of either what they've heard or they've never heard of them to begin with. It's very easy to drop in a $15,000 crate engine like a 502 cubic inch and call another engine bad without knowing anything about it.

The main arguments against the 366 seem to include a 'small' displacement compared to other big blocks, as well as supposedly using many parts that are incompatible with other engines. A lack of performance-oriented bolt-on parts is sometimes cited. A high cost to build them for power (relative to expensive crate engines) is cited as a reason. Some reasons just quote other sources as hearing that they were bad with no supporting data. Generally a lack of data prevails.

The background of the 366 is very important as the design factors were industrial-based from the outset. GM engineers modified the Mark IV big block with a focus on longevity under hard use. The goal was a sturdy, dependable engine that was long-lasting, relatively low-maintenance, and comparatively low lifetime cost. In essence: the ideal truck engine for most users.

The block deck height of the 366 is higher, allowing an additional piston ring without changing the connecting rod length or crankshaft stroke. This additional ring gives a better cylinder seal over the long term for piston intake and exhaust strokes under heavy use. It is also one more ring that must wear for compression to be reduced, which adds longevity to the short block. This modified deck height makes this engine part of the 'tall deck' family.

The taller block deck height forces use of a different intake manifold than the standard big block, or an adapted version. This is one of several reasons builders often give for not preferring this engine family. The intake manifold is designed specifically around the intended engine use, so it is not easily modified for uses outside the intended power range. Likewise the aftermarket is not eager to make a bespoke custom intake manifold for a specific engine family based around trucks rather than racing use. However when building a tall deck around the same power band as its initial design, this manifold works well. Almost all are a standard cast-iron design for use with a square bore four barrel carburetor. They have some advantages over other big block manifolds that are often overlooked.

The factory tall deck intake manifold has a larger coolant bypass than normal. It also is designed to flow more coolant, and comes fitted with dual thermostats and a larger output hose housing for this purpose. Special thermostats are used that, when open, close off the bypass circuit below at the same time, forcing all the coolant through the radiator. In addition to providing more coolant flow, another consequence of dual thermostats is redundancy. If one fails in the closed position the other can still provide (reduced) coolant flow to the radiator. This can provide a slower overheating cycle rather than instant overheating if a thermostat should fail.

Although it is often rumored (incorrectly) that a standard length distributer cannot be used with these tall decks, in fact it is only when a regular big block intake manifold is adapted for use that modifications may need to occur. The stock intake manifold distributer boss is machined lower, allowing the standard distributer to be used. When a regular big block intake is fitted, the distributor shaft may have to be lengthened or the intake boss machined down. This is another reason builders sometimes give for avoiding these engines, but in reality there is no issue at all if the stock manifold is used.

The 366 has a relatively small bore size for a big block engine. While this seems like a disadvantage from the start, it actually has many inherent advantages for strength and endurance. The cylinder walls are thicker than would normally be attainable in the standard large-bore big block, while also having more space around the bores for coolant flow. This also allows a slightly increased overall coolant capacity in the block. The small bore, along with the long stroke of the big block, is generally ideal for the intended low-RPM power range of the engine.

While the standard carburetor (for those units prior to fuel injection) fitted to the 366 is a complicated square-bore vacuum-governed Holly four barrel, this can easily be replaced with any other four barrel carburetor, and tuned accordingly.

The internal rotating assembly of the 366 is biased towards strength, using a forged crankshaft. The connecting rods generally use a slightly larger bolt size, and may also be forged. The pistons are standard cast alloy with an extra compression ring groove, and slightly taller to accommodate this as well. While this adds rotating mass to the assembly, the engine is not designed for high RPM use so this consideration is not highly relevant when keeping the engine in its intended power range. The heavier piston also makes it marginally stronger than a regular big block piston. When adapting a tall deck block using standard height pistons, a longer connecting rod may be used which is an advantage over standard height blocks (if longevity is not the ultimate goal). All 366 blocks have four bolt main bearing caps.

The cylinder valve size of the 366 is often criticized as small in relation to other big blocks, but since the cylinder displacement is also small in relation it is a poor comparison. The valves are actually well sized when using a proper yardstick. A better comparison uses the cylinder head of the 350 cubic inch small block, which is only 16 cubic inches different overall, for only 2 cubic inches different per cylinder. A good 350 head, which is widely regarded for good flowing valve sizes in relation to displacement, can use intake/exhaust sizes of about 1.94/1.6". The usual 366 cylinder head uses intake/exhaust valve sizes of about 1.94"/1.84". So although the displacement is nearly the same, valve sizes are easily better flowing, or just as good.

Some early (1966-1968) tall deck engines have a timing gear drive and reverse-rotation cam shafts, since only two gears are used. These have a bespoke distributor drive gear for the reversed direction. They are uncommon, but easily converted to a standard timing chain, camshaft, and distributer gear if necessary. They are less likely to lose timing position over use due to chain stretch (since there's no chain) and seem to have few drawbacks associated with them other than parts availability.

The block dimensions, such as bore centers, camshaft to crankshaft centers, bolt locations, etc. are all the same as a standard big block with the addition of the added deck height. The cylinder heads, although slightly farther apart due to the taller deck, are the same general design as regular big block heads. This can make some accessory brackets different between tall decks and regular decks when bolted to both block and heads simultaneously, but this is easily overcome. It is important to note that the main gasket difference between tall decks and regular deck blocks is the intake manifold lower gaskets. These are slightly longer on tall decks to accommodate the heads being spread slightly farther apart. Almost all other gaskets and bearings, etc. are interchangeable, in general.

Externally, most standard big block parts such as oil pan, rocker covers, exhaust, starters, and accessories are the same, making these engines easily adaptable for use anywhere a standard big block would fit. So many performance oriented big block parts are perfectly compatible with these engines, from exhaust to accessories.

Overall these changes make the 366 tall deck engine ideal for use in trucks where low-end torque is an advantage. Cooling capacity is increased, and the ability to operate at elevated RPM in the ideal power range is extended. Essentially, this means that this engine is well adapted for running all day, every day, on a job-site under heavy load. These characteristics make it a very good, reliable truck engine. Overall when building a cost-conscious truck engine, a well built 366 is a very good and cheap option when compared to an expensive larger-displacement, fancy crate engine that is designed for high RPM racetrack use and not low end torque and overall longevity.

We chose a 366 for one of our applicatons for all of these reasons. Cost was a big concern. The engine is generally common, being used in GM medium duty trucks for over three decades. They also have the distinct advantage of being unwanted due to the general disinformation and lack of knowledge. Being a bespoke truck engine, many people are unfamiliar with them to begin with, rather being focused on car and light truck engines. This makes them cheap, and relatively easy to locate.

Parts are generally easily obtainable, despite general opinion. Rebuild kits are plentiful because the engine is common in industrial settings. Big block parts are mostly interchangeable. Parts availability is not a disadvantage, but knowledge about those parts may be. This is easily overcome with a little research.

Sturdiness was concern number two. Rebuilding engines regularly is not a fun job (for us, anyway). The particular engine core we chose had actually never been rebuilt, and came off the factory line in 1972. We performed the rebuild in 2018, before which the engine served all its years regularly racking up hours in a medium duty wrecker. That in itself is a testament to this engine family's durability. Similar stories are not uncommon. The engine this replaced was a tired, truck-based four bolt main 350 cubic inch, which was not original to the chassis. Although it still ran, the necessary grunt power for a truck engine just wasn't there.

In summary, we were looking for a good truck engine, and relatively cheap. The 366 is the ideal solution.

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366/427 versus 454

This is more of an opinion piece, but it contains relevant comparison information from fact-based knowledge.

The 366/427 engines are designed for industrial and commercial use. The 454, while a 'truck' engine, is not designed to withstand abuse and torture under high RPM load for long periods. The cooling system of the 454 is not upgraded to handle this type of use, and the short block parts are not (usually) upgraded to handle this from the factory.

The 454 serves good use as a light duty truck engine in pickups and 1-ton and less sizes. But for industrial use it is not as well suited or long lasting as the 366/427 family. While the 454 served long use in RV's and some heavier vehicles, it generally was not ever intended for long duration heavy-duty use. This is quite evident in the quantity of insufficient cooling reports in RV's that were equipped with the 454. Without the upgraded internal parts, the engine also will wear much quicker under heavy use, reducing the lifespan.

While a 454 can be 'adapted' with heavy duty parts, there are some things that cannot be changed easily. Specifically the heavier-duty (larger/taller) pistons. Unfortunately, this is the primary reason hotrodders do not like the factory tall deck engines (except for the taller blocks alone). The factory tall deck pistons have more in common with diesel engines. A heavy, cast piston, with more rings, made for high temperatures and abuse. Since the piston is the business end of the internal combustion, it is really the starting point for a good heavy-duty short block. Racers and hotrodders don't care about longevity; they often do rebuilds every few thousand miles. But in an industrial/commercial setting this is not practical or cost-effective.

Another factor GM took into consideration was overall complexity. Adapting an existing engine to heavy-duty use would have required more bespoke parts, and more complications like oil-squirters directed at the piston undersides and skirts that are more common today. But today there are also very few, if any, simple heavy-duty gasoline commercial engines. The focus today is more on diesel for this application, while this was not the case in the 1950's and 60's. So complication versus simplicity and service life today is less of a factor in gas engines.

All of these reasons are why GM decided to invest in a dedicated industrial gasoline engine family for the Gen IV big block engines, rather than try to use an existing option. It is also quite revealing that the tall deck family lasted well into the 1990's with few changes.

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We found these resources valuable for original reference material of these engines:

     1971 Chevrolet Series 40,50,60 Medium Duty Truck Service Manual
     1972 Chevrolet Series 40,50,60 Medium Duty Truck Overhaul Supplement
     Carter AFB Rebuild Manual
     Edelbrock Performer Series Carburetor Manual
     Carter AFB Carburetor Tuning

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