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 Why bother with a 366?


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 actually specific to the tall deck engines. They 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

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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

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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

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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 probably remove some 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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Why bother with a 366?

This is more of an opinion piece than dealing with knowledge, 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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