Identifying Your Engine

Day after day, we get calls from people who have a Cad motor but are unable to tell what year/model they’re looking at. Since many Cadillac 472 and 500 engines use the same block, and the later (425 and 368) engines look similar, as well, you’ll have often to have a look deeper than the block casting numbers to finally determine which species of torque animal you have there. For a printable PDF engine ID guide, please visit the tech section of our web site.

Block ID:

The first step is to ID the engine block. The block casting number is found at the rear of the Cadillac engine, between the intake manifold and transmission bell housing, right next to the oil pressure sending unit. The casting number is the raised, cast in number on the passenger side. Note that the ’68-’69 (and a few rare ’70 472s) use a block that looks identical at first glance, but bears different casting numbers (ending in 6238), has smaller oil passages and does not have clearance for a 500 crank (rod bolt notches in bottom of cylinder bores). Fine for a mild street engine, but unacceptable for a serious performance engine.

One easy way to identify a 425 or 368 vs a 472 or 500 Cadillac engine block is to look at the rear of the block. The 2 smaller engines have a ‘box’ shape cast in near the oil pressure port, which makes identifying them at a glance fairly easy.

Another way to identify a 425 or 368 vs a 472 or 500 Cadillac engine block is by the shape of the timing cover or the opening it covers. The later blocks have a pronounced extra cutout toward the passenger’s side of the opening, and extra material on the timing cover to cover it.

Piston ID:

The ’68-’70 472″ and ’70 500″ engines have the 10:1 (or 10.5:1) pistons. Do not assume your high lift cam will work with those pistons! Those are compression reliefs, not valve reliefs. The Dish is shaped something like a figure 8 or ‘squashed peanut’, and is sometimes described as 2 overlapping round dishes. The ’68/’69 dish is a slightly different shape than the ’70 dish, and is slightly higher compression. This dish is about 3/8″ deep.

In 1971 the compression was dropped to 8.5:1. This dish is also about 3/8″ deep, but is rectangular (often alled the ‘soapdish’ or ‘bathtub’ piston).

The 1974 model introduced the new large chamber heads, and the 8.25:1 compression ratio. The ’74 472 sported a unique true flattop piston. This ’74 piston has no swirl chamber, no valve reliefs, not even the 8cc depression of the ’74-’76 500 piston (round dish, approx. 1/8″ deep). Note the ID picture is of the current replacement piston, which has valve reliefs not present on the original.

The Keith Black hypereutectic pistons are available in 3 configurations. There is a D-dish 472 piston that makes 9.8:1 compression, and a 500 piston that makes 10:1 compression with small chamber heads, as well as a true flat top 500 piston that makes 9:1 comrpession with large chamber heads and 13:1 compression with small chamber heads. There are also various D-dish and flat top style forged pistons out there. The KB pistons generally have the ‘KB’ logo and part number on the inside of the skirt. Most custom forged pistons have the name of the piston manufacturer on the inside of the skirt, as well.

You cannot interchange 472 and 500 pistons due to the different compression heights. The 472 and 500 pistons can be visually identified by the size / shape of the piston skirt. The 500 has a shorter, narrower tip on the skirt than the 472.

Crank ID:

500 crank casting numbers are usually next to the 2nd or 4th main journal. 472s are usually on the 3rd counterweight.

The 425 and 368 crank is the same stroke as the 472 crank, and is physically interchangeable, but this is not recommended due to balancing and weakness issues. The 425 and 368 crankshafts can usually be identified by a scalloped shape to the flywheel mounting flange.

Cylinder Head ID:

The cylinder head casting numbers can be found in 3 places. The full 7 digit casting number can be found on the top of the head, between the rocker mounting points. It is not uncommon for this number to be missing.

The last 3 digits of the casting number can always be found on the bottom side of the head, in a recess under one fo the center exhaust ports. This recess does not get machined when you surface the head, and hangs out over the lifter valley, so you can read it with a mirror after pulling the intake manifold, if it’s not covered with crud. Be carefull breaking loose the carbon buildup if you plan to use the engine without rebuilding it – that carbon grit you break loose will wreak havoc in the engine (bearings, etc).

The third place the casting number can be found is on the top of the left (driver’s fromt, passenger’s rear) exhaust port. Most 425 heads and a few late 500 heads have the number in this location. This is another quick way to ID a core as a 425 (head number 423) when junkyard hunting, etc.

From ’68-’73, the heads were of the 76cc small chamber design. These heads have one of 3 casting numbers. All ’68/’69 and ’71-’73 cars were equipped with a smog pump from the factory, while the ’70 cars were not. The ‘250 and ‘902 heads are equipped with built in smog rails, while the ‘950 heads were not. Not all ‘250 and ‘902 heads were drilled for smog rails, if they were installed on a non-smog ’70 car, even though the extra material is in the casting.

The 120cc ‘large chamber’ cylinder heads were introduced on the ’74 472/500’s and continued through 1976. These heads have one of 2 casting numbers (‘493 with smog rails cast in and ‘552 without).

The 96cc 425 heads are very similar to the large chamber 472/500 heads, and even use the same valves, springs, etc. Many people claim that it is a good idea to use these heads on a ’74-’76 472/500 short block, as it will raise your compression to 10:1. While this is technically accurate, it is not actually a good idea for several reasons.

The aluminum heads we have available at this time  for the 472/500 engines, and have a modern, closed chamber design, with 72-80cc chambers, and have raised ports with oversized valves.

For those of you considering power above all else, head flow is extremely important. Other head choice considerations include compression ratio choice. There is a high compression piston (10:1), we have in production at this time, for the 120cc heads. As for flow, the 76cc and 120cc heads flow within a few percent (all 5 castings) with stock valves and no port work. The ‘423 heads flow significantly less. When ported and equipped with oversized valves, 76 & 120cc heads perform well. Our un-ported aluminum heads outflow  ported iron heads.. The aluminum heads also have much more room for porting than the best iron heads, so they can be ported to flow much better. The power limit on the ported iron heads is well beyond what most bracket engines are built to make.

Other engine ID notes:

’68 – mid ’70 engines used the first generation heat operated choke, with an exhaust passage around the front of the carb. These intakes have a cast rectangular box-shaped choke well. Mid’70-’73 engines do not have the heat passage around the carb, and have a round choke well with what looks like a freeze plug in the bottom of it. Many ’68-’70 engines also had no EGR valve. All OEM 472/500 carbureted intakes are dual plane. Note that while electric chokes and HEI distributors were ’74-up only from the factory, it is not un-common to find them retrofitted to early engines.

Most parts outside of the block, crank, pistons, and head castings are interchangeable between the ’74-’76 472/500 and the ’77-’79 425 engines. Some parts require other parts be changed at the same time for compatibility.

Finding a 500 can be difficult due to the lack of external identification. While all ’70-’76 Eldorados and all ’75-’76 cars (except the Seville) came from the factory with a 500, it is generally very difficult to tell if the engine is original on a 30+ year old vehicle. As for the difference between a 472 and 500, there is very little. In most cases, if they are the same year, they will be absolutely identical except for the crankshaft and pistons. For instance, there is no ‘472 head’ and ‘500 head’. The head style is determined by the year, not the crank. The pistons must match the crank/head combination, or you can run into all sorts of trouble.