The Infopages (Cont.)

The 120cc ‘large chamber’ ‘493 cylinder heads were introduced on the ‘74 472/500’s and continued through 1976. Occasionally, you'll find a "late" 472/500 equipped with a head which has casting numbers ending in ‘552. This is essentially the same as the ’493 head, without the smog rails cast in. Like the small chamber heads, some cars shipped without smog pumps carried the ‘493 heads without the smog rails drilled. Unscrupulous (or unwitting) junkyards have passed off many a ‘76 500" as a ‘70 model because the ‘552 heads, like the ‘950’s, have no smog passage between the T-pedestal support towers. They do have a casting number, so keep ‘em honest!

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:!. While this is accurate, it is not a good idea for several reasons. The 425, having a smaller bore, has the water passages closer to the bore centerline, which tends to cause head gasket failure problems. Additionally, the smaller ports flow significantly less than the 472/500 heads, and the loss in flow will hurt the power more than the gain in compression will help the power. These heads can use the same oversized valves as the 120cc heads, but cannot be ported as effectively to thinner casting walls.

The only aluminum heads available at this time are for the 472/500 engines, and have a modern, closed chamber design, with 76cc chambers, and use the same valves as the 76cc iron heads.

For those of you considering power above all else, head flow is extremely important. Other head choice considerations include compression ratio choice. There is currently no low compression off the shelf piston in production for 500s with the ‘76cc heads. We have a limited number of them remaining, and not all bore sizes are available. There is also no off the shelf high compression piston in production 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, considering the same level of port work, the ‘76cc heads outflow the 120cc heads, making them a better choice for maximum performance. The un-ported aluminum heads outflow most ported iron heads, with the exception of the best small chamber iron heads available. The aluminum heads also have much more room for porting than the best iron heads, and as a result, the best option for serious performance is the aluminum heads.

Don't mix early heads with late pistons, or late heads with early pistons. You will get a compression ratio of 5-6:1 or 12.5:1. If you are building an engine that needs a compression ratio in one of those ranges, 30 year old cast pistons aren't likely to last long anyway. Beware replacement pistons, as well. With the exception of the Keith Black and custom forged pistons, if the top of the piston and the width of the skirt do not look identical (or extremely close to) your old pistons, they are not the correct pistons.

Other engine ID items:

‘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 intakes are dual plane. The ‘68-’73 472/500s have a 3 groove crank pulley, and all but the ‘70 have a 2 groove water pump pulley. The stock smog pump belt configuration consisted of 1 belt going from the front crank groove to the water pump to the smog pump, 1 belt from the water pump to the alternator, and 2 belts in tandem going from the crank, to the AC compressor, to the power steering pump. Non-smog cars have a single groove water pump pulley, the front crank groove further out, and a single belt going from the crank, to the water pump, to the alternator. The ‘74-79 pulley sets use the same configuration for the water pump/smog pump/ alternator, but have a single, wider belt for the AC and PS, and only 2 grooves on the crank pulley. If you have a smog pump spec crank pulley, eliminating the smog pump will require new pulleys or an idler pulley for the crank/water pump belt. Note that the stock 2 groove crank pulleys will not clear the early water pump (though the aluminum 2 groove pulleys will), and the dual AC/PS belt will not clear either the ‘74-up water pump or HEI distributor (OE electronic setup with coil-in-cap). If you are not using AC, the dual PS belt should fit with any water pump and distributor. Note that while electric chokes and HEI distributors were ‘74-up only, it is not un-common to find them on early engines.

Most external accessories (with the exception of the single plane intake and different rear AC bracket) are identical or very similar between the ‘74-’76 472/500 and the ‘77-’79 425 engines.

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 may run into all sorts of trouble.