Engine differences 73 and 75 500cui

[t2n]

Hello!

Found a 1973 500 eldo engine with alot of parts on it, which I thought I could use either as spare parts engine or overhaul it for engine swap in the future.

However, I have a 75 eldo.

What are the differences that one should think of before attempting this?
Lead additives? Interchangable parts? Could I put stock 73 engine in the car and put on 75 parts(carb, hvac, starter, generator and such)?
I know the carb wont match right away.

Thanks in advance!
//Filip

[TJ Hopland]

Most significant difference between 73 and 75 is the heads.  In 71 when GM mandated low compression the Cadillac engineers apparently thought it was going to be temporary so instead of designing new larger chamber heads to accomplish this they went to a very large dish in the pistons.   74 they came out with the new head and a more typical piston.  Pistons are available to get the compression you want from either head but if you have both heads available to you most people would go with the earlier head if they wanted high compression and the later head for low.  No other real differences in the heads for performance or valve seats, its all about the chamber size which. 

Other differences are pretty minor.  75 got the electric choke so the intake is slightly different.  Earlier intakes had a well in the exhaust crossover for the choke remote coil.  73 was the first year for EGR.    75 had HEI distributor so there isn't the little cast standoff on the intake to mount the coil.     

The HEI which was an option in 74 is what caused the change from the dual AC/PS belts to the slightly larger single belt.  The dual belt would rub on the distributor cap.   Pulleys and such are easily swapped.   75 often didn't have a smog pump so its got a different crank and water pump pulley.   If you are swapping pulleys around be sure to also take the timing indicator. 

70-74 the 500 was Eldo only so they will have a different oil pan, pickup, and dipstick than the 472's that came in the RWD cars. If you have a RWD car the Eldo pans usually have some value to swap people because the Eldo pan is the closest to fitting in place of a Chev engine.   Blocks were the same 70-76.  Difference between a 472 and 500 is crank and pistons.

[hornetball]

This reference has pictures and part numbers to illustrate the differences that TJ so accurately summarized:

http://www.cad500parts.com/catalog/page2.htm

["Cadillac Kid" Greg Surfas 15364]

Filip,
If I was in your position and location, what I would do is take the 1973 short block and rebuild it using flat top pistons.  Those pistons and your cylinder heads from the 1975 motor will give you a compression ratio of something like 9:1 which is a bit better than the 8.2:1 you have now.  When you need the motor I would pull yours and using the 75 cylinder heads and all your 1975 motor accessories installed on the rebuilt short block have a good solid motor that will be good for MANY miles and give you a bit more performance as well.
Greg Surfas

[t2n]

Hey!

Sorry for a little late reply!

Wonderful information! Great summary TJ, spot on was I was worried about! Some confirms on the thoughts in that link, and a recipe for a bit more powerful engine to sum it all up.


Great! Many thanks guys! For sure will see if this is worth a go!

//Filip