l72 flywheel

[william e snyder]

can any one tell me the part # for a 1966 L72 fltwheel? is the L72 internally or externally balanced? bill

[Warren F.]

Re: l72 flywheel

Bill,

I show the part # to be 3998281. 427's were internally balanced, 454's were externally balanced.

[Joe L.]

Re: l72 flywheel

william-----

All 427's, including the L-72, were INTERNALLY balanced.

The original PART # for the L-72 flywheel was GM #3889694. The current PART # for the flywheel is GM #3991469. Remember, these are PART numbers and NOT CASTING numbers. The PART numbers for the flywheel appear nowhere on the flywheel; they are only found on the boxes that SERVICE flywheels are supplied in.

There are, at least, several casting numbers used over the years. 3789733 is one. 3973456 may be another.

[Joe L.]

Re: l72 flywheel

Warren----

GM #3998281 was a part number, although not the original part number, for the flywheel for 1966 L-72 (and, many other) applications. It was discontinued about 30 years ago and superceded 3 times before being replaced by the GM #3991469 about 25 years ago. The 3991469 remains available today for about $215 bucks, GM list.

[william e snyder]

Re: l72 flywheel

if i have a 733 am i ok to use it or are there differences in machining? is there anything i should look out for and is it important to have it balanced before i use it? i just spent over 15k on my motor and want to be sure,bill

[Joe L.]

Re: l72 flywheel

william-----

I don't know if there were other flywheels manufactured from the same casting; that's the problem with trying to use casting numbers for a positive ID or "surrogate" for the finished part number. However, I really do think that any flywheel manufactured from the '733' casting will be just fine for your application.

I don't recommend using any surfaced flywheels and DEFINITELY not one that has been surfaced more than once. With ANY flywheel, new or used, I ALWAYS have it balanced and the clutch assembly balanced. ALWAYS. WITHOUT FAIL. PERIOD.

[Terry M.]

Re: l72 flywheel

When the rotating parts of the engine are balanced the flywheel, clutch disk and pressure plate should be balanced with the rest of the engine's rotating parts.

[Verle R.]

Re: l72 flywheel

Terry,

The man I use to balance parts build race car engines professionally.
He balances the flywheel by itself, then attaches the clutch and balances that assembly, making any change to the pressure plate.
He uses the bare flywheel (after balance) on the crankshaft when balancing the crank and makes no change to the flywheel, only to the crank.

Verle

[Terry M.]

Re: l72 flywheel

Far be it from me to tell folks with that kind of experience how to do it. But, that is not the way Chevrolet did it, but then there might be other reasons for the way Chevrolet did it.

[Verle R.]

Re: l72 flywheel

I suspect Chevrolet was not concerned with changes to the engine/flywheel/clutch.

The method I described isolates the flywheel/clutch balance from the engine assembly balance. If a change is made to the rods/pistons it does not require the flywheel to be rebalanced and vice versa.

How close did Chevrolet balance engines "back in the day"?

I know the LT5 engines were very carefully balanced and balance was checked on the complete running engine on the engine dyno. If it did not meet specifications they corrected it before the engine shipped to GM.

Verle

[Terry M.]

Re: l72 flywheel

Off the top of my head I don't know how closely they balanced the engines. I know those destined for Corvette were fired longer and balanced more closely than those for trucks or passenger cars -- I just can't remember the numbers. I might be able to look them up, but my best memory is that we were told how many inch-ounces when we toured Flint engine back when.
Chevrolet balanced the complete rotating assembly -- including clutch and flywheel -- at the same station that they hot fired the engine. That is why I said the clutch and flywheel should be balanced with the rest of the rotating assembly. What I don't know is if the various reciprocating components were weighed individually, and the rotating components balanced individually, before they were assembled. If they were, then the process is the same as you described.
But we do agree that the flywheel, pressure plate and disk need to be balanced.

[Verle R.]

Re: l72 flywheel

Absolutely they need to be balanced as well as the rest of the rotation assembly.

I seem to remember hearing they matched pistons and rods weight to some tolerance but don't know if they balanced the rotating assembly separate from the process you describe. How ever they did it, it obviously worked for millions of engines over the years.

[Jack H.]

Re: l72 flywheel

Foggy memory says the total balance spec for a Chevy engine was in the 5-8 gram range while other divisions, like Cadillac, held tighter tolerance in the 2-3 gram range... Typically, race guys work to fractions of a gram which is probably overkill for a street machine.

[william e snyder]

Re: l72 flywheel

my concern is that i assume that tom from cassic engine co balanced the L72, i sent him the fly wheel to balance with it but i am not sure this was done. if it was not balanced with the engine then i am assuming the answer would be to at least balance the clutch fylwheel assm. before i install it. i thought the harmonic balancer's job was to make up for out of balance issues. am i on the right track,thanks bill

[John H.]

Re: l72 flywheel

In the 60's, there was no balancing operation at Flint V-8; weights of the parts of the rotating/reciprocating assembly and dynamic balance specifications for the crankshaft were specified on the drawings, and the engines were simply assembled, with part weights checked on a statistical sampling basis by Receiving Inspection.

Many production engines today are done exactly the same way, including the 600hp Viper V-10 (crank photo below).