C1 Flywheel Question

[Christopher R.]

I've got a 1974 (I think.) Camaro engine in my 1962 Corvette. That's the bad news. Good news is that that engine is supposed to have the same flywheel, 3986390, as the 1962 Corvette. Took it out today. It is cast 3988999N.

1. Anybody know what a 3988999N flywheel is? I can't find it in my Chevy Parts catalogs. Anybody know what size clutch it takes?

2. If I have to go buy a flywheel, what are my options? It's got to fit under the original 1962 Corvette bell housing.

3. The 3988999N flywheel I took out has some gouges and scoring, but no more than 1/16" deep. Think that can be resurfaced out?

[James F.]

Re: C1 Flywheel Question

Chris,
I can't answer your numbers question. Very likely Joe L. will find it no challange! Will suggest a flywheel you may not have considered. Consider an aluminum one. Regards,

[Joe L.]

Re: C1 Flywheel Question

Chris----

GM #3986390 is a PART NUMBER for a flywheel. The number "3988999N" cast on the rear of a flywheel is a CASTING NUMBER (i.e. it's the part number for the CASTING, NOT the finished part). For Chevrolet flywheels, the casting number is RARELY, if ever, the same as the finished flywheel part number. More than 1 flywheel may be made from the same casting.

Anyway, the GM #3986390 flywheel is a 14", 168 tooth, internally balanced flywheel. It's the SERVICE replacement flywheel for 1957-62 Corvettes and it was used in PRODUCTION for some mid 70's lower performance applications like the 74 Camaro application you mentioned. It was also used for 78-79 Corvettes with L-48 and 4 speed transmission. I believe that GM casting number 3988999N is one of the casting numbers used for this flywheel. I believe another was GM #3729004 (this was also the casting number of the original flywheels used on 57-62 Corvettes) and there may have been others, too.

The GM #3986390 flywheel is designed for use with a 10-1/2" clutch. It MAY also be dual-drilled for use with a 10" clutch as was originally used for all 57-62 Corvettes. While I think that a 57-62 Corvette bellhousing will accomodate both a 10" and 10-1/2" diaphragm clutches it will NOT accomodate an 11" clutch.

If you currently have a GM #3986390 flywheel (casting #3988999) and it works in your bellhousing, as it should, you want to try to preserve it. The GM #3986390 flywheel is discontinued and difficult to locate. I don't think that this, particular, flywheel is available in the aftermarket, either. But, it's possible that it is; I have not really followed it.

If your flywheel is "first-run" (i.e. it's not been previously surfaced) and is in the condition that you describe, I think that it could be re-surfaced without much problem by a competent machine shop. That's what I recommend that you do.

[Christopher R.]

Re: C1 Flywheel Question

Just came back from the machine shop. The proprietor is sure he can refinish it. Scoring and gouges were not that bad. So, I'll stick with that. He's also going to balance it by itself, and then again with the clutch attached.

I'm ordering a 10 1/2" clutch. Those are the only mounting holes on the flywheel. You mention a 10" and a 10 1/2" clutch. Never heard of a 10". Heard of 11" and 10 1/2", but never a 10". Actually, my clutch measures about 10 3/8".

[Joe L.]

Re: C1 Flywheel Question

Chris-----

For 63+, the only clutches used on the vast majority, if not all, Chevrolet passenger car applications were 10-1/2" (actually 10.4", but often referred to as "10-1/2") and 11". However, 57-62 Corvettes originally used a 10" clutch. This was the Borg & Beck "3 finger" or coil spring style unit. I don't think that a 10" for Chevrolet applications has ever been made in a diaphragm-type clutch.

The other confusing part is that, for the most part, the 63+ 10.4" clutch applications used 12-3/4" flywheels and the 11" clutch used a 14" flywheel. There were exceptions, though. The mid-70s lower performance applications used a 14" flywheel and 10.4" clutch.

All 57-62 C1s used a 14" flywheel and a 10" clutch. This is what "fits" in the 57-62 style bellhousings. The mid 70s lower performance flywheel and clutch became the SERVICE replacement for the 57-62 Corvette applications when the original pieces were discontinued. Of course, even the SERVICE replacement parts have now been discontinued for quite awhile.

[Christopher R.]

Re: C1 Flywheel Question

Will the 10" and 10.4" clutches (pressure plate assy) bolt to the same places?

[Joe L.]

Re: C1 Flywheel Question

chris-----

I didn't used to think that they would, but I now think different. Since your flywheel is, apparently, a GM PART #3986390, since it's, apparently, NOT dual drilled, and since the GM #3986390 is the SERVICE flywheel for 1957-62 Corvette applications (without requirement for clutch type conversion), then I'd have to say that the 10" coil spring type clutch used for 57-62 and the later 1963+ 10.4" diaphragm units use the same cover mounting bolt pattern.

[Christopher R.]

Re: C1 Flywheel Question

That's what I think too. I've got a 10" clutch on order. When it arrives early next week, I'll let you know.