Flywheel / Starter / Bell housing

[Sean D. Philippe]

I have a 75 Vette in which I installed a new GM crate motor 350/ 300 hp. I installed a 10.5" clutch and 10.5" flywheel with 153 teeth.

When I went to install the starter it interfered with the bell housing. Why would a starter interfere with a bell housing? Shouldent the bell housings accomodate both 168 and 153 tooth starters?

In general, how does one match up flywheels, starters and bellhousings?

Thanks,

SDP

[Jim T.]

Re: Flywheel / Starter / Bell housing

With the 153 tooth flywheel you should use a starter that has the straight across bolt pattern. The starter nose with the offset bolt pattern is for the 168 tooth flywheel. Don't go buy a new nose for your starter from GM, just go to a recycle yard and buy a used starter and transfer the nose if you need a different one.

[Joe L.]

Re: Flywheel / Starter / Bell housing

Sean-----

I hope that you didn't use a 10.5" flywheel with a 10.5" clutch. GM never made such a combination. In any event, I assume that your talking about a 12-3/4" flywheel of 153 tooth ring gear and a 10-1/2" clutch. This is the combination that was originally used for all 63-68 small blocks and 78-79 L-48s. It was also used for 65 L-78s, 67-69 L-88/ZL-1, and 70-72 ZR-1.

Here's how it works for 63-81 Corvettes:

153 tooth, 12-3/4" flywheel works with 10-1/2" clutch and bellhousings GM #3788421 (63 only), GM #3840383, or GM #3858403. Starter used is (usually)aluminum nose unit with straight across mounting bolts with 1 long and 1 short bolt;

168 tooth, 14" flywheel works with 11" clutch and bellhousings GM #3872444, GM #3899621, or GM #464697. Starter used is (normally) cast iron nose unit with offset mounting holes using 2 long mounting bolts. An aluminum version of this nose was used for some applications.

Not all 12-3/4" flywheels are the same and not all are interchangeable. The same is true for 14" flywheels.

You can use a 10-3/4" clutch with a 14" flywheel IF the flywheel is dual-drilled for both 10-1/2" and 11" clutches. However, the use of the 14" flywheel ALWAYS requires the use of the bellhousings described above for use with a 14" flywheel. As far as 63-81 Corvettes go, the only ones ever utilizing a 14" flywheel with a 10-1/2" clutch were 78-79 with L-48. 1969 Corvette with MA-6 dual disc clutch and all 1971 LS-6 used a special 14" flywheel in conjunction with a 10" dual-disc clutch.

You cannot use a bellhousing designed for a 14" flywheel with a 12-3/4" flywheel and you cannot use a bellhousing designed for use with a 12-3/4" flywheel with a 14" flywheel.

Your 1975 was originally equipped with a 14" flywheel, 11" clutch, and GM #3899621 bellhousing. If you are using this bellhousing, then you cannot use a 12-3/4" flywheel. You can use a 10-1/2" clutch with a 14" flywheel IF the flywheel is dual-drilled.

[Sean D. Philippe]

Re: Flywheel / Starter / Bell housing

Thanks Joe!

[Pierre J.]

Re: Flywheel / Starter / Bell housing

Joe,
You mentioned in your post that not all 12-3/4" flywheel (153 tooth) are interchangeable. Why is that? what is the difference?

Thanks

Pierre G. Joly #34410

[Joe L.]

Re: Flywheel / Starter / Bell housing

Pierre-----

Some are for internally balanced engines (e.g. 327, 350, 396, 427). Some are for externally balanced engines like 400 small block, 454, and 502. Some are for engines with 2 piece rear seal-type crankshaft flanges and some are for engines with 1 piece rear seal type crank flanges. So, there are quite a few different 12-3/4" GM flywheels for various combinations of the above applications.