1968 Corvette 427 390HP Spec Questions

[Joseph M.]

Hello,

I have a 1968 Corvette with the matching # 427 390HP engine w a M-21 trans and I'm in the process of pulling the engine to overhaul it and update a few components. Can anyone tell me what is the the correct intake and exhaust valve sizes are for the orignial oval port heads that are on this car? Also, can you tell me what size clutch this Corvette came with? I'm assuming it is was an 11" clutch so I would like to upgrade the original clutch to a Centerforce 11" clutch and would like to know if it will fit properly inside the original bell housing and will bolt of to the factory flywheel. Thanks!

[Warren F.]

Re: 1968 Corvette 427 390HP Spec Questions

Joe,

The valve sizes should be 2.06 & 1.72 intake/exhaust. The clutch is a single plate 5 coil spring 10.4 inch diameter unit. As far as aftermarket clutches bolting in, I don't know. I only use factory equipment pieces.

[John H.]

Re: 1968 Corvette 427 390HP Spec Questions

The clutch has no coil springs - it's a diaphragm clutch, 11" diameter, and the CF 11" clutch will work fine in the existing bellhousing.

GM abandoned Borg & Beck-type coil spring clutches after 1962.

[Joe L.]

Re: 1968 Corvette 427 390HP Spec Questions

John and Joe------

I agree. Joe will note that the bellhousing on the car, assuming that it's original, will be a GM #3899621. The GM #3899621 bellhousing was always used in PRODUCTION in conjunction with a 14" flywheel and USUALLY an 11" clutch. There were some exceptions to the "11" clutch rule" in the late 70's when some applications used a "dual drilled" 14" flywheel in conjunction with a 10-1/2" clutch. There was also the case of "dual disc" clutches used for 1969 L-71 with MA-6 and 1971 LS-6 with 4 speed. These used a special 14" flywheel with a special 10" dual disc clutch. None of the exceptions apply to the 1968 model year, though.