C2: 64-65 and 66-67 clutch Fork

[Michael H.]

Would anyone happen to have both a 64-65 and 66-67 clutch release lever/fork? I would like to know the exact difference in the two. The 64-65 was a 3844493 and the 66-67 (and later) was a 3887177. There would have been a difference in the angle on the outboard end. The 493 would be the original part from a 64-65 and the 177 would be it's GM replacement for the 493. Thanks,

Michael

[Joe C.]

Re: C2: 64-65 and 66-67 clutch Fork

Michael:

That is very, very, very interesting. I have a strange problem with my clutch, which ONLY happens during powershifts at high R's. I swapped out an incorrect bell/flywheel/clutch/starter when I built my car, but retained the clutch fork. From what you just posted, I must assume that my fork is the later style. I will keep a close eye on this one!

Joe

[Michael H.]

Re: C2: 64-65 and 66-67 clutch Fork

Joe,

The only thing I know of on that one would be the "clutch pedal stuck to the floor" at high RPM operation. That's when the pressure plate diaphragm over centers and centrifugal force holds it in a totally opposite position. As the RPM drops, the plate suddenly pop's back out to the normal position. Is that the one?

On the linkage and pivots, there's probably a lot of 64-65 cars out there today with the clutch adjustment problem that we discussed in the other posts. I think GM was partially to blame for that because they discontinued the correct 64-65 T.O. arm, replaced it with a different number/design but never bothered to mention it in the parts book. There should have been a note that said "to be used with pivot ball part number *******. I'm still looking up numbers on thiis in the old parts book and will post the results as soon as I have everything rounded up.

Michael

[Joe C.]

Re: C2: 64-65 and 66-67 clutch Fork

Michael:

To explain further. My clutch works absolutely perfectly under normal conditions. No chatter, no slip, etc etc. Clutch freeplay @ 1/2", and adjustment rod nuts at about center of range.
Problem is this:

With powershifts above 6000RPM, there is a slight time delay between pedal sidestep and clutch hookup, (maybe 1/2 second), with a "Ba-Boom", and then slow/slipping engagement. It's frustrating that I can't break the rears loose when this happens. With all that power, I should be going sideways, but no!

Clutch assembly is a Borg, standard unit. Next project is to pull it, examine, and possibly upgrade if necessary.

If I can eliminate the disc/pressure plate as the culprit, then I would welcome that. What you said about overcenter travel sounds EXACTLY possible. Does that mean that I have to tolerate a super stiff clutch pedal for positive clutch operation during "spirited" use.

My L76 engine is only slightly tweaked, but essentially stock. If I 86 the Borg, maybe I should look at the Centerforce 2, Or Dual Friction.

Joe

[Michael H.]

Clutch Sticks to the Floor

Joe,

This was a very common problem on these and most other solid lifter GM cars of the 60's. The diaphragm spring in the pressure plate would "over center" at high RPM and would not come back until the RPM dropped below a certain point. In high performance pressure plates, the diaphragm fingers are bent towards the rear so the centrifugal force helps increase the holding power of the clutch at high RPM and also resists over centering but it still happens if the RPM is high enough. If you were to shift at 7000 RPM instead of 6000 RPM, the time it would take for the diaphragm to "pop" back out would be even longer. After several of these over centers, the pressure plate will start to loose some of it's tension and the problem will occur at even lower RPM's.

The fix (in the 60's) was to limit the travel of the clutch pedal to just slightly past the point of clean disengagement so the diaphragm would not over center. I think everyone I knew with a new solid lifter Camaro or Corvette had some sort of block attached to the bottom of the clutch pedal to prevent further travel than absolutely necessary. You might try switching your clutch linkage to the "fast" position which reduces a lot of pedal travel.

I don't know if todays racing diaphragm clutches are any different and I suspect they are not but you might want to check in with a few of the mfg's. Also, someone here may post and have some info on what is available today. If the clutch works well other than just the problem you described, I would probably try the clutch stop first just to make sure you have the problem nailed down.

Michael

[Joe R.]

Re: Clutch Sticks to the Floor

Michael,

I have several old clutch forks removed thru the years and have compared them with the 177 available today from GM. I can find no differences using a pin in the push rod hole to align them. I believe the new 177 is an exact match to the older clutch fork.

Regards,

JR

[Joe C.]

Re: Clutch Sticks to the Floor

Thanks Michael.

[Michael H.]

Re: Clutch Sticks to the Floor

Thanks Joe. That wasn't exactly what I wanted to hear but if you are 100% certain that the arms you used to compare were genuine 64-65 arms, then I guess we need to look deeper into this to find the problem.

The only thing that I know for sure so far is that if a 64-65 T.O. arm is changed and replaced with the 66 and later 177, the clutch adjustment never comes out correct. I suppose that if this change is made, the ball should be changed also as there was a different part number for 66 and the number is very close, numerically, to the 66 arm part number. I have to believe they are a set and made to be used together for some reason. If the O.L. length of the ball was changed in 66, there HAS to be some change in another part somewhere to make up the difference. If the length of the ball changes by 1/4", that would be a significant change in the location of the outboard end of the lever because the ball socket is much closer to the T.O bearing end of the arm.

I'll try to dig out all of the original 1965 part numbers and dimensions for the linkage components that could have had some effect on this. Will post later.