pesky clutch issue on C3 corvette

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tt 383
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Re: pesky clutch issue on C3 corvette

Post by tt 383 »

Did you measure the flywheel height? I think, it was a long time ago for me but I remember measuring the flywheel face to mounting surface and subtracting from a stock plate height and then that number from the ball height to get the proper ball height. I also remember buying a long adjustable ball haha.
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Re: pesky clutch issue on C3 corvette

Post by Belgian1979 »

It was a calculation like in this link : https://www.youtube.com/watch?v=22TwNOpXcPA&t=19s
It did involve the flywheel thickness, block plate etc.

Meanwhile I did the following : I disconnected the linkage at the fork and then pushed the fork back and forth so that the TO bearing touch the fingers then making a mark on the bellhousing where the fork is. I then reconnected the fork to the linkage which was still adjusted so that I reached the proper airgap and voilà it is clear the TO bearing is touching the clutch fingers. Probably has been the issue all along.
Meanwhile I ordered an adjustable TO bearing and a new adjustable pivot (for insurance). I guess I will be taking the trans out once more. :(

The adjustable TO bearing has the following adjustable ranges : 1.350/1.500/1.650/1.800 according to Mcleod.
I'm guessing here but the current one in there is 1.225'. Assuming I need to have .250" space from the fingers I guess I need the 1.500" length.
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Re: pesky clutch issue on C3 corvette

Post by zwede »

I don't know if this will help at all, but here goes anyway.

1971 Corvette. Using a quicktime bellhousing. TKO transmission. No block plate.

Stock fork, stock TO bearing. McLeod RST twin disc clutch. I set the pivot stud to stock (I guess the 4.75", can't remember). I'm using a hydraulic master/slave cylinder (although that doesn't matter for this).

Clutch works fine. I checked under the car with clutch pedal on floor and fork does not touch the body (although close).
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Re: pesky clutch issue on C3 corvette

Post by Belgian1979 »

Hi Zwede,

I think the fork touches the firewall because i had to cut slits in the angled part and bend it a little outward to prevent it from keep on hitting the Lakewood bellhousing flange (that I also had to cut to enlarge the fork window). Guess that move is making it touching the firewall.

It doesn't explain the issue with the clutch bearing free play, not enough movement. All linkages are original equipment btw.

Do you happen to know what size release bearing it was ?
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Re: pesky clutch issue on C3 corvette

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Re: pesky clutch issue on C3 corvette

Post by zwede »

Belgian1979 wrote: Wed Jun 12, 2019 4:56 pm Do you happen to know what size release bearing it was ?
It's what they had listed as stock replacement for a C3:

https://www.oreillyauto.com/detail/b/na ... 4018&pos=2
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Re: pesky clutch issue on C3 corvette

Post by Belgian1979 »

It doesn't say unfortunately. The image shows what seems a very short one 1.225' which is what I have now.
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Re: pesky clutch issue on C3 corvette

Post by Belgian1979 »

I just found a link saying that for a C2 somewhere around 1966 they changed the Z-bar on the cars. God knows what they did on the C3 and which Z-bar I have...

https://www.corvetteforum.com/forums/c1 ... stion.html
https://www.licorvette.com/clutch_cross ... rvette.htm

edit : seems that all Z-bars after 66 are the same and with a center to center length of 4 1/4 which is what I have in mine. So at least that seems to be ok.
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Re: pesky clutch issue on C3 corvette

Post by MadBill »

Belgian1979 wrote: Wed Jun 12, 2019 1:34 pm It was a calculation like in this link : https://www.youtube.com/watch?v=22TwNOpXcPA&t=19s
It did involve the flywheel thickness, block plate etc.

Meanwhile I did the following : I disconnected the linkage at the fork and then pushed the fork back and forth so that the TO bearing touch the fingers then making a mark on the bellhousing where the fork is. I then reconnected the fork to the linkage which was still adjusted so that I reached the proper airgap and voilà it is clear the TO bearing is touching the clutch fingers. Probably has been the issue all along.
Meanwhile I ordered an adjustable TO bearing and a new adjustable pivot (for insurance). I guess I will be taking the trans out once more. :(

The adjustable TO bearing has the following adjustable ranges : 1.350/1.500/1.650/1.800 according to Mcleod.
I'm guessing here but the current one in there is 1.225'. Assuming I need to have .250" space from the fingers I guess I need the 1.500" length.
I'd verify that number. Lots of clutch set ups are specced at anywhere from 0.050" to 0.125", e.g.: https://www.google.com/search?q=TOB+to+ ... #kpvalbx=0
Felix, qui potuit rerum cognscere causas.

Happy is he who can discover the cause of things.
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Re: pesky clutch issue on C3 corvette

Post by Belgian1979 »

Thanks for chiming in Bill. The .250" play from TO bearing to fingers is what Mcleod says in their instructions.

I still haven't taken the trans back out of the car. I did try a rod between the Z-bar and the fork that I made myself out of an old one I had and made that one adjustable. I increased it's length with .500 inch but aside from making the Z-bar angel different it didn't really change anything with the way the clutch frees us or where the fork arm hits the firewall. I think it's because the basic movement is determined by the the stroke in the Z-bar and the ratio of the fork.

Still in doubt whether to use an adjustable TO bearing as I would think this would not change a lot either or to adjust the pivot ball height and move the ball towards the clutch despite it being adjusted on the basis of the GM 4.75" required distance.
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Re: pesky clutch issue on C3 corvette

Post by zwede »

I don't understand the point of that much free play. I'm running a hydraulic clutch (master/slave cylinder) so I have zero free play and it works fine. I get that with a mechanical linkage you need some play, but 1/4"?
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Re: pesky clutch issue on C3 corvette

Post by Belgian1979 »

zwede wrote: Mon Jun 17, 2019 12:05 pm I don't understand the point of that much free play. I'm running a hydraulic clutch (master/slave cylinder) so I have zero free play and it works fine. I get that with a mechanical linkage you need some play, but 1/4"?
I agree with your comment, but I've read it in more than one instruction. Hays for instance says the same. It's also to account for some clutch wear if I understood it correctly. The finger rise alone is .12 I believe. So the remainder free play would be .13. Might get by with that and carefull keeping an eye on things.
However that doesn't help me as I need to put the TO bearing completely against the fingers to get the required .050" airgap.
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Re: pesky clutch issue on C3 corvette

Post by Belgian1979 »

An update to this :

I had ordered a number of parts to make the install correct :
- an adustable release bearing from Novak (https://www.novak-adapt.com/knowledge/clutches-etc/)
- an adjustable and correct shape pivot ball stud.

Even when doing the math according to the Novak site recommendations, it took a number of trials and taking out the trans before I got a workable setup.
Turns out that clutch manufacturers always supply the shortest release bearing possible, which in my case put the clutch fork at a too forward angle thereby reducing the force and stroke. This required me to put the release bearing closer to the diafragm (in fact touching it) to get the clutch to properly release. (original release bearings all are 1.225"). This is what damaged my previous bearing.
I ended up having to extend the bearing to 1.650". In that way it is against the base of the bearing retainer, the clutch fork is at the right angle and the air gap at the clutch plate is .030" and enought to properly release the clutch when fully depressed.
I couldn't get a .050 air gap as specified by Mcleod unless I would reduce the play of the release bearing to something like 0.075-0.100" which is a little to close for my comfort. The fact being that there is nowhere near enough ratio in the arm to provide suitable travel to release the clutch with an air gap of .050".

The .250" of throwout bearing play specified by Mcleod is taking into account : .0125" release bearing play and .0125" wear allowance apparently.
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