Clutch problem, Help please....

Clutch problem, Help please....

Postby Coop » Thu Sep 13, 2018 2:18 pm

Help....I have a problem with the clutch in my Corsa with a Crown conversion. The clutch won't disengage. I bought it with this problem, previous owner couldn't fiquire it out either. The car has a 3/4" Tilton master cylinder, a 7/8" slave cylinder. Chevy Camaro (60s) throw out arm, it had a Centerforce diaphragm plate and disc in it, I changed that out to a Corvette 11" plate and disc, thinking the plate was bad, or to heavy. The clutch pedal feels like a brake pedal, rather than a clutch. I set the pivot ball height at 4-3/4", like Chevy recommends. I'm confident the problem lies somewhere other than the clutch now. What are you guys running for a hydraulic clutch set up in your cars? How far should the throw out arm move? Cylinder sizes?
Scott Cooper
'65 Crown Coupe
Bainbridge, N. Y.
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Re: Clutch problem, Help please....

Postby Goodman Alabama » Thu Sep 13, 2018 10:33 pm

I am pretty certain my clutch master is a Willwood 3/4", and I'm using a RAM hydraulic release bearing. I have yet to drive the car, but from testing/observation, I am pretty certain it is good to go.
65 Corsa Coupe Crown Project, 350
65 GTO Convertible Tri-Power, 4 speed
67 Cutlass Supreme Convertible LS3-4LE85
70 El Camino SS396
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Re: Clutch problem, Help please....

Postby Richard Miller » Fri Sep 14, 2018 10:05 am

you say "The clutch pedal feels like a brake pedal " do you mean it stays high and solid ? does it travel down a ways before going solid ? I spent quite a bit of time designing the clutch linkage in my car ,, working thru design issues ,, ultimately I figured out why the factories do it the way they do ,, old expression about re-inventing the wheel applies ,,,and ended up sticking my head under dashboards and floorboards before realizing that fact ! its tough job to get right ..if your pedal is going solid before enough travel has been achieved to release clutch you need to see what is stopping it ,,I use same size master and slave ,, both 7/8 ,, your 3/4 to 7/8 gives you a softer pedal at expense of loss of travel ..release bearing should be close to clutch but not touching it to minimize travel needed ,,can you move clutch fork by hand to see how far it moves before hitting clutch ? shouldn't be too far ,, stupid question ,, does it hit crossmember ? my first Crown car the fork hit edge of crossmember and it had to be clearanced ,,does clutch pedal hit floor ? is master cylinder running out of travel and bottoming out before release ? somewhere your clutch is going solid before enough travel has been achieved ,, you have to minimize needed travel , no wasted motion ,, while making sure there is enough to release the clutch about .060 ,,when someone pushes the pedal can you see fork movement ? all a compromise between leverage and travel ,, you need about 100 to 1 leverage to get effort down ,, but that increases travel that same factor ,,how far the pedal travels . if your slave is not traveling full stroke you can try changing master ,,but if slave is bottoming out then adjustment is needed at pivot ball or slave pushrod ,, all comes down to finding what is stopping the travel
Richard Miller
West Covina, Ca
'65 V8 Vair
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Re: Clutch problem, Help please....

Postby Richard Miller » Fri Sep 14, 2018 11:05 am

I looked back for reference material ,, there was November 1978 issue of Popular Hot Rodding magazine , had an article written by late Tom Senter on clutch linkage basics ,, I don't know if this can be found online anywhere but it did a great job of explaining how force versus travel was worked out thru the linkage ,,one point I extracted is this , in his example using typical numbers ,, the end of the clutch fork sticking out of the bellhousing will need to travel about 1.125 inches to release typically ,,I just ran the numbers , roughly ,,7/8 slave needs 0.68 cid for this travel ,,3/4 master needs about 1.5 travel for this cid ,, times about 4 to 1 pedal ratio and your pedal would need to move about 6 inches
Richard Miller
West Covina, Ca
'65 V8 Vair
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Re: Clutch problem, Help please....

Postby Rick Andersen » Fri Sep 14, 2018 11:40 pm

One thing to think about is the mechanical (hydraulic) advantage you are losing in having the slave cylinder being a larger diameter than the master cylinder. Typically the master cylinder is a larger diameter than the slave cylinder. I currently run a 7/8" master cylinder in place of the 3/4" that I used to run just to get more travel. I am running a hydraulic throwout bearing in place of the traditional Crown slave cylinder setup. You will find a different feel when using hydraulics vs mechanical linkage.
Rick Andersen
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'65 Crown V8 Corsa Convertible
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Re: Clutch problem, Help please....

Postby Randolph Galek » Sat Sep 15, 2018 8:21 am

In addition to the points already mentioned, I’d strongly suggest you also use a clutch stop.

I had numerous strange problems with a race car years ago until I realized that too much pressure can cause a lot of damage which then looks like not enough pressure.

Chet
Chet Galek
Randolph, New Jersey
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Re: Clutch problem, Help please....

Postby Coop » Sun Sep 16, 2018 8:56 pm

All straightened out, took it for my first drive today. I had a multitude of issues:
1.) Too big of a master cylinder,(1"), went down to a 3/4", much softer pedal.
2.) Too much free play between throw out bearing and clutch fingers because of wrong pivot ball depth. My car has a steel scatter shield on it with the back plate behind the flywheel, that back plate adds 1/8" depth to the pivot ball. I removed the back plate, I'm not racing, just street driving.
3.) Poor alignment of slave rod to throw out arm, pushing at an angle instead of straight on.
4.) This is a big one, probably 75% of my problem...there was deflection of the cross shaft under the dash. When the clutch was depressed, the shaft deflected back away from the master cylinder, I wasn't getting the travel of the rod into the master cylinder I thought I was getting. I came up with a support to keep that cross shaft from moving.
5.) The clutch pedal was bottoming on the floor before the master cylinder was at full travel. I reworked the arm.

The clutch feels good now, catches 1/3-1/2 of the way off the floor from full up.

I'm happy with the clutch, now I can move on to some other minor issues; front and rear alignment, brake bias tuning, a bushing on the shifter that won't stay in the tube, and some minor cosmetic stuff. I'm just excited to be able to drive it a little before the weather gets bad here in the NE. Thanks for all your replies!
Scott Cooper
'65 Crown Coupe
Bainbridge, N. Y.
Coop
 
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Re: Clutch problem, Help please....

Postby Louisville Lanning » Mon Sep 17, 2018 9:11 am

Glad to see that so many people weighed in on your clutch problem. Roger, the previous owner was plagued with this problem for some time. Looks like another sharp Crown car is back on the road. Thanks to all our members who offered Scott some advice. This was the main purpose for making this Forum available to members from the beginning.
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President - Derby City Corvair Club
'65 Corvair / Mid Engine Cadillac Conversion
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