Technical Hydraulic clutch slipping? Or pressure plate no good?

So I’ve got a bit of a dilemma here.

I’ve got a somewhat complicated clutch setup in my ‘34 coupe that was built in the 60’s. The engine is a 1960 Pontiac 389, with a Schiefer flywheel, Chevy clutch disc and fork, a Ford/Long 11”, and a 57-59 GMC truck throwout bearing. The engine is mated to a Chevy T-10 4 speed through an old Trans-Dapt adapter. According to an old catalog, this is the correct set-up.

On the hydraulic side, the dual brake/clutch master is off a 1960 International pickup, 1” bore. The slave cylinder was something sold through a Moon catalog, it’s 1 1/8”.

So I’ve finally got everything****embled and in the car, but the clutch is slipping. Bad. Especially in 3rd and 4th. I’ve tried about every adjustment that I can. Shortened/lengthened the slave push rod, same with master push rod. There is a clutch return spring, and I’ve added and removed tension, and taken it off completely. When I add more tension to it, the pedal feels soft, and the slipping gets worse.

The Schiefer flywheel is NOS. Clutch disc is new performance Organic. The pressure plate was used, but in good shape. Tried a new pressure plate from Mac’s but the fingers stuck out to far and couldn’t be used. The master and slave cylinders were both rebuilt by White Post.

A friend and I tried troubleshooting for hours, but to no avail. If I squirt Brake Kleen up in the clutch, it does grab better- for a little while. There’s no grease or oil on the flywheel or clutch disc that I can see.

Any last thoughts? Or do I have to bite the bullet, yank the engine and tear it all apart again?

 

Does the slave have a little bit of freeplay? Not necessarily the pedal. But the slave itself.
If it does not, adjust it so it does. If it does, the trouble is the clutch itself

 

Can’t you just yank the trans? Why don’t you just look for a Pontiac bell housing and ditch the mix and match clutch set up? Chevy or Pontiac, the trans is the same.

 

A clutch/brake rebuilder should be able to replace the springs in your pressure plate, I'd bet that's where the problem lies.
First hint was linkage adjusted 'loose', still slips. Brakekleen on the disc improved for short time."
I was surprised in the early '60s when a friend with a slipping clutch brought it ('58 Chev, 280 H.P. 348, Saginaw 3 spd.) into my boss's shop. 11" was hastily replaced with a Borg & Beck 10-1/2". Boss Red Mayfield said it should stick now.
He was right. Stiffer springs in the 10-1/2" made the difference.

 

Yes, slave does have a play when I put tension on the return spring. But that’s when I lose my pedal and slipping is worse.

Nah definitely no room to pull the trans. Not much room in ‘34 frame with stock center crossmember that’s been hogged out. 55-60 Pontiac manual parts are very scarce. Plus I couldn’t use my Schiefer flywheel.

The pressure plate is our main suspicion…. But how could it be that bad? The fingers has plenty of tension when I tested it. After fooling around with different adjustments, I can barely drive it without slipping. Maybe the clutch disc has glazed over from trying it too much?

 

The clutch master and slave cylinders are only to release the clutch, as long as you have it adjusted so there is no pressure on the clutch release fork it will not be the cause of the clutch slipping. It might be a cause of not being able to get the clutch to release all the way, but not cause it to slip. The problem has to be in the pressure plate.

 

Agree with the above. If you are certain that the TO is not engaging the pressure plate at all when relaxed, the problem must be the PP. If the TO is even slightly engaged, that's the problem.

 

Just a thought adding to above: Maybe your throw-out bearing is too long [what the bearing fits on and fork engages]? I think in GM there was two or three lengths, depending on pressure plate. Diaphragm needed longer throw-out, B&B three finger shorter. Don't know where the Long fits in. With clutch released look in through clutch fork opening and make sure there is some clearance between face of throw-out and pressure plate fingers.

 

Do we know for sure the bearing is fully off the fingers? Cheap endoscope on your phone should be able to see it. Doesn't need to be running to check this!

Chris

 

Thanks for the advice guys. I’ll bring home a small snake camera from work just to double check before I yank everything.

It seems odd that last night I could barely drive it, in any gear. Slipping terribly. How could a pressure plate get so bad? Even within a couple days.

The throwout bearing is the same as the one that was in the car, but I got an NOS one. During****embly, there was probably 1/2” at least of free play between the bearing and pressure plate fingers.

 

Been doing a little research, and 11” Flathead clutch discs are .365-.385 thick, and Chevy clutch discs are typically .310. Is that enough difference to make it slip like crazy?

 

Is it possible the disc is not contacting the flywheel correctly? Like the disc hub hitting the flywheel bolts or pilot bushing?

 

surprised no one mentioned possibility of disc in backwards?

 

Hydraulic clutch master/slave cylinders typically fail in the closed position.

 

on the engine's I've worked on, it's usually pretty obvious if the clutch disc is backwards, it'll hit the bolt heads that hold the flywheel on. That said, I've never worked on a Pontiac.

 

This comment doesn't make sense to me. Are you SURE you're adding tension to the return? More tension on the return spring should absolutely add resistance to pushing the pedal.

 

Yes, I am SURE that I’m adding tension. I drilled a hole further out, it definitely adds tension. This is the part that baffles me the most. How could more spring tension make for a softer pedal and more clutch slippage?

 

sumpin aint rite

Ben

 

Reach down on to the fork and actuating linkage while it’s sitting and rattle it with the return spring on the fork off. It should rattle and by hand YOU should be able to move the fork at least 3/8”.
If you can try it again after driving if it’s the same and slipping it’s the pressure plate because you proved it was released.

 

A used but in good shape pressure plate? The pressure plate and clutch disc are designed together for proper holding pressure and easy to use the clutch on a 60 to 80 hp motor. The thinner disc is like putting a well worn disc in. How old is the pressure plate and how much spring pressure does it have?

You need a new pressure plate that will keep more pressure on the disc. The pedal will be a little stiffer but that is where you compromise a little to have a good solid clutch setup.

 

The thinner clutch plate will mean that the pressure plate fingers will be out further (away from the flywheel). This could be causing a clearance issue with the bearing.

Or the thinner clutch plate could be trying to make the pressure plate work in a range that it simply won't and thus maybe won't grip. Just a thought. I wonder how thick ( thin!) a Ford clutch plate can go to and still be just operational? A Ford pressure plate would not need to be designed to be able to operate at a clutch plate dimension lower than that.

Chris

 

When I first put my car together years ago I had a stock never been apart 55 Pontiac 287 and a 3-speed. If I had somebody in the car and sidestep the clutch the tires would not spin the clutch wood. That was with some skinny 75016 rears and 411 gears. Upgraded to a new pressure plate and was good then I rebuilt a 1957 347 motor with some goodies in it and added a posi and it started slipping again. So I stepped up to a center Force 2 pressure plate and can drop the hammer in second gear at five grand and it'll boil the tires.

 

Well I brought home my small snake camera from work, and right away saw that the throwout had no free play- we were mistaking the free play of the fork itself on the pivot ball.

Then I noticed a spring up in the pressure plate. I removed it, and then found another one! The smaller smooshed spring was lodged in there pretty good, but the other spring was floating freely. Must have been stuck in the pressure plate? Anyways, removing them didn’t make much difference.

I thought the stiff spring must be from the clutch disc itself, so I bit the bullet and started wrenching. I did manage to pull just the trans, although I’m not sure it was any easier.

Anyways, there was a lot of pressure from the PP fingers on the throwout bearing. I know when I****embled the motor/clutch/trans that I double checked the throwout bearing freeplay, and there was definitely movement. So what happened? I measured the clutch disc, and it was .327” thick, and the Flathead discs I have were less than that! How could the fingers be so far out? Stepping on the PP fingers, there is plenty of resistance and they move smoothly.

 

Well, the first one looks like a tension spring, which is confusing. The second looks like it might have escaped from an old disc and lodged in the PP; that makes more sense.

 

Maybe one of those vagrant springs was hanging up the pressure plate when you checked the clearance on original****embly, only for it to dislodge subsequently resulting in the fingers changing (eliminating) the clearance? Other possibilities are possible but I don't want to suggest that you may have funked up on that initial measurement. Oops, I just did! Whatever, it seems like you're getting somewhere .

Chris

 

Once you bust the tires loose , the clutch isn't working very hard .

 

The thickness of the clutch has to match the pressure plate, if you use a clutch disc that is thinner than what is required of the pressure plate. You are in essence fitting a brand new worn clutch

 

If the flywheel is also drilled for a Borg & Beck pressure plate I would change to one. It looks like a new pressure plate is needed in any case. I’ve used that style disc with the larger heavy springs and it doesn’t give much room between them and the flywheel bolts. You may look to use a lighter duty disc. They work just fine and are not part of the slipping problem.

 

Is the smaller spring one that is used to retract the throwout bearing?

 

Post #8.