Clutch Master Slave Cylinder Question
 

What I need to know is the distance or travel the slave cylinder needs to push open the throw out bearing fork to work correctly.

The car isn't running so I can't start it and see.

The clutch master cylinder is a small Girling; the slave is 7/8" bore.

Chevy 350 with Borg & Beck 3 lever clutch and Muncie 4 speed.


Thanks,

Jim

We have a lot of clutch experts on here, blykins being one of them, but it doesn't look like any of them are going to reply to you. That may be because there is not one single answer to your question. If you are asking, "how far do the fingers of the pressure plate need to be pushed in before the driven disk will spin freely" then the answer will be different for different pressure plates (and for different thicknesses of the driven disk on those plates). My PP opens up with just about .5" of finger travel. In our local club, we have seen other Cobra PPs that need a good .7" of finger travel. We learn this little lesson by swapping in the latter for the former and then the clutch won't disengage after the swap like it did before the swap. With that in mind, your TOB is going to have to move that needed disengagement amount, plus any air gap. So then how far does your fork have to move? It will depend on the fulcrum point of the fork on your bell housing (just like your rocker arm ratio on your valve train). Now to answer your question, how far does your slave have to push the end of the fork so everything works right? The answer is "far enough to do all of that, and pick up any slack in the process." And, in my experience, the only way you're going to be sure is to have it up on a lift with someone pressing the clutch pedal while you stand under the bell housing watching it all work and spinning a tire to make sure the disk is spinning freely when the clutch pedal is all the way in. You just can't do it with a pencil, paper, and ruler. :cool:

Thanks,

I understand. I can test it like you suggested on my lift by having someone push in the clutch while I spin a wheel, but can't see the clutch assembly as it's inside a blow proof bell housing.

I was trying to get an idea of how much travel the slave should have.

I can tell if it's releasing by spinning a wheel, but won't be able to tell if it will slip under power.

Thanks,

Jim

You can peek inside the fork hole and kind of see it all working. Testing for hold is actually fun and easy. Just go to an open parking lot, where there's nothing to hit, and from a slow roll with your foot completely off the clutch pedal, give it a quick half-throttle and your tires should break away smoothly and quickly almost as if you're on packed snow. Take a look at your marks and you should see two nice, even marks, assuming you have an LSD. Now, if you try that and smoke starts coming out of your tunnel, and your revs go up and your tires hold, then the clutch ain't holding.:LOL: As to how far the clutch slave and fork should move, here's a video of mine:

From memory I think you typically need a slave cylinder that has at least 1.1" of piston travel to safely disengage most clutches

Thanks guys,


The car isn't running yet, is on my lift with the wheels off. (been working on the brakes).


I put the transmission in gear, pushed in the clutch and tried the get the rear hubs to move.


They won't move with the clutch engaged. From what I can tell, the slave is moving in and out over an inch.


The clutch pedal doesn't feel right to me, so I'm going to re-bleed the clutch master and slave and see if that helps.


I'm reasonably sure the pressure plate, disc and TO bearing were installed correctly.

Jim

I have had a few cases of stuck clutch plates, new plate on a new flywheel, assembled for days/weeks by the time you are ready, cannot select gear etc.

I started one of my cars, jumped along in 1st gear, and as soon as the engine fired, the clutch operated normally.

Gary

You can kind of "rough estimate" the typical travel distance of my fork from that video. See if your fork is moving a similar guesstimated distance. The problem could be something as simple as air in your lines.

Thanks,

Jim

Or there could be too much free play at the top of the pedal before the TO bearing makes contact with the pressure plate. You should not get much perceivable movement if you grab the end of the fork by hand and try to yank it rearward, before it is stopped by the pressure plate fingers.

Also, I own an old Corvette too and they have a shallow foot box and limited room for clutch pedal stroke so to get full release of the clutch and a bit of free play when released, required careful adjustment. They have lots of problems with clutch discs that don't release with the clutch pedal fully depressed. One of the experienced mechanics on Corvette Forums started setting a variety of pressure plate/disc and flywheel assemblies up in a hydraulic press with a dial indicator. He then measured how far the TO bearing had to be pushed to fully release the clutch disc (.050" clearance). He found out there is a lot of variation between flywheel manufacturers. Some were so bad they just can't be made to work in a old Corvette. McCleod clutches were generally very good. Some of the favorites like LUX and Centerforce were iffy. So, could also be a problem with your pressure plate depending on the source and style.

Hmmmm, so it's possible that your parts might not work together and can never be made to work together no matter how hard you try. Well, that's encouraging.:LOL:

I copied my Wilwood clutch and brake pedals, made them out of steel. They had to be shortened from the original length to clear the bottom of the foot box. I wanted them to mount in the top of the footbox rather than from the bottom. Not sure where that puts me as far as stroke is concerned.

Bled the clutch master and slave again, can't tell much difference. I plan to turn the motor over using the crankshaft pulley bolt tomorrow and see what happens. If it's stuck, maybe that will loosen it up.

I had a local shop rebuild the Borg & Beck pressure plate, reline the clutch disc and turned the flywheel.
Also bought and installed a new throw out bearing that he supplied. The shop owner said the pressure plate was "one of the good ones" for motors under 400 hp.

Just curious: how strong should the return spring be?


Thanks,

Jim

Just strong enough to pull the fork back a bit over the force of the slave. It does not have to be like the springs we had back 50 years ago when you could not pull them back at all with one hand and a set of vise grips. And I can still remember when somebody taught me to put a few dimes in to the spring to spread it and then pull the dimes out one at a time -- that was a like a magic trick to a young genius mechanic.:LOL:

If I may, as Patrickt has said, "discussed many times", this is a nice physics problem that might be a "sit at the table with a pencil" type of chat.

Two cylinders - one the master the other the slave are drawn as a matter of volume. So in a balanced system, each must have the same pressure and amount of fluid.

SO, if you know how much you need in the slave, you can adjust your master to fit.

Once you know what is necessary for activation, you can "adjust pressure and travel" to deliver it.

Each system is unique, so asking for specifics may not fill the bill.

I hope this helps and if someone else has said about the same thing - then I am sorry for the repeat.

Tru

So that others understand, we are changing the master cylinder bore diameter, or the stroke via the pedal pushrod ratio.

Gary

Turning the motor over by hand didn't make any difference.

Now, I'm thinking about pulling the motor and trans to get a real look at what's going on.

Thanks,

Jim

Jim,

Are you turning the engine my hand, while the clutch is depressed, AND transmission in gear?

Gary

OK, what exactly is it doing wrong?:confused:

The slave moves about 1 1/2 inches, but the rear wheels won't turn when the clutch pedal is depressed.


I tried turning the motor over by hand with the transmission in gear and the clutch pedal depressed, and it was very hard to get it to move; was afraid I'd snap the harmonic balancer bolt, so I stopped.


Thanks,

Jim

Sounds like the clutch is stuck to the flywheel, but will release when the engine starts if clutch started.