Thoughts on clutch acuation

[jlaird]

Haveing had a few hours, like days to think on this, and to have been shown the way toward a solution in my case, I have a few thoughts.

The piston in the slave cylinder of course actualy moves the clutch fork and throwout bearing against the pressure plate which releases the clutch. OK that out of the way here goes.

A piston in the slave cylinder is pushed toward the rear of the car when the pedal is depresed. How much you say and what can be adjusted.

The pedal of course can be adjusted at the MC and must be so that the MC gets a full bite of fluid to push to the slave. Note that if you make more threads show on the clutch/MC pushrod you get not only a higher pedal but also a longer throw. Clutch pedal should of course be the same height as the brake or preaty close anyway.

Now, assuming that is adjusted properly it is time to adjust the slave cylinder. What you say there is no adjustment there. Of course there is and it is the length of the small rod that goes from the back of the cylinder piston to the clutch fork. They come in two sizes, the 1275 about a quarter inch longer than the other. What does this mean?

Ok lets go to how the slave cylinder is built. There is a cirlclamp that keeps the piston from comming out the back so the piston can travel from the front of the cylinder to the cirlclamp inside and in my opinion should never touch either, close but no touchee.

So when the push rod is inserted it should push the piston all the way to the front, almost, just as it fits into the clutch fork clevis hole.

Oh, but how long a rod does it take you say? There are only two provided as stock sizes.

New manufactured clutches work fine but their specs are not quite the same as the orginals, in my set I noted that the throwout bearing casting was thiner than the orginal, much thiner, ahhhhh longer rod so it can push the fork in far enough to release it. Yep. some cases stock rods do not seem to be long enough even with a 1275 rod in a 948 application.

My take on all this, is weld a half inch on a rod. Push it in the slave till the piston is all the way in and take the slack out of the fork and measure the difference between the fork clevis hole and the rod clevis hole. Grind that much off of the end of the rod and just a touch more maybe 1/32 inch and you chould be good to go.

I would suspect, it is my opinion, that many trannys are trashed ie grind a bit going into gears because the clutch is not really disengageing enough to really stop the thing from spinning under pressure.

If I got this wrong someone please correct me, in addition an adjustable slave push rod would be a wonderful item?

 

[RickB]

I see you have had some time to ponder these things...

Take a look at this, admittedly this is my Nissan setup on my Midget - but it's not a bad idea.

 

[PeterC]

Jack, just insert a 5/16" nut into the depression in the slave piston. See if that helps.

Peter

 

[jlaird]

Oh, there is a trick for testing, thanks Peter.

 

[piman]

Hello Jack,

many clutches are self adjusting, and how this is achieved is by making the slave cylinder much longer than the cylinder action. Basically with a new clutch the pressure plate actuating fingers are at X position relative to the flywheel. As the friction centre plate wears the actuating fingers move further away from the flywheel (clutch engaged of course)
In the first instance with a new clutch the slave cylinder piston will be nearer the rear of the cylinder and in the latter it moves deeper into the cylinder so there is no need of an adjustable length push rod.

Alec

 

[PeterC]

That's true. I'm pretty sure that the only time you see an adjustable slave pushrod is when the car has a release arm return spring.I know it was an improvement on Land Rovers to remove the return spring. Clutch lasted longer.

Jack, out of curiosity, and since I've forgotten and never saw, Answer these few questions, please.
1. What engine back plate. Cast or stamped
2. What slave cylinder, 948/1098 or 1275
3. What clutch? (pressure plate)948, 1098,1275
4. What throw-out bearing 948, 1098, 1275
5.What master cylinder.. stock tandem bore?

Thanks

 

[jlaird]

I did that Peter, but it still is not perfect.

Next time I have the engine out I will replace the fork bushing although it is not really loose.

Drives fine really at present including shifting, but I would not want to race it.

 

[Hap Waldrop]

OK, back to a really simple question here, you sure you didn't install the throw bearing arm backwards, also, they made several different arm over the years for the different engine and clutches, it might be possible y8ou have the wrong one of you engine and clutch. Also ther are tow differnt slave cylindere design, one for the 948/1098, and another for the 1275, and the bore sizes are different.

 

[jlaird]

Hap I know the throw bearing arm is not backwards for sure. In fact when I looked specificly this last time I found the open ends of the throwout ears pointing forward so they were pushing the bearing. I decided that it would not work backwards as the throwout would just pop off.

Now, is the fork the correct one, no clue, it came out of another smooth case I have but any old PDO could have changed it.

The slave cylinder is what Moss provided for a 948 which means nothing I supose (Lockheed) guess I will measure the bore size next I am underneeth and see what it is for sure.

Note: No way I am going to pull that engine again in the near future for this. Change a slave cylinder, sure, engine no.

Thanks for the comments, always nice to have your insight.

 

[PeterC]

I have seen quite a few bent t/o arms.

You can't put the wrong slave in without doing some "plumbing".


Peter

 

[jlaird]

It would not suprise me at all if that fork was bent from the wrong clutch that I tried earlier in this crazy engine pull I was through. In fact I guess I need another fork on the shelf for the next engine pull. At the very least one that I could compair this one to. That could just be the answer for sure, goodness knows I put a lot of presure on it, hard and I mean hard.