Moss Motors, Ltd

I'll ask and hope someone can educate me on a clutch componet for a 79 midget.  Specifically the pressure plate and what is normal resistance on the fingers that the release bearing pushes on.  Have had issues with a clutch that would not disengage.  Engine and Transmission are now out of car.  Prior to have inspected Mstr and Slave , bled system , checked free play and wear of all componets . Now with everything exposed release bearing spins freely and fork works like its suppose to .  The only part in all of the disassemby that showed any wear is the fingers on the pressure plate. I realize that the fingers are spring loaded inside the plate - but pushing on them seems to be very stiff.  In fact I can't move them , unless ( and I have not tried it ) I REALLY put force to them? Is it possible that this is the problem with failure to disengage ?  What would tell me normal from defect ?  As in most things once you know the right way a part feels and works you can determine a problem - but without that I have no idea what is right or wrong.  Thanks in advance and I really appreciate you folks who spend your time helping us- the guys who need it. 

I can't answer your question but it's pretty standard practice to replace the pressure plate, clutch disc and throwout bearing whenever doing clutch service.  Most auto parts stores commonly sell a clutch 'kit' that includes all of these.  Of course these cars are old enough and you probably won't find replacements at your normal auto supply companies, but the practice is still wise.  If the clutch was slipping, that could likely be from a bad disc or a weak pressure plate.  Failure to disengage is more often related to the hydraulics and the linkages.  But it could be from a bad pressure plate.  I'd install a new set rather than try to diagnose the pressure plate.  The cost is worth it when compared to the effort of R&Ring the engine & transmission just to get at it.

I played with the old clutch from my MGB. I could only activate the pressure plate "manually" by bouncing on it as if it were a springboard. I weigh about 190lbs... If you think about the pressure one normally applies to the pedal, and multiply that by the mechanical advantages of the pedal arm, Hydraulic system and release arm combined, you can get a feel for the amount of force involved.

The advice to replace the whole set is very wise.

Cheers,

Ed

---

I want my MGB

Guys, Thanks for the suggestions !!!   I have ordered a " kit " for clutch replacement.  While that maybe true I am the kind of person that wants to know how something works and your bouncing on the fingers sure tells me this part has lots of pressure on it.  I think that the real cause for the failure was , and is, the friction material itself.  The flywheel had a imprint of friction material , not scored into the metal but actual pieces of the friction plate ,  to the point that even on a slant of 70 degrees for the engine the plate did not just fall out of the assemby. Had to use a screwdriver to gentle pry the plate from the flywheel. Sure did not look like what I assume should look like a disk brake setup.  All clean and shiny.... The sad part is that the parts don't seem to wear out- time just makes them give up . Thanks , again.....   

yearslatter:

Had to use a screwdriver to gentle pry the plate from the flywheel.

Well, there's your reason why the clutch was not disengaging!  I've never seen one stick like that.  As you said usually they are scored into the metal or the friction material is worn down to the rivets.

I understand your interest in wanting to understand how the pressure plate works - I went through the same thought process.  It does take a massive amount of pressure to move the fingers and I wonder how much force this puts on the clutch disc?  If one had a properly instrumented press, you could measure it.  But you wouldn't have any specifications to compare it with, so you'd still be in the dark.  You're on the right track - good luck.

Wanna see how much force it takes? Place the pressure plate, face down, with a clutch plate installed, bolted to a flywheel. Now place that flywheel in a shop hydraulic press on top of a heavy duty postage or shipping scale. Then engage the press and see what it takes in pounds just to deflect those fingers inward about 3/8ths of an inch, which is what it takes to release the disc. You will be STAGGERED, I promise.

Remember, the entire inertial weight of the vehicle depends on that pressure to propel the vehicle, uphill in fourth gear, even! Imagine what that needs. The actual figure varies greatly with the weight of the machine and the power of the engine, but it's huge...

---

Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon