'70 clutch still broken, help needed to make race

[Grady Clay]

Juan,

What disc is in the clutch? What Sachs P/N? How thick is it uncompressed? How thick compressed? What is the history of the pressure plate? Are you absolutely sure the pressure plate diaphragm spring isn’t cracked? (It can crack from the circular relief outward from the fingers where it is difficult to see.) Did a rubber center disc ever come apart in that pressure plate?

Something is wrong here and we just haven’t found it – yet.

Best,
Grady

[logician]

Warren, I saw a few of your old posts on clutch problems that said the pilot bearing can be at fault. Of all the possible explanations, that makes the most sense to me, because the amount of drag seems independent of clutch pedal position. I can lift the pedal a few inches off the floor, and the drag seems not to increase, up until the clutch engages. It seems to me if somehow the problem was due to the disk draging against the flywheel/pressure plate, that pusing the pedal down further would have to release some of the pressure. The clutch seems to have a reasonablely solid engagement, when it does engage. Can you elaborate on some of the symptoms of the pilot bearing vs. other drag problems?

Grady, I don't have the part number for the disk. I'll know when I take the engine out again. The thickness of the disk and flywheel were close to new. I don't know the history of the pressure plate. It is an aluminum one from a later model year (SC?) Why do you mention a rubber center disk coming apart? I find that interesting, because I have a second pressure plate that I could use, and I've been puzzling over how dirty it is. It seems to have a lot of black stuff in it, maybe the rubber that you are referring to? Are you going to tell me that I should really clean that out?

Thanks guys for all the advice. I'm puzzling over it so much because I took it apart once already, and didn't really find anything wrong. I want to be really well prepared to know what to look for when I pull the engine again, lest I miss identifying the problem again.

-Juan

[Grady Clay]

What have you found?

Best,
Grady

[logician]

Grady,

Thanks for the inquiry. I don't have a lot to report, as the engine is still in the car. But I have driven it a few miles on the road, and I'm pretty sure from the symptoms that the problem is with the pilot bearing.

I think I will almost certainly install a 915, rather than invest any more $ the 911 gearbox. The 915 is a better choice for my racing application. I will report on what I find when I finally pull it out some time this winter.

-Juan

[Grady Clay]

Juan,

If so, send me your type 911 and I’ll do a diagnosis on it for free. Just send a pre-paid return shipping label. I’ll return it as ready to race as I received it.

I still want to know about your pedal assembly clutch lever arm. What is that measurement?

I’m one to try and solve a problem and learn from it rather than the shotgun approach and replace everything, hoping that will solve the problem. You have a fairly unique set of symptoms, I’d like to see why. Everyone will benefit.

Best,
Grady

[Bobboloo]

I can only imagine your level of frustration Juan. You might take Grady up on his generous offer. I just hope the problem isn't somewhere other than the tranny (pilot bearing, flywheel, pedal assem. etc.) otherwise you'll be wasting time and money with a 915 replacement or the 911 shipment.

Is there a chance that there might be another tranny in your area that could be used temporarily for a process of elimination?

[logician]

Grady,

I will measure that lever next time I'm at the shop.

Thanks for the offer to look at the tranny! I'm curious to debug the problem too, as I would like to contribute to the PP knowledge base. But I think that the problem isn't with the Tranny, but with the clutch parts. I happen to have all the clutch parts that I would need for the 915, but would have to buy a TOB for the 911, so the economics favor the 915.

Bobby, it seems that all the 911 transmissions that I know of are blown up. Thje two other Toyo Spec cars that use 911's have broken them. That's another reason I'm thinking I really should just pull the trigger on the 915, lest I be stuck mid-season with a broken tranny.

All roads seem to be pointing to a 915 for Christmas...

-Juan

[Grady Clay]

Juan,

I agree; the problem probably isn’t in the transmission. I’ll still inspect it for free if you want.

I don’t want to see you convert to a 915 and experience similar problems. Finding the actual problem is critically important.

Let’s carefully lay out the symptoms:
-Clutch won’t release fully.
-The throw on the fork is less than the specification.
More?

I think repeating the previously recommended tests may provide useful information. There has to be something that has escaped our attention or be changing. What is it?

These kind of problems can be solved by bruit force – replace everything. I prefer to actually diagnose the problem, solve it and learn to not repeat the difficulty.

Best,
Grady

[logician]

Grady, all,

OK, so I finally had time to pull the engine today to inspect the clutch. It turns out that the pressure plate was bad. There are three leaf springs on the periphery of the PP that retract the pressure plate when the clutch is depressed. The rivet securing one of them had pulled out. See attached pictures.

This is almost certainly the problem. But that's what I thought when I found the broken pedal cluster. So I'm not counting it until I test it with the new PP.

-Juan

Bad:



Good:

[MobileWrks]

Glad to see you have found the problem.. Bet you learned alot doing all the basics...

[Joeaksa]

Juan,

Hope it works now and really agree that you now know a lot more than you ever expected about this system! Course now it probably will not cause any more problems for you either...

Merry Christmas...

JoeA

[Grady Clay]

Juan,

Good eyes. I’m sorry it took so long and so much effort to
diagnose this while missing PRC events. At least you have
the Spec Miata for lower level thrills. Well … maybe more
excitement depending on your field. Hehe.



Actually both images show distortion in that locating spring
system. Look at a new pressure plate.

These springs are in tension when you apply power or start
off from a stop. However, if you suddenly release the clutch
with the revs low when downshifting (or a missed shift into
a too low gear) the springs are in compression. They aren’t
designed to operate in compression and can buckle with the
results you see. The locating pins (to the right in your images)
aren’t strong enough to support the compression loads and
they can bend. I’ll bet you will find these “wallowed out” in
the circumferential direction

The usual symptom is an RPM dependent vibration from the
pressure plate moving piece being loose or off center.

This brings up how important it is to inspect clutch parts every
time the opportunity presents itself. The outer housing can
crack (red arrows) on the outside or inside. The diaphragm
spring can crack (usually out from the root of the fingers where
it is difficult to see.) The bearing can be failing and damaging
the tips of the fingers where you can’t see unless you remove
the bearing. Of course the issue with the locating pins and
tension springs you found. The locating pins (green arrows)
should be clean and rust free so the moving piece doesn’t
stick, particularly under load.




Since the clutch release bearing is not a sealed unit, it
shouldn’t be cleaned in a parts washer. You should
remove it and thoroughly clean inside the pressure plate
so you can inspect it. Using the shank of an appropriate
size drill bit, check the pressure plate moving piece for
concentricity in the housing. After clean, dry and inspected
thoroughly douse it in WD-40. Include inside the bolt holes
and particularly the diaphragm spring and pivot. Blow off
any excess and wipe the friction surface only with lacquer
thinner.

Installing or removing the pressure plate from the flywheel
should be carefully done with the nine bolts turned only a
little at a time. Getting it cocked during removal or
installation tries to bend the locating pins.

You probably now have the most currently analyzed
2.2 clutch in the world.

Merry Christmas.

Best,
Grady

[logician]

Fixed! I took the car out for a test drive, and all is well with the new PP.

Interesting observation: my car had always had a bit of a rattle upon decelleration when in gear. I had thought it was the exaust rubbing on the body as the engine torqued. But now the rattle is gone with the new PP. So it seems the rattle was due to the PP vibrating, as Grady suggested happens when the leaf spring fails. The fact that the rattle happens upon deceleration correlates really well with Grady's explanation of the the way that leaf spring works.

Thanks everyone for the help!

-Juan

[rw7810]

Grady - Getting ready to put my 901 back together after 3rd ballpin break in 12 years andd about 150kmiles. Yes, bad design, but in any event, what do you recommend for a lubricant between the ballpin and the clutch fork??

[Bobboloo]

Great news Juan.

It's nice that you posted the results with the solution. It completes the data in the archives for future troubleshooting.

[randywebb]

you know there's a little nylon cup in there that should be replaced right?

I used a sticky red Bosch grease... interested to hear what Grady says....

[Grady Clay]

Bob,

The updated pivot usually didn’t have a problem; I would
carefully look at everything else. If the pivot breaks,
something is causing too high stress. The first place I
would look is the clutch release bearing and guide tube.
Of course the pivot-to-arm ball connection must be
proper, lubricated and operate without binding. The
guide ring on the transmission differential side cover
also needs to be in good condition.

The lube at the pin-to-arm is best a moly grease.

Look at how far Juan had to go solving the problem.
Don’t overlook anything.



Rube Goldberg is smiling from his grave at our addressing this.

Best,
Grady

[rw7810]

Randy - A nylon cup sounds like a good solution for reducing friction, but I've never seen one shown or sold. This is for the 70-71 design. Thanks Grady. I'll check. I don't think I have lubed the guide tube in the past, but will do so now.

[logician]

Here's a neat trick from Thomas J for repairing a worn 911 fork. After one racing season, the heim joint shows no sign of wear and is still tight. The pivot support is modified from the standard parts to get the right height. Note that the bolt head is shaved down a bit, as this is necessary to clear the pressure plate.

-Juan

[rw7810]

This looks like a great idea. Can we get a picture from the other side?? More info on how it was done??