Clutch/Flywheel Analysis

[Ferrino]

I currently have my 1984 3.2 engine and 915 transmission out, so thought I would check the condition of the clutch and flywheel:

- The pressure plate has "84" stamped on it, which I presume is the year of manufacture, so it's original. The friction surface of the plate looks OK - no scoring or grooves.
- The clutch disc has approx. 7.25mm of thickness (I don't know exactly how many miles are on the engine/transmission, so it could conceivably be original too).
- The throwout bearing feels like it's toast as it spins very loosely and can easily be pushed around (in the plane of the pressure plate). The guide for the throwout bearing: not sure what it should look like, but it seems very dry to me and the inside of the throwout bearing looks pretty ragged.
- The flywheel looks OK apart from a 1.5mm wide annular groove that lies near the outer edge of the surface that mates with the clutch disc. There is a complementary ridge on the clutch disc. The groove is so smooth and neat that I thought it was actually machined into the flywheel as some sort of "wear-indicator", but I'm pretty sure that's not the case.......

I've uploaded pics to an album HERE.

Would like to get some feedback on recommended course of action while engine is out. My guess is that I should just buy a new Sachs clutch kit (pressure plate+clutch disc+throwout bearing) and have the flywheel surface machined. Should I also replace the guide tube if replacing the throwout bearing?

[bfunke]

IIRC the clutch disk limits are 8.1 +/- 0.3mm. Appears your flywheel needs to be resurfaced although that's a pretty deep groove and you didn't specify if ever done before so check the remaining thickness to ensure it will still be in specs. Those grooves are usually the result of disk wearing too far down to rivets. Here's a pretty good primer on clutches.

http://www.g-w.com/pdf/sampchap/9781605252131_ch07.pdf

[Ferrino]

Thanks. I don't know if the flywheel has been resurfaced before. I would probably get it checked out by a machinist for thickness and any warping before committing to a resurface.

Isn't 8.1mm more like the thickness of a new clutch disk?

Regarding the groove - I don't understand how a disk wearing down to its rivets would result in a groove that lies outside the radius of the rivets?

[James Brown]

Would like to get some feedback on recommended course of action while engine is out. My guess is that I should just buy a new Sachs clutch kit (pressure plate+clutch disc+throwout bearing) and have the flywheel surface machined. Should I also replace the guide tube if replacing the throwout bearing?

there is your answer. and do the guide tube. also clean and lube the TOB lever (release arm)

[RWebb]

how about the guide tube, James?

I don't replace, just look at it...

[Walt Fricke]

Well, I think the way to approach this is to decide what your style is. If, like me, you have your engine out every few years for something or other (or transmission work), I'd consider reusing the flywheel and pressure plate as they are. I might scuff up the surfaces to break the glaze some, just for good measure.

That is, if your clutch was working well enough (not slipping, anyway) before you pulled the engine.

That groove wouldn't much bother me, as it doesn't reduce the surfaces significantly (and, when the disk wears some, it will grip the groove as your old disk did.

A new disk is a kind of obvious thing to do. Same with the TOB. I once disassembled one, thinking I could clean it, repack it, and reuse it. But it still sits in a box, as reassembly proved to be quite a challenge.

The guide tube is something I'd probably leave alone. The TOB's inner race should be more or less fixed to the fingers of the clutch diaphragm spring and not move relative to it. The TOB's outer race should be more or less fixed relative to the throwout fork, and the guide tube. The rotation between things should all happen in the ball bearings inside the TOB. So the TOB should only slide back and forth on the tube. At least when the TOB is new and clean inside. I suppose you could turn the tube upside down, if it looked like there was more wear on top than on the bottom? It looks like some of the plating has worn off. But is the wear deeper than that?

It looks like the seal on the transmission input shaft is good? No oil coming out? If you replace the guide tube you'll probably want to replace that seal as well, as it is inside the guide tube.

But what do replacements cost? If inexpensive enough, why not?

You might want to pull the pilot bearing and inspect it. Maybe clean and regrease? Or just replace (and grease). Despite centrifugal force, those normal wear particles you can see in the photos may have gotten into this bearing.

On the other hand, if you view this as sort of a once in a lifetime deal, or want to get a good 200,000 miles out of this before you have to think about it again, them the full monte looks more attractive - pull the flywheel and see what a machine shop thinks of it. It looks like there may be some hot spots (blue color) from local overheating. These are said to be problematic when it comes to resurfacing, as they are harder than the rest of the surface. There are specs, including a minimum thickness which is in part related to the geometry of the release mechanism. Get a new pressure plate just to be sure. And not have to make decisions.

[johnsjmc]

It,s been a long time since I did a clutch on a 1984. I wonder if the groove is there to indicate max refinish thickness if outside the swept area of the disc. Looks like the disc and pressure plate could be re used with a new throwout if noisy or loose. A stock full kit isn,t much though. I recall about 75000 mi from a 911 clutch is average life. last one I did was about that. It needed replacing because one of the fingers cracked and fell off the plate spring and it started chattering.

[Ferrino]

Thanks all. It looks much better value to just buy the complete clutch kit, so I think I will get that. The throwout bearing moves a lot within the center of the pressure plate.

Regarding the flywheel - that groove lies on the mating surface, so I don't think it's a wear indicator. I will probably pull it and have it checked by a machinist. Is this something a good machine shop can do or should I consider shipping it to a specialist?

[Walt Fricke]

I've not heard of Porsche flywheels having a wear indicator groove.

I think a machinist who does flywheels could deal with one from a Porsche with no problems. The only caution I have seen is that the cut out by the outer lip be quite square, so that the pressure plate bolts all the way down onto the flywheel pressure surface.

The TOB is, by design, kind of a loose fit into the center of the PP. That funny small diaphragm spring washer is what keeps it in place once it has self centered. I don't think the TOBs wear externally to any significant extent. It is how well or poorly they spin on their bearings which is the determining factor.

It is common to see those shiny spots on the tips of the main diaphragm spring, and they get there because of relative motion of the TOB. Not bad up to a point, though I don't know of a spec. A new PP resolves worrying about that, though.

[Ferrino]

What do you think about the clutch release fork? There are no visible signs of cracking at the stress points, but the ends that push on the release bearing are quite worn - the profile has changed from a "lobe" to a "flat":

[Walt Fricke]

I finally remembered to look at a spare tranny in my garage. Its throwout fork looks rather like this, though I didn't measure things. But the wear surface was distinctly lower than the highest part of the small semi-circular protrusion.

I guess I'd say this is the surface which pulls on the TOB (since that is what the overall mechanism does), but I suppose in terms of wear a push by any other name is just as sweet.

I've never purchased a new fork, so I don't know if they start out with the wear surface the same as the protrusion, and then wear like this, or they start out as a forging from the die with a full width semicircular protrusion, and then are machined down like this so that the contact area is larger.

I have some trouble seeing the TOB flange as being able to wear the fork contacts down a millimeter or more, at least without it getting paper thin in the process.

Were this mine, I think I'd just put it back in. But that might be due simply to my ignorance, and the fact I take out my engines myself and with some frequency, so I can afford to fix mistakes, so to speak, if it turns out I have made one.

It seems the problems we have with the fork part have to do with it breaking, or with its arms getting spread due to over vigorous attempts to mate engine and transmission with the fork not positioned just right to slip into its proper groove.

[KTL]

I wouldn't worry about the contact surfaces as much as the locations on the fork where the curved arms meet the splined tube. That area is the fork's achilles heel. Replaced a friend's fork with him earlier this summer on account of the arm cracking problem.

[Ferrino]

Thanks for the feedback. I'd be curious to see the contact points on a brand new fork, if anyone has one? It definitely appears to be wear, though.

[Reiver]

Call your local indy Porsche shop and see who they rec for resurfacing. Ask about the FW min depth spec at that time.

[911 tweaks]

a good tip I do on all repairs I have & have not performed before is to compare new parts to what I am disassembling...to me, this technique speaks volumes...look closely at everything... measure the wear, stress & moving area(s) of the parts...
Think of the time you have spent to get to this area/repair & do you mind going back in there to save x amt of $$ on any parts...reminds me of the clint eastwood movie, I think it was Dirty Harry, when he says, "so...after all of this action I too have lost track of how many bullets I have spent...so punk(not calling u a punk)...do you feel lucky...??
ALSO, if you have a failure from trying to skimp by using an old part and it ruins a bunch or even 1 new part that you just installed, that will make a lasting impression on you for sure & you will probably change your technique... :-)
Finally, do yourself a favor, buy one of the Porsche little spec books for your model/yr car...it is amazing how much tech data is in those books...
carry on & good luck... all of this is JMHO...

[Walt Fricke]

The "little white spec books" are just great references. And I believe no longer in print, so harder to get than they used to be? But there isn't a spec in there for everything, and in particular not for the TOB fork. Neither is there a spec in the factory manual.

If this were mine I'd just take it to Stan Richardson at GBox. He knows about everything having to do with this kind of thing (though he is not the only one). But that's because I live in the town where his shop is.

I bet you could drop in at Black Forest and maybe get an opinion from John Rickard.

I looked at the one I have handy (installed in a transmission) again. It looks like it has somewhat less wear than yours does. Your pictures make it appear that the hump part is worn down to where it is even with the main surface from which the hump rises. Mine looks to be worn maybe half-way. On the other hand, the remainder of your hump is wider. What still sticks up on mine is quite thin.

I'm not sure this matters one way or another. The clutch adjustments should be capable of dealing with this. Ideally, with the clutch pedal not depressed, nothing much is going on with these parts.

[Ferrino]

In looking for photos of new forks, I found this company that offers a service to weld the worn area and regrind to the original shape: here.

[spuggy]

I'm told that clutch forks have been known to crack. Seems like a fair amount of force acting on the part to me - and it's a wear item.

Guess the question comes down to how much you drive the car, and how comfortable you are with the idea of either being stranded or dropping the trans again in a couple of years to replace a $125 part.

[KTL]

That's a nice service Zims provides. But until the forks are truly no longer available new (Pelican sells them), I don't think it's economical.

New fork is $125. I think the extra $35 is worth it to start with a fresh fork that is less likely to crack any time soon?





The fork pictured above came from a '79 SC (not mine) that was driven sort of infrequently

[Ferrino]

Kevin - I think the cost of the Zims mod is $35 when you recover the core charge. Regardless, I've decided to buy a new one and might weld and grind my old one as a spare. I have also decided to go with a Sachs power clutch kit (with the aluminum pressure plate) and am having my stock flywheel resurfaced. The throwout bearing guide looks good, but I will check how it rides on the new throwout bearing when I receive it.