Chris White

I posted an inquiry about the 930 clutch disk a week or so ago – I thought I would share the end results (to spare someone else in the future!!)

The project was to replace a failing 930 disk (in a 951) with a new 930 disk and a KEP pressure plate (Supplied by the customer). I installed these along with a Firdenza flywheel and when finished the clutch would not fully release, the pressure plate would drag at full pedal.

Take it apart and check everything – the 930 disk is 1mm thicker than the 951 disk (BTW – it has about the same contact area as a cup clutch). After consulting the list and measuring everything I come to the conclusion that the thicker disk is the issue compounded by the different pressure plate, the customer runs up to Canada and picks up a pressure plate and Kevlar disk for Markus (Blaszak Precision).

Install the second set of parts. Well, it’s a little better but still some drag with the pedal all the way down. Debate possible causes – come to the conclusion that the $5 pilot bearing has failed and the shaft is spinning with the clutch engaged (BTW – use the SKF bearing – I have heard of several initial failures of the cheap bearing. The Porsche part is $45, SKF is $30).

Take it apart (third time) and the pilot bearing looks OK (replace it with SKF anyway). Resurface the flywheel just incase…measure all clutch components – all with in spec. Assemble again….still the same problem. Measured the slave cylinder position and travel – 16mm – right on spec

I was getting a little grumpy about now. Spent some time contemplating the system, chatted with Markus as a sanity check. We came up with the only other possibility – excessive crank movement. Keep in mind that his engine has great leakdown results (just did that – 2,2,4,3%) and the oil pressure was at 3.5 bar or better.

Yep, the crank was over .030” of end play - double the allowed wear. The crank was moving back and forth with the application of the clutch. I have pulled apart quite a few engines and I have never seen this much wear on the thrust bearing. The well worn clutch that was being replaced masked the problem quite well – but with 20/20 hindsight it does explain why the release point was high with the old disk.

I did the last round at no charge to the customer because it would have been too painful since he has to contemplate an engine replacement sometime. After adjusting all the free play out of the pedal the car can be put in gear at idle. Once moving it works fine.

Just something to file in your strange happenings file.

Chris White

Tom M'Guinn

Hmmm... wonder if you could make a slave cylinder extension to get a little more range?

Is riding the clutch the only cause of thrust wear?

theedge

Thats really odd, I had massive amounts of play on my crank, 0.41-0.42" actually, and my old clutch was fine (dont think it was even that worn either), yet I cant get my KEP Stg 2 to feel as stiff as it did with a new crank and all new parts... Im confused as hell

Ill post some pics of my crank and thrust bearing if you want. Quite something, the thrust bearing melted and looked like one of those things where you pour paint on a spinning sheet of paper and it all spins out Crank was all grooved too. Your customer might wanna see them.... Hell probably accuse you of trying to scare his wallet open though

theedge

Actually theres some already on my server... Ill upload some more/better ones later.

Thrust surface of the crank, this is the one that faces rear (ie, the one that takes the abuse from the clutch).




Thrust bearing

Tom M'Guinn

Thinking about it, isn't this a pretty good reason not to use any stronger pressure plate than you really need?

Chris White

Wow, that’s pretty ugly. How was the oil pressure before you took it apart?

The problem with the master/slave system is that it is self adjusting – there is nothing you can do to get more range – it only develops a certain amount of movement and it will adjust to clutch thickness – not position. An extension would not help except for the first cycle!

When the KEP stage 2 was in it had about double the pedal pressure – which should be about right since it has double the clamping force. So keep in mind that you are pulling on the back of the crank with that much force while the clutch pedal is depressed.

Chris White
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theedge

Oil pressure was awsome, always nice and high. No weirdness there, no clutch weirdness with the old stuff, nothing. Took the engine out, noticed it could slide back and forth, thought to myself "Hmmmm this is a BAAAAAAD sign" and went out and got the dial gauge to see how much

theedge

I think that more than that its a lesson to not hold the clutch down at lights I imagine thats what happened. Hard to get 0.42" of wear any other way id think...

K-bit

This is my car -- Chris was fantastic to work with just wish he had better news than what we found out. I am now trying to decide whether to go into this engine and do a complete refresh or to look for an engine in good shape out of a wreck. As Chris stated it will go into reverse if you have the pedal all the way to the floor - first is a little knotchie but the other gears are pretty good as long as you are moving. Chris said that it may get a little better as the clutch seats and the flywheel and pressure plate burnish the edges off -- I am hoping to put off any more repairs until this fall. I guess the original components (Fidanza flywheel, KEP2 and 930 disk)might have worked together if it had not been for the thrust bearing wear. Anyone have a lead on an engine?
Thanks,
Jack

Tom M'Guinn

I have a good 2.5 crank (from an 89 n/a) I'd sell now if helpful.