Clutch pressure plate appearance; how to tell?

[Ted Idlof]

Could a bad pressure plate still look good?

I've changed out the clutch a few times over the past 10 years and seen a few used but functioning pressure plates. On another occasion World Speed swapped out the diff case for me and sent back the pressure plate marked "good spare". And the only time I actually had to change the clutch, the pressure plate fingers were sprung and obviously distorted. So I thought I knew what a good vs bad pressure plate looks like.

The big picture is that the clutch simply won't completely disengage. And it happened overnight: it worked perfectly on a Friday test day followed by a 3 session Saturday race day, then did not work at all first thing Sunday morning. No way to get it into reverse and only into first with a lot of grinding and "gnashing of teeth", both it's teeth and ours. After doing all the adjustments and checks we and some 20+ year FM guys could think of, no change. So we raced it 3 sessions on Sunday with the problem.

I have now gone thru the entire system, reinstalling the pressure plate and disk since they were fairly fresh (8 hours) and looked good to not only me, but other FM guys and my local speed shop guru. I concluded the problem was the throwout bearing since they always look funky to me and because I had not replaced it last time. Everything else looked fine and it went back together normally.

So when I jumped in it for the test, (on stands in the garage, motor completely cold), it worked fine, (well nearly fine since I've never compared the clutch to a Swiss watch.) Warmed it up just a little, drove it down the street and within 4-5 shifts (into 1st then reverse), it stopped working again.

The master cylinder is fairly fresh (8 hours), not a drop of fluid lost and I swapped out the slave cylinder with my buddy's perfectly good one. Also I've adjusted the linkage to both extremes. The pedal pressure is normal. And still no difference.

So I'm wondering if it is the pressure plate even though it looks good. I'll call Kennedy on Monday, but I thought I'd check in here.

So, what am I missing? And yes, I am ready to slap my forehead.

Hopefully, TIA.

[David Ferguson]

Perhaps the real problem is a bad pilot bearing or bad input shaft (groves in the splines don't let the clutch disk move).

normally pressure plates are checked for flatness to determine if they are good. Overheating (due to too much slipping) will warp them.

[Ted Idlof]

I did have a good look at the input shaft and the pilot bearing; all seemed good.

[formulasuper]

Ted, I'm not familiar with the FM clutch design, does it use the external clutch slave cylinder or concentric slave cylinder?

[Ted Idlof]

The slave cylinder is mounted on the outside of the diff case. It actuates a lever arm that moves the throw out bearing.

I'm not familiar with a concentric slave cylinder.

[mikey]

does the flywheel have a friction plate screwed to it or is it one piece? I only ask because I had a Fidanza flywheel in another car that I tried to re-use the friction plate insert on the aluminum flywheel but it was warped and clutch wouldn't disengage..

[Ted Idlof]

No, we don't have a separate plate.

The flywheel looked smooth but somewhat discolored, so I had it resurfaced after getting the allowable dimensions (I think they called it the step?) from Kennedy for proper engagement. And, for my buddies ready to protest me, it is still comfortably over the minimum weight after resurfacing.

Pretty sure that if there was a problem, it would have made the step too deep and slippage would be the result? Just guessing.

[Fred Michael]

Sounds to me like the slave is not extending far enough to allow the T/O bearing to depress the fingers on the pressure plate.
Look for wear on the arm as well as bent parts. Make sure the slave has full travel.
Surfacing the flywheel may have made it worse for you. (moved everything farther from the T/O bearing)

[Ted Idlof]

Kennedy did say that a pressure plate (pressure ring specifically) can look just fine (well at least to the untrained eye) and still have failed. Not really sure what that means or how that occurs, but I sent a complete blow by blow of the situation and pics and am waiting a bit right now to see if they have any additional thoughts before I tear it apart again.

Fred, I adjusted the slave cylinder(s) travel to both extremes with no change whatsoever. The slave cylinder on there now came right off my buddy's car.

Here's some pics if anyone is interested.

https://www.dropbox.com/sh/om8hrvt7y...WWJm4UZjUYqw5a

Someone remind me to post pics if I can visually spot the failure.

[Hohndo]

Hi Ted, your flywheel shows excessive heat. I'n my opinion, the pressure plate failed to one degree or another due to heat/fatigue. The slave cylinder is a fixed throw fairly easy to measure.
Just my two cents to an old friend.

[Nardi]

Checked the slave arm to cross shaft for proper teeth engagement or wear?

[formulasuper]

The slave cylinder is mounted on the outside of the diff case. It actuates a lever arm that moves the throw out bearing.

I'm not familiar with a concentric slave cylinder.

Have you tried disengaging the clutch by moving the lever arm manually with the slave cylinder removed?

note: A concentric slave cylinder is one that mounts around the input shaft inside of the bell housing. I'm surprised FM's don't use one.

[Modo]

step to deep Ted and u will not have enough travel to release clutch disk, more pressure ......maybe that's it, the step is not correct or errors on the side of deep ......should have a lot of clutch grip in that case but fingers now will bend more so throwout travel needs to be more ............ don't know if that's a step in the right direction ...... am puzzled why it worked during practice and next day not, dual issue maybe ... clutch step then bleeding/slave set, dunno?????? look to what was just done then problems ....... Good Luck!! ................... ain't broke, don't fix it type of deal, maybe not .... LOL .......... step maybe not too deep but deeper than before .... reset slave movement again if u can, need more travel

[Ted Idlof]

I will look more closely at the lever arm itself. I thought I'd try moving it manually but didn't have a big enough cheater pipe. I'll get one. Visually it appears to be moving the full range at least from what I casually recall seeing before.

As for the step depth, the problem was there before the flywheel was resurfaced, and although it may have possibly made the problem worse, in my estimation the problem is no better, no worse than before. And I have faith in Kennedy specs and my trusted speed shop that did the work and more importantly knew to ask the question on step depth.

I don't think I've mentioned this but Kennedy said the PP could look OK even to them and that they might have to take it apart to know for sure if it's bad. If true, the takeaway is to always replace the PP with new every time. This one only had 8 hrs in service.

More later.

[Modo]

Yea Ted, he probably maxed out the step for top notch engagement as well as resurfacing, both will put the clutch a smidge further from your throwout bearing, I would just reset the clutch pedal stop and adjust at the slave if there is an adjustment there, have someone pull-push on chassis a little and hit clutch pedal in a gear, pedal stop a little past chassis rolling/release ......... I can do it myself hand pulling on clutch and watching for chassis roll and hitting the stop .......... sounds like you know this and have probably done this already but stumbled into problem at track ...... best of luck!! oh, yea look for the difficult last, bad clutch pressure disk while visibly looks ok ........

this is just a guess, if you increased travel and still no luck, then maybe sprung the pressure plate, dunno

[Ted Idlof]

well I better document the resolution just to save someone else some head scratching some day.

There are 6 riveted buttons at the outer perimeter of the PP that serve as the fulcrum for the fingers to pivot on. They have to be tight. All 6 of mine were equally loose and moved at least 2 mm.

Here's a video https://www.dropbox.com/s/gij4rbde3cokeml/DSC_0029.MOV

Kennedy said, "yep, not good" and rebuilt the PP under warranty (including shipping back to me). I got the impression someone wasn't too careful during fabrication and didn't get the rivets in as tight as needed. This PP only had 8 hours on it.

Clutch now works perfectly. So when you have the PP out, check those buttons for movement; there should be none.

[Kazis31]

Amen to that.
Good lesson for everyone.

[Bob Wheless]

Thanks for that , now i at least know what to look for ......bobo the large