65tripleblack

I agree, and that's why I want more eyes on it.

It's hard for me to accept that the heat can be transferred from a hot PP, across the disk, which is not a good conductor of heat, to the flywheel, with all due respect to Westolorn who always has good information.

Apparently it started with a fault in the PP, but how it transferred a mirror image to the wheel is still not resolved in my head.

No symptoms under normal driving. Smooth engagement, etc. No symptoms under moderate abuse. Only when I'd lay into it, like hard shifting at high RPM, it would slip for less than 1/2 second..............the R's would not drop immediately with the backward surge of positive engagement.

Westlotorn

A clutch slipping under hard acceleration would create heat very fast. I have been in cars that slipped the clutch, in one 1971 Challenger I would have sworn we were smoking the tires based on the sounds and acceleration but found out later we only smoked the clutch. I would guess that car had a 440HP 340 in it. Headers and 3" exhaust big cam, ported X heads, made it loud at WOT. RPM was up close to 6,000 and the car was moving.
I did not get to see his clutch when he upgraded to see if it had hot spots.

Rich Yanulis

In my mind I can see a very logical explanation.

Lets start with an assumption that the pressure plate is defective.
For some undetermined reason it is applying uneven pressure, concentrating all the force on three specific spots.

The pressure plate is bolted solid to the flywheel. Therefore all the clamping force is being concentrated on the same three spots on the flywheel.

When you accelerate from a dead stop, the clutch disk is slipping on only six spots......three on the pressure plate, three directly opposite them on the flywheel.

In theory........I can see where you might end up with the six blue marks that you have.

GTOguy

You can analyze and theorize until the cows come home, but until you physically pick up the pieces and measure them, you are spinning your wheels. You need to measure flywheel run-out and thickness, pressure plate run-out, trueness and thickness, and the clutch disc itself. Putting 'more eyes' on your warped clutch is not going to determine how out of specs it is.

R66

Agree with the need for finite measurements to determine the high spots on either the PP or FW.

As for theory, the clutch disc is a flexible item and will conform somewhat to the surface it is pressed against. However, if the surface has high spots, the disc will flex at those points, but attempt to return to the original flat surface it was built to upon passing those high points. The opposite surface will see the high spot transmitted through the somewhat flexible disc and thus the additional point contact and heat transfer at the high hot spots.

Just redneck theory, and don't know if I am getting the point through, but I can visualize it being a possible diagnosis . Get out your straight edge and feeler gauges if you don't have more accurate testing equipment available.

I am betting on the PP as I have had more problems with the PP than the FW.

Good luck. Betting RAM will tell you you screwed up the installation. Were all six PP bolts tight when you disassembled the assembly?

65tripleblack

All 6 required a breaker bar to loosen. I used Loctite on the ARP bolts.

Donny Brass

Joe,

I thought about this, could it be that the loctite has holding the bolts tight, but they were not tight or had bottomed out in the flywheel ?

vetrod62

I do not know the exact reason the hot spots form like that. But I have seen this pattern before on flywheel and PP. And it was not from a bad clutch. Below are pictures of a high end slipper clutch I put in a guy's car. The same set up I run, and I pull wheels 1st and 2nd every run.

This guy was a rooky driver. He did this on the burnout!!! He went though the water box , revved it up and released the clutch too slow. Engine cranking away, wheels not turning. A $1000 of parts in the garbage.

This was a single pass. My only guess is it might be some sort of chatter harmonic.



Only .005 wear on disc


Heat marks on rear of flywheel, now that is HOT

65tripleblack

Donny,

People who know me really well..................and that's almost nobody..............and all bolts that I come into contact with, KNOW, that I have an obsession with tightening them (the bolts, not the people).

So, that said, I don't think so, and I appreciate the thought. The bolt holes in the wheel, BTW, are not blind. And that doesn't mean that they can see, you unnerstand. That fact comes in handy when tightening and loosening because if one bolt is removed, you shove a screwdriver through to hold things still while cranking on the old wrench.

Excuse me I have to go tighten all the lids on all the jars in my house, which I do once a day.

65tripleblack

Understood. I'm not saying that I'm the worlds best driver, so maybe I'm at fault. Or maybe not.

It goes toward clearing things up, although still trying to understand how or why overheating a clutch sometimes results in a pattern equally spaced around the PP/flywheel.

In your picture of a B&B it's easy to see that where there are concentrations of springs, in areas between the three pivot points the plate has flexed outward toward the disk causing the hot spots.

With a diaphragm spring, the correlation is tougher (for me) to make.

vetrod62

I was not suggesting that the driver was at fault in your case. I was just showing that the hot spots were similar in placement. I would think you did not have enough clutch or the disc wore too thin and you might have been slow to catch it.

Easy Rhino

You sound as OCD as me.

Do you use a torque wrench on those jar lids?

(I assume you do on all bolts - nothing to be ashamed of, I do)

Ausbrian

Improper break in procedure to the clutch assembly can cause these hot spots. Until all surfaces are bedded in properly avoid high r.p.m.s etc,

Nev Williams

I think you are onto it.
I have seen clutches on overloaded industrial gearmotor driven equipment with this sort of pattern.
If you are beating on it with powershifts at 6 - 7000 rpm and getting slippage even for 1/2 - 1 sec thats 116 rpm the flywheel has gone round with the pressure plate probably only having done a few rpm's !! Thats alot of energy with that sort of clamping load.

427425

The diaphragm clutches have been notorious for not completely releasing at high rpm forever. If it goes over center above 5500 rpm, it can exhibit this pattern. The engine is revving during that tenth of a second, and 'bam' the clutch releases and it is trying to stop the disc.
The clutch that Vetrod uses is probably the best compromise for all round performance with high rpm engines. It is designed to slip off the starting line as well as on the gear changes. A data logger is needed to tune the clutch perfectly at the track. The down side is too much pressure will show the weakest link behind the clutch.
To, Neville from NZ, your car looks great.

427425

I forgot to add that disc thickness will contribute to this pattern. As the disc wears, the diaphragm fingers start to flatten, reducing clamping force.

The clutch that Vetrod uses is adjustable, and requires a bellhousing that allows adjustment of base pressure as well as counterweight. BTW, Vetrod, I would like to know if you still have the trashed parts. I can probably do something with them, if you want to get rid of them.

vetrod62

The flywheel is gone, sold it to someone who did not care about the heat damage. I still have the 471 clutch. I guess a new pressure ring would fix it. An almost $500 clutch. Yours for shipping.

I had a long learning curve to get the car work (as in repeatable passes year after year) with a high HP engine and a super high traction system I built.

Some of the clutch parts

427425

Sent you a PM.

Sky65

What type PP fingers do you have? Curved or straight? Maybe that is the cause of the slipping which caused the hot spots?

https://www.corvetteforum.com/forums...nt-finger.html

Tom

65tripleblack

Tom,

I use a RAM pressure plate which has bent fingers. I had a Borg PP in there at one time and the thing would overcenter with pedal stuck to the floorboards. I don't remember whether the Borg used straight or bent fingers, but it was ordered for a Corvette so I'll guess that it had bent fingers as well.
Joe