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 1275 clutch bits 
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After we finished running at the newly-reopened Thompson (Conn) race event I noticed the clutch pedal needed to go to the floor before I got full disengagement of the clutch.

But the week after Thompson, we had rented a house at the beach. We had a dozen family members joining us that week (kids, grandkids, siblings, etc.) so the car only got a cursory safety inspection plus oil change. We had a great time at the beach! :mrgreen:

When we arrived at Pittsburgh International Race Complex (Pitt-Race) for the first weekend of the PVGP, the clutch pedal seemed a little worse. There was no air in the system and I hypothesized that the clutch release arm is bent or the throw-out bearing is worn. The system is self-adjusting so there's no issue there.
We make it through the Pitt-Race weekend with some grinding but we do OK. I actually got a podium in one race (third) plus the Worker's Choice in another session (note to non-racers: Worker's Choice is like winning a prize for Miss Congeniality). Anyway, we win a case of beer for each of those so we are well stocked.

But I'm worried about the clutch. It's now at the point that I can barely get it in gear without rolling the car and even shifting into the second, third and forth are rough.

The second weekend of the PVGP puts us in Shenley Park which has a very tight paddock area. I worry about the clutch in these tight quarters and tell my friends I hope I have "one Shenley's worth of clutch pedal left".
There were a few very ugly shifts but we made it through the weekend and had a great time. I ordered parts from Moss while in Pittsburgh in anticipation of pulling things apart when home.

I pulled the motor last Wednesday and found the problem. This is one I don't recall seeing before; a very worn bushing in the clutch release arm. I had a good, used spare arm so in it went. But I wondered what had caused it? Then I realized my throw-out bearing was the "modern" roller type. These cars normally come with a carbon throw-out bearing but I had forgotten that I'd fitted this roller type unit a couple of years ago. Actually, I didn't specifically buy the roller bearing; it came in a used trans trans that I'd picked up. When looking at the roller throw-out bearing vs. the carbon throw-out bearing, there is an obvious difference in thickness. This, in turn, probably changes the fulcrum point of the load. I wonder if this caused increased load on the release arm bushing? It's hard to say, but since I had ordered a normal carbon bearing, I installed it and all the arm geometry should be back to normal. If I were to use another roller bearing in the future, it would have to be with a modified arm to keep the geometry correct. Or the roller bearing thickness would have to be thinner.

One other point; with the incorrect geometry caused by added thickness, the throw-out bearing was not "on center" with the pressure plate. The amount of error is probably minor but still......not good.

Since I was pulling it apart anyway, I decided to fit a new clutch disk. The old one was "biting" fine but a new disk is cheap and it seemed like a useful idea.
Actually the old disk was still decent but the pressure plate was about to break apart! It was riddled with radial heat-stress cracks. I'm glad I decided to replace the disk as it gave me a chance to see this. That pressure plate had been fine about 18 months ago when I installed it.
The funny thing is, I consider myself pretty "easy" on clutches. I never do any drag race starts and my current engine is very mild so it doesn't require a lot of slipping to get launched.

I put in a good, used pressure plate but I will be keeping an eye on it from now on. It's all back together and seems to be working fine. Next weekend I'll be at New Hampshire Motor Speedway so we will see it it's really fixed.

Worn clutch release arm bushing:
Image



Thickness difference; roller throw-out bearing vs. carbon throw-out bearing:
Image



Radial heat stress cracks in pressure plate:
Image

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Sun Jul 27, 2014 9:10 am
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Nial, Interesting stuff ... FWIW the throwout arm bushing in my MGA always wears out quickly, and the pivot pin that it rides on also wears quite a bit. I always figured it was just a tough environment, no lubrication, and we shift a lot more while racing than most people do on the street. I always inspect the clutch parts whenever I have the engine out, and almost always have to replace the bushing in the arm AND the pivot pin, but normally the carbon throwout bearing is OK and the clutch disc and pressure plate are almost always OK. Eventually I replace everything just for peace of mind, like probably about once every 6 or 8 years. Really -- my clutches seem to last forever (AP "heavy duty" MGB style clutch disc and pressure plate).

I don't like ball or roller-type throwout bearings. I tried one once and it failed, which rendered the car undriveable. The carbon-faced bearings last a very long time for me, so it's just not a problem that needs fixing. And when the carbon bearing starts to "fail", you get a warning since it wears gradually over time -- you can usually limp along through another race weekend and fix it afterwards. When the ball/roller bearing fails, you're done for the weekend.

I'm guessing that the heat checking you found on the pressure plate was a result of your throwout problem. The clutch wasn't fully disengaging, which means that every time you shifted, up or down, the clutch disc was spinning at different RPM with light/moderate contact against the pressure plate. That would generate a lot of heat, every time you shifted. That's probably what toasted the pressure plate. Good that you checked it (guess you could say you performed a heat check check ... always check for checking ... whatever)

See you at NJMP in September.

Mark


Mon Jul 28, 2014 9:33 am
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I seem to remember that I tried a roller thrust bearing when we first built the car, oh some 25 years ago. We had the same experience Mark did, the bearing disintegrated rendering the clutch useless. The added thickness also meant that the clutch was always a little engaged, probably why the bearing failed. I think we shortened the slave push rod but I might be confusing that with another clutch issue.

Anyway I agree with Mark, to my knowledge I've only changed the carbon throw-out bearing once in 25 years, and that was because I thought I ought to, not because it was worn out.

Happy you're going the New Hampshire, see you there though we'll be later that 6, more like 7:30

John

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Mon Jul 28, 2014 11:31 am
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Nice triage of the issue Nial. Good stuff and thanks for sharing and documenting with photos.

Also, glad you got to run the whole weekend at Schenley, especially considering how much nicer Sunday was v. Saturday.

Regards,

Ian

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Mon Jul 28, 2014 12:01 pm
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Interesting post Nial - thanks. It makes me want to yank my engine and give things a once over before the next outing. Findings like this after the fact always intrigue me and I always wonder how much life is actually left until it let's go.

How difficult is it to replace the lever bushings?

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Tue Jul 29, 2014 2:51 pm
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I understand the roller bearing T/O bearing is available again. I ran one for 15 years and it never wore out! Come to thin of it I have one on my street Bugeye and that one is 20 years old! I sold a lot of them to racers and was told they had better longevity than the friction based units. For what it's worth!

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Tue Jul 29, 2014 6:36 pm
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Just to be clear here:

The roller throw-out bearing was fine when I pulled it out.

But I swapped it with a new carbon bearing because it was much thicker (by about 0.275") and I suspect (but cannot prove) that the added thickness changed the fulcrum point of the lever (possibly causing the throw-out bearing to be slightly off-concentric.....this would be a bigger issue on a roller bearing than a carbon bearing).

Also.....I've never really noticed wear on the release lever bushing before on a Spridget (but yes.....I've seen it on an MGB). So I was wondering if maybe the changed fulcrum point due to the thicker throw-out bearing had some effect on the bushing?

I imagine it would be pretty easy to replace the release arm bushing, but I had a good spare arm so it was quicker to put that in. If I have time over the Winter, I'll make up a new bushing and press it in.

And......the old clutch disk I took out was fine. The clutch wasn't slipping; I am sure of that.

The cracked, heat-checked face of the pressure plate is sort of a mystery to me. It looked fine when it went in and the old clutch disk I removed didn't show any signs of undue heat or wear.

The "new" pressure plate I installed in a used one so I will keep an eye on it.

John: I have registered for New Hampshire. Car is ready...see you there!

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Tue Jul 29, 2014 9:42 pm
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