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MG MGB Technical - Carbon clutch release bearing, how much life left?

I've currently got the engine and gearbox out of my 72 Roadster for a refresh. The plan is to inspect and replace parts as necessary or where prudent.

When I rebuilt the engine approx 12 years ago, I reused the carbon clutch release bearing. I did this rather than replace as the quality of replacements at the time left a lot to desired. I'm now at the same point again and have the same dilemma.

I've measured the height of the carbon face above the carrier and it's 4.2mm. Without a new one to compare it against I don't know how worn this and hence should I refit or replace?

If anyone has a new bearing they could measure, it would be appreciated. If it's less than 50% worn I'll probably stick with it, failing that I'll replace it. This leads on to my next question, does anyone know of a supplier of reliable carbon release bearings.
R.A Davis

4.2mm sounds like a lot. If it was me, I'd re-use it.

Dave O'Neill 2

Measured a brand new old stock BMC one I have here---- 4.4mm---
William Revit

Thanks, if my calculation is correct it's only 4.5% worn, in which case it'll be going back in.

Bob
R.A Davis

Out of interest if this one has been in 12 years since being reused how many miles has it done in total?

There was a batch of bad graphite bearings a few years ago when they changed from bonding the ring in the carrier to pinning it and they could fail in very short order, which is why many go to 'roller' bearing type. However they depend on there being decent alignment between the release arm and the first-motion shaft, OEM applications like the V8 and midget 1500 have a guide tube to ensure the two are concentric. Mine was way out for some reason (I've been told they all are to some extent) and the bearing broke up in pretty spectacular way as the load was all on one side of the bearing and the clutch cover release ring, which made the biting point low as well. Even though I corrected the mis-alignment I fitted a new graphite, as ever time will tell, 12k so far ...






paulh4

Paul-
Yes, the carbon clutch release bearings have been known to have their own problems. The AP-manufactured carbon clutch release bearing is fine, but the Indian-made one has had problems with its carbon ring fracturing and even falling out of its housing. A problem that can occur when purchasing an off-brand kit is that there is no way of knowing what you are going to receive. Both the Borg & Beck and the AP kits include the AP carbon clutch release bearings. However, some retailers will piece together their own kits having both an AP clutch driven plate/friction disc and an AP pressure plate assembly, and then include a cheap, junky carbon clutch release bearing in order to maximize their profit. Instead of enduring this risk and being in doubt, just purchase a complete genuine Borg & Beck clutch kit (Borg & Beck Part # KT9694).
Stephen Strange

Paul, I've no idea how many miles this bearing has done. It's the same bearing that was in the car when I bought it, 12 years ago. I'd estimate I've done in the order of 30 - 40K miles since I've had it but it's anybody guess how many mile it did before that. I could look up old MOT's to get an estimation but A) life's too short and B) I've changed the speedo since I bought it.

As you may be able to make out in the second picture, it says AP England and the number opposite that reads, I believe, QS74-75150. Admittedly it could be an Indian copy that used an AP one to create a casting mould but who cares, it's lasted this long and I try to treat the clutch with respect. So it's going back in.

Bob






R.A Davis

Bob - I was interested from the minimal wear vs the number of years, driving style makes a huge difference on how long a graphite bearing will last. Respect from me also - not riding it or sitting at lights or stuck in traffic with it held down, that roller bearing had only done 50k and the friction plate was barely worn.
paulh4

Paul, I'm a bit anal about these things, if I'm at the lights or junction for more than a few seconds, it's into neutral and foot off the clutch.

I believe the guys that came up with this method knew what they were doing. What they couldn't take into account was poor modern manufacturing process's and poor driving habits. It's common practice to use the clutch as a handbrake substitute.

Bob
R.A Davis

I would consider the release pad and the carbon bearing to be "wear parts". Like gears, if one is changed, the companion should be also as they have bedded in together.

I recently had a carbon bearing start to fail on a cross country trip. It had about 75,000 miles on it and showed no symptoms of failure when I left. 1,000 miles out, it started making a metallic noise so I returned, put the car in the shop and bought an airline ticket to my destination.

75,000 is not bad for the carbon set up. Good to have the car back and ready to go without that concern.



Glenn Mallory

That's as badly offset as mine was but in a different direction. Don't use a roller bearing ...
paulh4

This thread was discussed between 21/10/2023 and 24/10/2023

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