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MG MGB Technical - Premature failure of clutch thrust bearing


This is a carbon thrust bearing that failed in a friend’s car (MGB with 4 sync overdrive gbox) after less than 10,000 miles. Clutch was working fine then one day just grating noises and no clutch, engine out and found this. Thrust ring on pressure plate also ripped off.
Supplier/manufacturer of thrust bearing unknown.
Any similar stories?
Mike


Mike Ellsmore

Mike, Go look at the recent posting on the MGA thread. I don't know if the problems are related

Frank Bruns
Bruns

Come across several like that. One of the causes was when they started pinning the graphite ring into the casting instead of bonding, but they should have all been used up by now. In this one (8k miles) the whole graphite ring had disintegrated and covered the inside of the bell-housing, the pin is shown.


paulh4

In this one the casting has disintegrated leaving chunks in the bottom.




paulh4

Another cause can be because the release bearing fork is not concentric with the first-motion shaft, so the release bearing is continually sliding back and fore across the boss on the cover plate, causing rapid wear. It's why some people fit roller bearings instead of graphite, but the same things happens, and they break up as well, as mine did (attached). People say that they are used on modern cars so must be better, but on those (and the V8 and midget 1500) they slide on a tube which keeps it concentric at all times. As my roller bearing replaced a carbon that also broke up I investigated carefully and found it was off-centre by about 3/16", and I machined and spaced the release arm on the pivot to centralise it - time will tell. The cover plate was also badly worn and gouged, even though the friction plate was less than half worn.



paulh4

I started the thread on the MGA BBS. In my case the MGA release bearing had fallen apart in the bag, so at least I was spared the double engine removal. In that case the manufacturer was Powertune, retailer was Brown and Gammons - who confirmed they have no more in stock and replaced immediately FOC with a different supplier's unit.

The AP MGB roller bearing is a piece of cr*p too, failed in about 300km.

As I have a T9 box and MGB clutch in my own MGA, I will be installing the Burton concentric hydraulic cylinder next time the engine comes out.
Dominic Clancy

I have used the thrust release bearing form a Saab Turbo.
I think it is the one used on the Burton kit.
It has a curved front face not flat, which helps it to self align.

So I am told!

Colin
Colin Parkinson

Mike-
Inspect both the pivot hole inside of the forked end of the actuating rod of the clutch slave cylinder as well as that of the pivot hole inside of the clevis pin for signs of wear. It is common for both the actuating rod of the clutch slave cylinder and the clevis pin to have to be replaced. Next, inspect both the plain shank of the pivot bolt (BMC Part # 11G 3196) and the pivot bushing (BMC Part # 11G 3195) of the clutch withdrawal fork (BMC Part # 22B 56, Three-synchro transmission; BMC Part # 22H 1056, Four-synchro transmission) for wear, and replace them if you find any. This will greatly reduce slop in the action of the clutch pedal and make engagement more consistent. In addition, if either the pivot bushing and/or the ends of the plain shank of the pivot bolt are unevenly worn, the clutch withdrawal fork will then tilt when under pressure from the actuating rod of the clutch slave cylinder, causing both off-center running and uneven engagement of the carbon clutch release bearing. This will cause a grinding sound.
Stephen Strange

Pivot pin and bush on fork like new. New carbon thrust, clutch cover and clutch plate installed. We will wait and see how long it lasts!
Mike
Mike Ellsmore

Having had both a graphite bearing and a roller bearing break up when the second one failed I looked carefully at the release arm and it's relationship to the first-motion shaft - and found it misaligned by about 3/16", and spent some some 'machining' it to bring it back in line, and fitted a new Graphite bearing. The other effect of the offset was a low biting point, even after modifying the pedal and master push-rod to give more 'throw'. This was because the release bearing was pushing more on one side of the cover plate boss, so the pressure plate wasn't lifting as far off the other side to fully release the friction plate.

People say that if roller bearings are good enough for modern cars they are goof enough for ours. But that ignores one important fact - which is that OEM roller bearings as on the 1500 Midget and the V8 incorporate a guide tube which keeps the bearing concentric at all times. Simpy fitting a roller-bearing to a 4-cylinder gearbox will not have that, and any misalignment that is there before will still be there afterwards. And even if there is no fixed lateral misalignment as the release arm moves the bearing it will slide across the cover-plate boss, The graphite bearing is designed to slide to some extent, but the roller bearing isn't. In some cases you may well find a roller bearing works OK, in bad cases like mine not. The V8 front plate with the tube will directly fit the 4-cylinder gearbox - if you can find one.

If the hydraulic bearing comes with a guide tube then that will solve that problem, but personally I wouldn't put hydraulics inside my bell-housing given the problems accessing them.



paulh4

The misalignment


paulh4

The V8 arrangement, fixed guide tube arrowed. This keeps the release bearing concentric with the shaft both laterally and as the release arm moves through its arc.


paulh4

There are roller bearings with just a domed face intended to run on diapragm spring fingers - a la Triumph which are NOT suitable for MG clutches with the release plate fitted to the spring fingers.

The ones that work with the MG setup have flat trust faces which have a teflon pad affixed. These CAN take the sliding motion caused by the arc of the fork.

Clutch riding or failure of the fork to fully release from contact with the release plate will wear out both carbon and roller bearings in quick order.
Chris at Octarine Services

Clutch riding and failure to release not applicable here! As I recall the roller bearing - MGOC - did have a flat face with some kind of composite surface.

John Twist advises against roller bearings, as it happens.
paulh4

After a couple of premature carbon release bearing failures I decided to go for the roller bearing. I bought a complete three piece kit off a trader on ebay but with the arrangement as described by Chris for Triumphs ie the domed face bearing meeting the diaphragm fingers. I was a bit sceptical at first, particularly as the plastic bearing housing couldn't pivot in the arm as it had squared off pivots. Said trader assured me this was how it was supposed to work, but as I couldn't get my head around the engineering theory, I filed the pivots round as per standard, so bearing could pivot. I was also a bit worried about the bearing being in constant contact with diaphragm and played around with additional pull off springs at first, but then put my trust in it. A little noisier - with a constant slight whirring sound, but other than that has been fine (I reckon 15 to 20K miles since). When I last looked he was still selling the kits.
Peter Allen

The bearing definitely has to pivot relative to the release arm. It doesn't move far, so doesn't pivot much, but has to be able to do so otherwise the force of disengaging the clutch will be taken on one side of the bearing.

My roller bearing started making noises after a few k, which was when I read about the pull-off springs. Supplier said they had never heard of then. Noises came and went over the next 50k or so until it was obvious something was definitely not right, and the fluid level started dropping. Which was when I changed it. Friction plate barely half worn after 60k+. For that matter V8 friction plate was still not quite down to the rivets after over 100k.
paulh4

This thread was discussed between 02/02/2018 and 12/02/2018

MG MGB Technical index

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