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MG MGA - Clutch problem

Two years ago, my bog-standard clutch friction disk disintegrated. Upon disassembly I found that the pressure plate arms were “over-throwing”. In other words, the release bearing was pushing too far forward causing the pressure plate arms to come in contact with the clutch disk. Hence the center spline became detached from the friction disk. Fast forward a year. I replaced the clutch components and flywheel with MGB spec parts. That included a correct throw-out arm, release bearing, pressure plate, 10 spline clutch disk and an MGB flywheel. Seemed to be working like a charm.

Within a couple hundred miles I again had a clutch problem. This time I could not disengage the clutch. (30 miles home stuck in 3rd is another story). Essentially the same problem as last time, the clutch release is moving to far forward and coming in contact with the friction disk. This time the center ring in the pressure plate disintegrated. Different designs of the MGA and new MGB pressure plates caused different failure points, but the cause seems to be the same.

Static measurements appear to show that the release bearing isn’t moving too far forward. There is a small amount of play in the linkage between the clutch pedal and master cylinder. Correct length for the slave cylinder push rod. The only thing I have not been able to test is if a quick double clutch action is somehow causing the release bearing to move too far forward?



John Backman

1. Why are you double declutching?

2. When did you last change the flexible slave cylinder hose?

If there the hose has deteriorated internally then it may be slowing the return of the fluid.
Chris at Octarine Services

John
I am a Sprite & midget owner but some years ago I had the back ring detach from the diaphragm pressure pad as yours has done. I have to use Quinton-Hazel clutches on my car. I found that the ring was held onto the pressure pad by a couple of swages (indents). To overcome the problem I tack welded the ring to the pressure pad. Several years and many miles since.
Alan
(Another double declutcher).
Alan Anstead

Hi John, You also need to match your bell housing to the clutch components ie:use an MGB mk1 gearbox housing when using MGB clutch components.
I Hazeldine

You do not need to use an MGB bell housing.
You just need to fit three dowels to the MGA flywheel in place of the existing two,
You need an MGB diaphragm type pressure plate, an MGB operating arm, an MGB thrust bearing with clips, and an MGB gearbox front cover plate.
I have found that recently some MGB pressure plates can touch the bell housing slightly. A very small amount of metal may need to be ground away inside the bell housing in some cases.

Mick
M F Anderson

Did You use MGB3_brg clutch fork/cover?
Dirk Van Ussel

Thanks for the comments and questions. The clutch hose was replaced in 2006. Double clutching mostly for hard down-shifts into 2nd gear.

I don't think that tack welding the pressure plate ring will be enough. There is evidence that the ring was hitting some tabs on the friction disk enough to slightly bend them.

I forgot to mention the MGB front cover plate, new oil seal and a new lever bearing. A small side benefit of the clutch failure is that I discovered that the MGB front cover plate was not flat and leaking badly. I also had to grind away a bit of the bell housing near the starter for pressure plate clearance.

I am thinking my next steps will be to replace the clutch hose, slightly reduce the length of the slave push rod, put a bit of white paint on the inside ring of the pressure plate and reinstall the engine without all of the peripherals. Then see if I can spin the engine and recreate the problem. Any other suggestions would be appreciated.



John Backman

What is the casting number on the gearbox front plate?
Chris at Octarine Services

So, thinking about this-
You have the same overtravel issue with both the original setup and again with this diaphram set maybe
or
is it just the throwout pad on the pressure plate has disintigrated and left a mark on the drive plate---I'm thinking this is what has happened
If that pad had been bottoming out on the plate you would have been having trouble selecting gears as the plate would have been dragging on the flywheel when the pedal was right down

Shortening the pushrod makes no difference to the travel, all it does is move the slave cylinder piston to a different starting position in the cyl and possibly, if too short allow the piston to pop out
If you can push the fork towards the slave cyl and get some travel back into the cyl. then the pushrod doesn't need shortening--the only time you'd shorten it would be in the very rare case of the s/cyl piston being bottomed out leaving no possible free travel in the arm in the rest position

There is another possibility though--
How high does your clutch pedal sit in the rest position---If it has an adjustable pushrod on the master cylinder, someone might have adjusted it too far out giving too much travel all up
William Revit

Chris, I double checked, the front plate number is 22B55.

William, in both cases there is very clear evidence that the MGA pressure plate arms and the MGB throwout pad were hitting parts of the drive pad. The MGB damage to the drive pad is limited to slight bends in three tabs, but the engine side of the throwout pad failed since it wasn't designed to come in contact with anything. Your point about selecting gears is a good one. If the over throw condition was only happening when I double clutched, it may not have been all that noticeable? I never heard or felt anything when shifting.

I will check on the travel of the slave cylinder pushrod.

There is an appropriate amount of free play between the top of the clutch pedal travel and the master cylinder pushrod. The master cylinder has been rebuilt and the pushrods replaced. I also fixed the brake and clutch pedal to master cylinder pushrod holes with new bushings. So the whole system from clutch pedal to the flywheel has been rebuilt.
John Backman

John
You seem to have done everything right, but something doesn't add up-
Maybe the offset of the clutch plate spring housing is the issue and is more rearward than an MGB clutch plate---it would be interesting to measure both and compare
You might have to go to a different brand that doesn't have them little tags and the raised section around that area
I take it that you have the bumpy style plate as in pic one but there are other flatter style plates as in pic 2 that would provide more clearance for the p/p rubbing pad to travel






William Revit

Beware. Willy is spot on.

I had exactly the same issue of overthrow on my very first MGA clutch. The levers on the pressure plate were contacting the springs in the friction plate but shortening the slave pushrod only allowed the slave cylinder piston to leave the cylinder and lose all clutch function completely. It may seem counterintuitive, but if the MC is providing a given volume of fluid to the slave, reducing the length of the slave rod will be taken up automatically by the hydraulics in the next few pushes on the pedal, and all you will have done is move the slave piston closer to the end of the slave cylinder. This is the automatic wear adjustment in extremis. Make the rod too short and the piston will fall out

Motobuild Hounslow (in those days) had sold me the clutch and never did discover the cause - in the end I was given another new clutch assembly and everything was fine. All the returned parts measured to spec and we were all just happy to have resolved the problem.

When I fitted the 5 speed, I switched to a B clutch as well. I too had to relieve the new bell housing by the starter, but have always found the MGB clutch much more positive in operation (with a lightened B flywheel helping along as well.)

You can reduce the travel on the pedal by adding a rubber hose along the the pedal stop bar - the honourable Mr Firth of these parts can provide photos as IIRC he wanted to get his pedals to move down a bit.
Dominic Clancy

This might be a red herring but I offer it for consideration.

I started getting severe vibration on gear changes with my MGB clutch assembly in my Type-9 conversion. I investigated before any significant damage was done. I found badly worn bushes and bolt in the operating arm pivot caused by not tightening up to the correct torque on installation. This caused the thrust bearing to run off centre, applying asymmetric pressure to the thrust plate. Not sure if this would have gone on to produce your type of catastrophic failure.

Steve






Steve Gyles

Strangely enough this has happened to us recently. MGA with 5 bearing and MGA box. Clutch bearing disintegrated so had new bearing, plate and pressure plate, release arm resleeved. All clutch parts uprated from Moss - AP. Once back on the road had a large oil leak from rear engine seal ( 100 miles per pint) so all out again and found pressure plate same as Johns. No other signs of a problem and car ran perfectly...New pressure plate fitted and old one will go back to Moss.
David Brenchley

As an addition to Steve's comment, Gio Delicio commented to me that all modern fork bolts are undersize, and the sideways slop Steve mentioned is a consequence - IIRC he was commissioning correctly sized bolts to eliminate the issue
Dominic Clancy

Still having the same problem. The clutch seems to be over-throwing and I can sometimes feel something hitting the flywheel when the clucth pedal is pushed all the way. It seems to happen after I have been driving a bit, particularly in city traffic. The only thing I can think is that with frequent use, the master cylinder is not fully releasing the clutch. In other words, unnecessarily retaining some pressure (I don't have the brake non-return valve on the clutch side).

John Twist suggested adding some gasket spacers to allow the MC clutch piston to travel further forward thereby ensuring that the piston seal clears the small hole between the piston chamber and the brake fluid chamber. Still no luck.

So today I removed the MC for inspection. I had this MC resleeved many years ago. Removing the MC top I noticed that in addition to the two holes that are drilled through to the piston chamber, there are what appear to be two holes further back that are not drilled through (see attached photo). Are there supposed to be four holes through to the piston chamber, or just the two in front?

Thanks, John



John Backman

This thread was discussed between 22/08/2020 and 08/08/2023

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