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MG MGB Technical - Clutch puzzle?

I am visiting from the MGA section as my problem relates to MGB components used on my car.
Has anyone got a comment on the following clutch puzzle? I have had a problem with the clutch on my MGA Coupe where at high revs the clutch does not engage sometimes when the clutch peddle is released after changing gear. I need to let the revs fall and wait 1-2 seconds before the clutch engages then off I go as if nothing was wrong.
My Coupe has 5 brg B engine (1950 cc) with a MGB all syncro overdrive gearbox, MGB comp clutch and pressure plate and MGB master cylinder (no easy adjustment).
This problem doesn't occur when driving on the road but only during competition (operating at high revs) - hill climbs and sprints - where I am shifting up anywhere between 5,500 and 6,500 rpm.
The clutch is not slipping - it just doesn't engage.
I found that if I didn't fully depress the clutch it seemed to be okay - very difficult to do when the red mist comes over you!
I have tested for slipping on steep inclines in 3rd and 4th gear. Even starting in 3rd gear with the handbrake on and 3000 rpm - let the clutch out, pour on the juice and the engine stalls immediately.
I have checked the clutch arm throw several times from underneath while someone activates the clutch - the arm moves at least 10mm and returns sharply. The arm is not catching on anything externally, hydraulic hose appears in good condition (thought this maybe restricting oil return but arm comes back instantly). Have inspected inside the clutch housing using a flexible shaft camera - thrust bearing wear is minimal, clutch pressure plate diaphragm appears fine.
I did find one reference to this problem in the MGB archives where it was suggested that if the clutch diaphragm is depressed too far it can go over centre and can be held in this position due to centrifugal force at high revs.
A club member has suggested I limit the clutch travel by putting a block under the pedal.
I have done this by Superglueing a wooden block (12 mm thick) just below the clutch pedal pivot point - this keeps the clutch pedal about 40-50 mm from the floor. I can still change gears okay and it seems to have cured the problem but the only testing has been on the nearby freeway/expressway which is rather restrictive! Has anyone had this problem, is it possible for the pressure plate to hold in due to centrifugal force?
Mike Ellsmore

I think that you may be experiencing boiling of the fluid in your clutch hydraulic system. I've had similar problems when driving inland, in California, during hot Summer months. I bled my system with DOT 4 and the problem hasn't returned. Worth a try. RAY
rjm RAY

After a lot of internet research I am sure that my problem relates to the clutch diaphragm going over centre and being held in the disengaged position by centrifugal force at high revs (+5,500). Many references to this happening on Mustang, Camaro and Ferrari BBS sites. See description on the following site
Mike Ellsmore


FR Millmore

Does Tolja mean "Not met this problem, but it is plausible"?
Mike Ellsmore

Ah ya, second phrase.
Plausibility being here based on plain physics and experience, it is more reliable this time than usually!
The newer designs with the bent fingers are self releasing, nice, but the evils of overcentering a Belleville spring are still there. So I take the "failure" as evidence of a defacto release system fault, and a warning you can hardly ignore. And the wood block is perfectly corrective, if the pedal pressure is OK with you.

FR Millmore

I am going to test the set up at high revs this weekend (hill climb competition) - I will let you know how it goes.
Mike Ellsmore

This thread was discussed between 04/07/2012 and 09/07/2012

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.