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MG MGA - Clutch puzzle?

Has anyone got a comment on the following clutch puzzle? I have had a problem with the clutch on my 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

Not met this problem, but it is plausible. It would be the result of over travel due to changes in the combined hydraulic/mechanical ratio of the pedal/hyd/release arm. If it is all MGB parts, that only leaves the difference between A & B pedals. If it is over traveling, it seriously compromises the diaphragm, which does not like going over center one bit; it will be weakened (pressure) and maybe break.
Note that over travel also results in small but finite increases in clutch reaction time, possibly of import. Leave the block in, or fit a smaller MC (or bigger slave). I automatically reduce my foot travel to minimum required; with a reasonably worn in clutch, that is about 2" maybe, don't like new clutches much.

FR Millmore

On the very first clutch I fitted to my car during the restoration, the clutch overthrew to the point that the pressure plate springs wore grooves in the clutch cover levers. It sounded strange but worked fine until I finally got around to changing it out a few years later. We checked everything - cylinders, operating rods etc, and it was all absolutely spot-on. On comparing that cover with another one while replacing it, all looked and measured identical. The replacement worked just fine.
dominic clancy

I found some interesting stuff on clutch pressure plate diaphragms going over centre at high RPMs on this site - seems this may be my problem!
Mike Ellsmore

This thread was discussed between 03/07/2012 and 04/07/2012

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