Very strange MGB clutch issue

clutches

#1

My car is a 1979 MGB. It has a lightened flywheel (on not sure how they did it, as i never saw it, but te work was dont by a very reputable machine shop.) when the car has been off for more than say, 7 hours, the clutch will not fully disengage. If I let the engine warm up before I touch the clutch, it has no effect on the problem. If I drive the car 1/4 mil or so, the clutch resumes totally normal operation, and continues to for 5-6 hours after driving it. In the first 1/4 mile of driving it, the catch point of the clutch gradually moves from the pedal needing to be on the firewall, back to its normal catch point at about 5/8 pedal throw. I have blead the clutch, but I havent moved beyond that… Any ideas?


#2

I see a clutch job in your future… and you’ll need to get that flywheel inspected.

When cold, if you could reach the slave cylinder and the clutch lever, the clutch lever should have a bit of free play. If this is the case, and the clutch is still slipping, the problem is inside the bellhousing. The transmission will have to be dropped, and the clutch components properly inspected. Personally, if I go thru that much trouble, I’ll just replace the pressure plate, clutch disk, pilot bearing and throw-out bearing, and maybe remove the flywheel and have it resurfaced.


#3

My guess would be a hydraulic fault in the clutch master or slave cylinders. When one is replaced the other should also be changed because whichever one you don’t swap will go bad the next week. Murphy’s Law.

Hopefully whoever did the machine work on the flywheel did it correctly.
I usually take issue if a flywheel has been surfaced like this one.
http://cgi.ebay.com/ebaymotors/MGB-1800-FLYWHEEL-RESURFACED_W0QQcmdZViewItemQQcategoryZ10076QQihZ020QQitemZ300147533835QQrdZ1QQsspagenameZWDVW#ebayphotohosting

Note the surfaced area does not include the clutch cover mounting bolt pad area. This means that area and the surface where the disc rides are not in relation to each other as they were originally.

This can play havoc with the clutch operation and can be made worse if the engine has a fairly decent amount of crankshaft end-play in it due to worn main bearing thrust surfaces.

I would examine the hydraulic end of this first and hopefully the problem will lie there.


#4

I think you may find that the “lightened” flywheel is made of an aluminum alloy. It has a greater coefficient of expansion than cast iron.

You may be able to get some relief by adjusting either the position of the slave cylinder or adjusting the length of the rod. (MGB’s had hydraulic clutches?)