clutch mystery

[chapelfarmer]

Hello 
I'm hoping a description of a problem may spark some thought from somebody which will help me avoid having top remove the engine from my special!
It runs well for about 20 minutes and then the clutch starts to slip. If I drive on it will normally stop slipping (after another 20 minutes or so). The clutch is fairly new (less than 10k miles) and I have tried reducing gearbox oil to between half and 2/3 pint, reducing engine oil level to bottom of dipstick range and oiling the clutch bearing release through the tube provided. Box is 4 speed synchromesh (with close ratio gears), engine is earlier bacon-slicer type (1930 I believe)..I don't suppose the fact that a Dante fiend sump is fitted would have any significance? Do these symptoms spark any theories? 
(please!!)
Thanks
John

[Ruairidh Dunford]

John - I have had similar issues with a number of cars, the cause was the new posts jamming in the cover plate holes when they expanded as the engine warmed up.

I now check and reem these carefully on every build.

Very careful lubrication of the posts in situ, with the starter motor off, allowed the clutch to operate until sufficient wear occurred.

Another cause of slip can occur if the car is used less regularly, the lining glazes and will slip initially until this layer is ‘burnt’ off - glorious smell in the process.