Composite gaskets fail because of inadequate clamping force. The old BSA Service Sheets say to tighten the head bolts and "Make them really tight". OK for the backyards of the 1950's, but in these more enlightened times what sort of torque figures are folks working to?
Goes without saying the barrel threads need to be clean and the blind holes cleared of anything to cause the bolts to bottom. Bolts need to be lightly lubricated and fitted with nice substantial washers. Parts book ref shows them to be 64 Thou thick. Later A50 and A65 models with higher power outputs all moved over to the solid copper type.
Composite types (copper/asbestos) were the mainstay of the UK Auto Industry for many years until the spectre of asbestosis and compensation claims gave us the cardboard and aluminium types we see today.
If you ever come across a WW2 Gas Mask, don't try it on. The filter medium was asbestos. Brought to light by claims from workers at the Boots Company in Nottingham, who made gas masks under a Government Contract. In those days the harmful long term effects of breathing in the dust of this widely used mineral fibre were not appreciated.
Swarfy.
