I regard a 1/4" thick insulation spacer (Tufnol or similar and used with an additional standard gasket) to be essential if fuel vapourisation and the attendant starting and running problems are to be avoided...
Wartime and postwar parts lists show only a standard gasket (66-127)...
Interestingly a tufnol spacer is shown for M33 and B31/33 models post war...(65-1238) 'carburettor washer [tufnol]'...(1949-58 BSA parts list)...
That's surprising, as though the B Series engines run far cooler than the M Series variants BSA considered the OHV motors were the ones that needed it!
My advice would be to fit an insulator and lengthen the studs as required..Ian
Thanks everyone. I served as a DR in the 42 field reg artillery during the early fifties in Germany I remember the M20 as an easy starter hot or cold but then the leg was a lot younger, I had 2 of them and not sure about spacers. Top speed about 65 MPH I could get 68 to show on the clock with chin on the tank Normal convoy speed with the quads pulling the guns about 30 Thanks again Bryon
Some form of thermal insulation should be used to prevent heat from the cylinder head from heating up the carb. This is not required when the bike is running as vapourisation of fuel keeps the carb cool. Neither is it required when starting from cold. If you stop the bike and wish to re-start it after a few minutes, some form of thermal insulation will make it easier to start. I use a 3mm thick gasket with a coating of hylomar.