"The shim measures 0.0195 inches and the space it goes into is about 0.034 inches deep". Quoted from Gordo below.

http://www.britbike.com/forums/ubbthreads.php?ubb=showflat&Number=668717


1961 BSA DBD34 Clubman
1966 Triumph T120TT
1972 Rickman 250MX