Hi nimrod,
I seem to remember the orignals are 3/32in. ???? =0.093in
4mm is 0.160in app and 6mm is 0.240in
The extra thickness will cause the case to interfere with the clutch and primary chain
I have made the spacers from alloy plate, machined to thickness required, the orignals are steel
the gaskets usually supplied are about 20 thou thick, adding an extra or leaving one out makes quite a difference when setting up the case position
John