Service Bulletin: Increase A65 Model Piston Clearances
Original BSA Service Bulletin #198 10-26-1965
In this original BSA service bulletin #198 dated October 26, 1965, BSA indicates that pistons (for A65 650 models) should have a skirt clearance of approx .0035" - .004". This is a very interesting sheet as the title has a very important keyword, "increase". For those who have have built an A65, or any late model British motorcycle, typically a skirt clearance from .004" - .0045" is an ideal place to aim for.
I could only assume that BSA must have been experiencing seizures on the A65 models for them to "increase" the skirt clearance. I wonder what was there skirt was clearance before? Perhaps .003"? In my opinion, .0035" is pushing the limit. For .0035" to work (which it can) your cylinder bore must be perfectly round, your jetting has to be spot-on, ignition timed correctly, and the break-in procedure must be done correctly. Cylinder material and piston material also play a key role in how close you can run the skirt clearances. Although this service bulletin was dated 1965, most pistons today by JCC or EMGO for example typically recommend a skirt clearance of .004" - .0045".