RE: Os fp engines
My old FP40 is fitted with 6 BA bolts. These are replacements but I thought the originals were 6BA.
A 6 BA thread is 2.8mm dia x .53 pitch .
An ISO coarse M3 is 3.0 mm dia x .50 pitch
So the difference is slight but important.
I used longer bolts than standard and used them like studs with a nut, so that the tightening is done by the steel nut on the steel thread instead of in the alloy of the silencer. This is a good trick if the threads have worn in the silencer.
Of course if you asked this question it seems that you may not have used a thread locking compound! Even a rubbery type of contact glue can be used if you do not have proper thread lock.