I have in the past retapped them to 4x40 standard, The M3 is close but just a little smaller with a slightly finer thread, If you can get a tap cheap it will make it so much more fun finding muffler screws. Heck if you feel you want to retap I would not mind sending you a tap, I work in a machine shop and and could get one for you jsf135, Just PM me if you want one since you only in CT and I'm in MA it will get to you fast.

Dauntae