Believe it or not, I haven't had this happen to me. Of course, I have mousse cans on all my B and 2610 engines, and they use 4-40 bolts. I also am a big fan of locktite on the smaller engines.

That said, I know guys routinely drill out and re-tap their muffler bolts to a larger size. Guys use 4-40 bolts on the .15 engines that use the 2.5mm and 2.6mm bots. And I think they use 6-32 on the engines that come with 3mm.

Oh, while I haven't had it happen on the muffler, I've now re-tapped all my Magnum carb mount bolts to 6-32, after I had a couple of the 3mm ones strip. I just had a carb party one night, tore down and re-did 5 carbs, including tapping for 6-32 and drilling the crankcase mount.

I'd use 6-32 because the corse thread gives more metal in the threads. I know that in theory fine threads are better for high-vibration envrionments, but when you have a metal as soft as the cast aluminum used in many of our engines, it makes sense to get as much metal surface involved as possible, and 6-32 seems to work well.

So, if it was me, I'd tap to 6-32. You can get good quality 6-32 taps and drill bits at your local hardware store, and by all means, use quality socket head cap screws and locktite so you can get that thing torqued down so it never comes off until you want it to.