For going on 15 years now I have been using Lock tite 272. The screws and threaded holes are cleaned with acetone before installation. I also take and block the muffler flange first. I have had very few issues with mufflers working loose. Usually I have to hit the screws with a 140 watt soldering iron to get them loose. It appers that there are several products that will work as there are several different methods mentioned in this thread. IMO the key to keeping the muffler from working loose is to carefully balance your prop and spinner and more importantly do not run your engine too hot. In all cases the cylinder and muffler are different alloys and the screws are steel, all different expansion rates thrown in with most thread locking compounds don't work past 300 degrees well.