I've got an OS91 four-stroke, bought from a guy who used threadlock to re-assemble. If I could find the guy again, I would get him to dis-assemble it, because I've got the head of a ball-driver broken off in the socket of one of the headscrews. The engines don't come with threadlock. I've flown engines by the dozen, most of them disassembled and reassembled one or more times, never one with threadlock. Just tighten them up with an L-shaped allen wrench, tighten to the first little squeak, and they won't come out unless you stick a wrench to them. Teflon tape is the way to go on muffler threads.