Anybody use aluminum NPT pipe? I've had the problem with iron NPT pipe rattling and chewing the threads out of my flathead Briggs & Stratton blocks, which are aluminum also. I'm thinking of a snug aluminum adapter fitting (something stubby) first installed in the block and then extended with more aluminum pipe and end adapter to receive a Briggs muffler. Could sort of patch the thread problem at the block, and relieve the stress from weight of iron pipe.
most of the bike supply firms offer a pre- bent tubing kit that is far easier and less expensive than most other options.. that is if you don.t get extreme enjoyment form making things difficult... the link below is just an example of what is available to us builders... look around