F.Y.I. APRA Rules Enforcement At Whittier
Max, To the best of my knowledge, Bill's interpretation of the rules is correct regarding using sheet metal screws to hold the muffler together however I have not had this confirmed by the APRA rules committee. Possibly Jim Allen, Gary Schmidt or Stan Douglas from the Arizona based APRA rules committee could jump in on this thread and enlighten all of us, "From the Horses Mouth" so to speak. The problem we run up against in such things is how strick does one interpret the phrase "Unmodified or Box Stock" Obviously the muffler does not come new out of the box with sheet metal screws in it. AMA rules say you can change bearings, gaskets and bolts. Does this mean adding additional bolts/screws beyond what the manufacturer shipped it with (?) or is the criteria based on whether or not a modification could or would result in a performance advantage? Clearly this is a gray area and needs a definitive answer from the powers that be on the APRA rules committee. As Bill pointed out above, unlike a welded muffler, you can always take the screws out and still disassemble the muffler for internal inspection if the need arises.
Another solution to this problem, which has been discussed in a previous topic on this forum, and I know is legal, is using Permatex Hi-Tem RTV silicone Gasket Maker material. I personally like the "Ultra Black" however Don Stegal found "Ultra Copper" which evidently will withstand higher temperatures. I've used both and ended up going back to the Ultra Black just because it seems to have much stronger adhesive characteristics after it sets up and is completely cured, which is what we're mainly looking for in this application. I've lost tail cones too and just the same way as you, the bolt sheared inside the muffler. I haven't had the nerve yet to try running one without the bolt at all and only using the Ultra Black however I can tell you it takes quite a pull to get them apart for cleaning/maintenance after they've had a few flights and the stuff is completely curred.
Stan Shoun
CD- SGVRCL