Tell you what works better than finding an exact match, and will work a lot better than the original bolt:

Get a Length of fully Treaded 6-32 rod & 2 6-32 Lock Nuts or Nylon Insert Lock Nuts(Nylocks)
Cut it 1/2' to 3/4" longer than the length of the silencer/muffler
Drill out the holes on both ends of the Muffler the same diameter as the Rod
Screw onto one end of the rod one of the Locknuts
Insert the Rod through the Muffler
Screw on the other Locknut
Snug down the Locknuts and your done

Gauranteed not to come apart, I use this method on my Pylon and Combat planes, I can't afford to have the Mufflers come apart in the middle of a Heat.