ORIGINAL: ED STEFAN
I don't mess with small metric bolts. [>:] I go to Lowes and buy a threaded 12", 6/32 rod. Drill out both ends of the muffler, install the rod (cut to proper length) and double nut each end. Suggest using locktite on the threads/nuts. This baby won't be coming off unless you want it off.
Ed has the right fix. I did this with my muffler and never worry about it coming off.
Pete