hey, i had the same prob only it was a muffler that threaded in that would unthread itself. NEVER HAD A PROB AFTER MAKING THIS...
I assume your engine has a threaded exhaust port. Mine is for 3/4" black pipe. (homedepot, lowes... evreybody has it.)
this type of pipe is confusing, but 3/4" refers to inner diameter.(the outer diameter is around 1")
OK here was my fix... i thought why not utilize the threads and the screw holes??? (it only works if you can screw in your muffler and it wont hit the tank or watever else is there, or you could take all that stuff off, put/screw the muffler on, then put the stuff back on.)
if your header doesnt have a threaded end on it, buy a pipe nipple (small ~2-6" long section of pipe) and cut off one of the threaded ends and weld it onto the part of the header that goes to the exhaust.
now cut out a flange to match the exhaust port BUT MAKE SURE THE PIPE THREADS AND A LITTLE BIT OF THE HEADER CAN SNUGGLY FIT THROUGH THE CENTER.
see the picture of my flange...
ok put your new flange over the nipple threads/header, and screw the header threads in.
once it is screwed in all the way, move the flange toward the block, so that its flush with the exhaust mount area, and the screw holes line up. screw some screws in to make sure its right.
now, with the exhaust still attached as above, tack weld the flange to the pipe that is sticking out of it.
unscrew the flange screws so that the the exhaust can unthread from the block. finish weld the flange to the pipe.
see my other pic...
if someone doesnt understand, i dont blame ya!
just ask for pics cuz i did it to my minibike engine's exhaust(briggs5hp).
hopefully this isnt true...
good luck,
BAMBY--PICS LINKS IN POST ARE SAME AS ATTACHED ONES