I would agree with the all thread route. There is one big problem with some of these OS engines and their mufflers. That is loosing the back half of the muffler. This results from the bolt that runs through the muffler breaks in flight. 9 times out of 10 this bolt breaks because it is too tight. What happens is that the metal of the muffler expands as it heats up and snaps the bolt, and gone is the back half of your muffler. The only thing you can do now is to replace the entire muffler as you can't buy just the back half of it. If you put washers on this as indicated above then it's only a matter of time beforf this happens. If you want to avoid the all thread route you have one other option. First clean the mating parts of the muffler and put the back half in position on the muffler. If needed use a hammer to lightly tap the muffler so that it is tight on the front half. Now assemble the bolt through the muffler, and get it snug (not strong arm tight). Now, where the muffler halves join drill two small holes 180° apart from each other and screw in two small sheet metal screws. This will stop the muffler from rotating.

Ken