Frank,

You should try it and let us know if it works after your next....er, maybe we better not go there?

I would not recommend notching under the screw head for an installation like an OS that has threads in the muffler and full length holes thru the engine case. The bolts may break but they will likely bend at the manifold face and you may bust up the case anyway. when the plane hits the ground this joint is in shear and wants to slip. That would bind up the bolts in the case.

I would take a round patternmaker's file and neck down the screw exactly at the muffler/engine interface location. The screw probably has threads there. File a round notch until you go a bit below the threads. If you like the engine a lot, neck it down more. You know you went too far if they snap when you install them or if they start failing in flight. A muffler that comes loose in flight may pivot on the remaining--now also loose--bolt and bang around on things, even get into the prop. Expect engine damage from that. Frangible bolts for mufflers may have unintended consequences for you.

You will have to reduce your installation torque a bit for reduced shank bolts. Since the prevailing locking torque will be less, I would use Loctite on the threads when you install.

Good luck and happy.....er, well I said I wouldn't go there.

Divot McSlow

Some motorcycle racers have the problem of unstable ground flinging them off too . . .
So as to get moving again , when theyve broken off the cluch lever , they pre-empt this by filing or sawing a slot at the front , 2 inches out ,
so it breaks off there , and despite the difficulties theyve still got something to hang onto , with two fingers .

Employing the same priciples , on the part its not screwed into , 3/32 or maybe 1/8 out , so theres something left to get a hold on ,
either filing a light 'V' , or a notch or two , will create a ' break joint ' . Screws being cheaper than mufflers ,
if youve the problem of the ground moving into your aeroplanes path , it could keep you in buisness .
Even been employed on engine mounting bolts .

The ideas to leave a fair bit , but create a stress raiser , where ' itll go ' . A bit like a carrier catapult hold down , under severe duress , rather than a lottery as to what snaps .