Getting my motor competly rebuilt from the bottom up everything. It's been taken a part idk how manny times and re torqued im sure those threads are getting weak what's the biggest anybody went went im a machinist so I can put it on the mill and do it the right way not with a hand drill. So I know it will be straight I do t wanna go to big and make the block weaker than it already is.
Just wanna put new threads cause I'm sure they are weak and don't wanna do all this work to it and something that would have take me a hour end up failing cause I can't stand when my motor leaks oil

What makes you think they are weak? If the bolt holds torque it is fine bud!
Its an 8mm bolt now, Next size would be a 3/8" But that might bee too big...

The case leaks when the case cover shifts. The holes in the cover are larger than the bolts which allows some play and the first thing to contact is the dowel pin at ~3o'clock position and this is where they always crack. MB has some sleeves which they insert in the 11o'clock and 5 o'clock holes - the hole in the block is 10.5mm and the one in the case cover is 11mm so the sleeves are different diameters on each side. These add more support to help prevent shifting.

Being a machinist, you may look at additional solutions to eliminate play in that joint to help prevent it from shifting. I agree with the post above that I doubt the threads are worn and need to be drilled out unless you have something cross threaded.

That cover has been taken off at least 12 times then threads Gota be getting weak cause after I run it a while I can go and tighten then back up and I don't use a torque wrench and I'm sure I over tightened them and just scared they pull I could helicoil them but I'm not a fan of heli coils

I was gonna make some bolts with the same diameter as the hole in the case so it wouldn't have slack but then if the case and block don't line up perfect it won't bolt up and if you tighten it with it in a bind it's deff gonna crack this is gonna be my 4th cover tired of replacing coves the cover alone is $380

Embed grade 8 studs into the block and use shoulder nuts on the cover.

Helicoils will hold better than the threaded aluminum.

Well funny you say that im actual looking for a place to send my crank and flywheel to get balanced know of anywhere that can do it

Who balances these things

Tim's Engines in Denham Springs.

Know about how much it cost

I just did $185

Still briggs junk.

What's the point if it's on a rattling ass GTR?

If you wanna pay for all my repair bills I won't get it fixed who knows I could have the worse one ever made tired of the vibration and the motor is competly apart might as well can't hurt

I'm saying it could be the prop, planetary gear, etc...

Well Ima start with this since its apart right now nothing on the frame cracks it's always on the engine

I hear ya, I put a new prop on one weekend and by the end of the weekend I had two broken air dams, a cracked exhaust and a leaking case. The Briggs is the weakest past. Hahaha

It also dont help that when you shim the motor and its just sitting on the 4 shims in the corners it not sitting flat on the frame. And that's causes more if you would use a solid plate to shim the motor it would have more area to displace the vibration I might do that to machine a plate so the whole motor sits on the shim

The main source of vibrations are the flywheel, Crankshaft, and Propeller.