Quote (Slaytanic454 @ Oct 28 2013 08:21pm)
I've always had good luck with helicoils. Being a machinist might help with that though.
I don't think a helicoil is the solution here though, there's a LOT of damage there; one tap size bigger wouldn't cut it either, I don't think.
One thing you could do would be to buy something like a male 1/2" NPT to female 1/8" NPT fitting, drill out a monster hole, tap it 1/2" NPT, then thread the fitting in permanently. Use a 1/8" NPT male plug as your oil plug. NPT (National Pipe Thread) is tapered, so it would be self sealing on the aluminum case without any need for teflon tape/pipe dope.
Like this:
http://www.suremarineservice.com/images/products/detail/FIT535.jpg
You'd have to drill a huge fucking hole to thread the 1/2" NPT plug in, but it would get rid of all that damaged material. You can g et 'em made out of aluminium too, there's all kinds on eƁay.
If you were to see the cases from the inside it would appear much different. Think of the area around the threads as the same width as the sealing surface for the crush washer. That piece protrudes into the transmission. If you drilled that away there would be nothing to tap. The metal is only 1/4" thick or so outside of that area.
They make rubber expansion plugs to fix this too. Not a big fan of those either. It's hard to tell without being there if the sealing surface that is cracked will hold any pressure. If it will he could thread seal the bolt and it should hold. The torque spec is like 10lb/ft... There are many bikes running around with weeping drain plugs that have RTV or thread sealer holding the oil back.
I prefer the loctite metal putty to JB weld. It is thicker and easier to mold into the shape you want. Sets faster too.