Seems previous owner of my ATV overtightened the drain plug and stripped out the plug hole so that the plug will no longer tighten. Had anyone had this issue and find a fix for it? I've read not to try self tapping plugs as they might crack the engine case. The plug that's in it is a 12mm bolt with a crush washer.

 

Looks like there may already be a crack. I have done a few Honda 300/350s which had a peculiarly bad sump plug hole, with half the aluminium threads missing to start with, due to the shape of the sump. The first couple I did by drilling for M14 threads and tapping to that size. Made things a bit awkward, finding a suitable short bolt and washer, and the socket is nearly too big for the hole in the bash plate. So, got an M12 thread insert kit and drilled and tapped to fit that and go back to standard on the last one or two. If you don't want to go to the expense of the kit, motorcycle repair shops usually have the common sizes, and can fit a thread insert for you, as can engine machine shops.

 

Thank you, I'll have to crawl under to get a better look at that crack but I think the insert might be the best way to go.
I did find a kit on Amazon that taps a 12mm drain plug to a 13mm an comes with the plugs though insert might be better if it's already cracked.

 

I have only tapped a few holes for helicoil and for me the toughest part was keeping the drill and tap perpendicular to the hole. Maybe some kind of simple jig to drill straight and start tapping straight.

I have also heard to grease drill and tap really well and back out frequently to clean shavings away which are caught by the grease.

there are other options but I am not sure which one might work better than helicoil. Timesert I think is one.

 

I ordered a 12mm oversized self tapping drain plug from Dorman, gonna grease the threads and slowly try to go at it

 

I upsized mine to 1/2-20 because I already had the tooling and the drain plug was way cheaper than anything ending in mm. So far, so good with a year on it. Just did an oil change and the threads are still in good shape.

 

So I ended up going with a Dorman 12mm-1.5 oversized self tapping plug and slowly worked it in to make new threads, Gotta flush it out now but hopefully should be good.
got a better picture of the cracks too.
Probably gonna put some automotive high temp silicone on the plug threads on final install.