When working on used vehicles it is almost certain that you will run into a damaged thread situation at some point, especially with threads in aluminum – it doesn’t take a huge amount of force to strip threads in aluminum. The latest issue for me is stripped threads in the drain plug hole in the clutch cover of the KTM 125sx – not damaged by my hand!
Drill and tap oversize
Go to the next size up, drill out the hole to the new thread size and tap new threads. Have to be careful about reduced wall thickness and if you have to go to a larger bolt head, ensure that you still have enough room to fit a socket and/or wrench or consider an allen key head.
Install longer bolt with nut
When I found the threads damaged in a triple tree fork clamp, this was the easiest fix. I found a longer bolt and installed a nut on the end, fortunately there was enough room for the nut without fouling anything. Obviously this is no good for blind holes.
Fill with copper wire off-cuts
I found this recently on YouTube where a guy had stripped threads in his steering wheel hub. The holes were approximately M6 size and about 10mm deep, the guy cut about a dozen strands of copper wire an inch long, folded them in half and inserted them in the hole before installing the bolt. The bolt jams the copper wire against the ID and then cuts new threads in the copper resulting in quite a strong repair, good for times when you are not concerned about sealing and/or frequently removing the bolt.
Fill and re-tap
If you have access to an aluminum welder, this could be the best option. Need to be careful about heating around the affected area, ensure that there are no seals nearby that could be damaged and the part will not be distorted due to the heat.
If the part can be heated, aluminum brazing can also be considered, however the repair will probably not be as robust as a welded repair.
Epoxy like JB Weld can be used but the resulting threads will be relatively weak, ok for a repair where you can epoxy the bolt in place and not have to remove it again … ever.
Heli-coil type thread insert
This involves drilling out the hole and tapping to a larger size but then installing a thread insert that looks like a coil spring that provides a thread of the original size. The inserts usually have locking features but can also be further secured with thread locker (Loctite) and are often steel providing a very strong repair. The downside is that the kits that have the special size tap and insert tool and are usually quite expensive but once you have them, the inserts are relatively cheap. Might not be cost effective for a one off special size repair.
Similar to the Heli-coil type inserts, a threaded insert is a cylinder with threads on ID while the OD can be threaded, plain or have other anti-rotation features. The hole would be drilled out to the size of the OD of the insert, then the insert would be pressed in or could be epoxied in place.
Replace damaged part
Last resort if nothing else is suitable.