At Home Depot, the Milwaukee bits are in SAE units, but they show their metric diameter on the front label. One of them is IIRC like 6.2mm or thereabouts and that is essentially an M6 hole.
I just looked it up and the correct drill bit size for an M6 tap is 5 mm. 6.2 mm would be way too big:
Tapping drill size chart for coarse and fine metric thread screws.
Those are what I use. If you have the money, you can't beat a good step-drill bit.
Step drills are for drilling holes in sheet metal; wouldn't work here, because I need the hole with some depth to it.
I have another Milwaukee kit that has two of them in it but thats about $60-70.
I work at Snap-on, so I don't buy Milwaukee. They are our main competitor and they are patent trolls.
But if drilling all the way thru, there's no need to go up to M6. A thru bolt and nylock is going to be waaaay stronger. Its oval tubing but a washer on each side will keep the nut and socket cap from digging into the frame just fine.
Through hole would be stronger if we're just talking about resistance to it pulling out, but on the other hand, it would weaken the part unnecessarily, in this case.
Vibra tite is much better on something like a bicycle. Get it on Amazon. Comes in an easy-to-apply gel. The blue stuff. It never hardens. Originally developed for the aviation industry for resisting severe vibration. Its the locker of choice for competitive long range shooters and their delicate scope ring bolts and mounts, that suffer from major recoil shocks.
This sounds perfect; like something that should be in my toolbox anyway. I'm going to get some; thanks for the recommendation.
I have never done a rivnut. But I worry about tapping M6 and those threads in alloy that has already failed once.
Drilling out and re-tapping seems like the least ghetto way to do this. It's not like it's a piece that was bent, then bent back; the material that failed is gone; ripped out by the old fastener. I'll remove a bit more material by drilling, then cut threads in the virgin material.
By the way, I think I just found the perfect wrench for getting to the axle nut on the wire side of the rear hub. The protective cage only allows about 1° of throw with my small crescent wrench. My metric combo wrench sets don't have 18 mm, for some reason.