Repairing stripped threads on a nut type bottom bracket I recently bought a used 2011 diamondback outlook mountain bike for $30 on craigslist. 

The bike however needed some significant work. The bearings in both wheels need to be replaced, the shimano 7 speed freewheel was rusted to crap and best of all, the rear brakes were missing. I guess that's why it was $30. The previous owner also decided it would be a great idea to coat everything in some very heavy automotive grease. the only good thing about that was the chain was really dirty but just needed a good cleaning. Somehow that was intact...

Anyway here's what we are getting to. I was removing the right crankarm to replace it with different ones that weren't so beat up and the threads are now stripped. Not the threads inside the crankarm, the threads attached to the bottom bracket that allow the retaining nut to hold the crankarm into place. 

Picture:


As you can see the threads have been almost ground down. The threads at the bottom closest to the square taper are what they are supposed to look like, and above that is what the threads look like now. 

The nut also was stripped when I was removing it, so I ordered a replacement 14mm nut for this type of bottom bracket online since I can't find a nut like that in the fasteners section of my local hardware stores. But I can't easily do the same with the bottom bracket, so I need to fix it.

My first thought is to wrap some thread tape around the damaged threads. They still seem to be "sort of" there, just sanded down. 

My second thought is to use a ton of threadlocker red on both the nut and the bolt, then tighten it in. This should hopefully create a good enough seal and the new nut won't move. 

Third option is to buy a thread tap and try to carve new threads into the bolt. The only problem is I would need some sort of hollow thread tap that would fit over the bolt, and I don't think that even exists. I also don't know the appropriate thread pitch tap to buy in the first place. 

My fourth choice is replacing the bottom bracket altogether. That will require me to buy more tools to remove the bottom bracket like a shimano bottom bracket removal tool, and costs the most. So I really want to avoid this if possible. 

Finally, is it possible to remove the threaded portion of the bottom bracket entirely by unscrewing it and just install a new threaded rod? Can that even be done?

Any ideas?
The thread repair tool that is used in this sort of situation is called a die. The common size for crank nuts is M8x1.25.



However, too much material appears to be lost here for that approach to be reasonable. Crank nuts/bolts are one of the higher torque items on a bike, meaning you don't really have much leeway for severely damaged threads. Also, hardened steel threaded parts like this don't take as well to thread reformation as softer steel parts. Taps and dies have some capacity to shove material on damaged threads back into place, but here it's likely to break off instead.

You don't indicate whether you have a cartridge or cup and cone BB, but nutted BB spindles are almost always (maybe literally always?) found on cup and cone BBs. If so there's nothing stopping you from just replacing the spindle; basic cup and cone BBs generally let you do that freely since they tend to all use JIS spindles. You need the same length and, if applicable, offset. Pull the existing one and hopefully you'll find a code on it like the one in the picture. Match the new one with that. Shops don't tend to just have them anymore, since the common approach is now to just replace everything with cartridge, but they are still able to be ordered, and are cheap.