Dammit’s adventures in mountain biking

With Helicoil, or relying on the remaining thread alone?

What in earth did you use to cut the bolt down?

Last time I did it-
Thread a nut on to the length you need.
Thread another nut on leaving enough gap for a junior hacksaw.
Cut using the nuts as a guide,
Theread off the nut on the side you need, which will neaten up the end.
Deburr the thread using a triangular file following the thread.

Came out straight, didn't cross thread.