Adjusting headset on a damaged steerer tube.
A friend asked me to look at her bike because the headset wouldn't stay adjusted. Upon disassembly I found that something put a big cut and mangled some of the threads near the top of the 1" threaded steerer tube. Every time I try to tighten the locknut it only gets so tight and then gets loose again. The headset looks healthy and currently consists of the threaded race, 2 spacers and the locknut.
The bike is a fairly nice Bianchi with Columbus tubing, so I'd prefer to save the fork. What are my options to address this issue? How important is the locknut? Would adding a keyed spacer and lockring between the race and locknut keep things tight and safe? Should I resort to Loctite? Cutoff the top of the steerer tube and remove the spacers? Other ideas?