Oldguyonoldbike

I have an early 80s frameset with a nice Columbus SL fork that has French threads. A previous owner had tried to install an English threaded headset and damaged the threads on the fork. I now have a French threaded Velo Orange headset installed and it fits fairly snugly, but the lock nut tends to work its way loose over the course of several hours - not necessarily over the course of a single ride, but after, say, eight hours of riding. I could simply tighten it before every ride, but frankly I don't always remember.
So, the question is, would it be okay to apply some locktite or similar to the fork threads? If so, any recommendations on type or grade? What are the down sides that I might not be thinking of?
If possible I would like to keep the fork, because it is light and pretty and the bike rides like a dream.

JohnDThompson 

Try wrapping some teflon pipe tape on the steer tube threads. If you're lucky, that will be enough. Locktite might also work, but it will make servicing or replacing the headset a PITA. I've fixed damaged steer tube threads by melting brass into the damaged threads and then cutting new threads into the brass. The threaded section of the steer tube isn't painted, so you don't need to worry about paint damage. The one caveat is the French thread. You need to find someone with a 25mm x 1mm die, and preferably a piloted die stock. Take care with the VO headset; the aluminum threaded parts can easily be damaged if ridden loose; the steel steer tube will eat right through the aluminum in no time.

dsbrantjr

Before you try something like Loctite®, which might make future servicing problematic, try less-aggressive Vibra-Tite®: https://www.vibra-tite.com/threadloc...c-3-threadmate
I specify it for use in truck-mounted military SATCOM products where severe vibration is encountered, but fasteners need occasional removal/replacement.

Oldguyonoldbike

JohnDThompson Thanks. I have some Teflon tape, so will try that first.
SurferRosa Yes the adjustable cup has wrench flats. I've hesitated torqueing down too much, but will give it a shot. If that and the Teflon don't work, I'll give the blue Loctite a shot.

easyupbug 

If all the excellent suggestions above don't work Blue Loctite softens at 250º F so there goes the headset plastic seals, grease, etc. Super glue can be dissolved with acetone but again issues.

63rickert

JDT’s suggestion above to fill the threads with brass is the best long term solution.

If this is an aluminum locknut the threads are very likely already shot. Try turning other locknuts on the steerer and note how they feel. Even steel locknuts may not all have the exact same threadform. If there is a locknut that plain feels better than what you have been using, that’s your new nut.

Teflon and loctite are good, if the aluminum threads are too far gone that is not enough.

Moe Zhoost

How well does the stack height of your VO headset match the fork length? Perhaps you have a situation where the lock nut is not engaging a sufficient number of threads.

HillRider

If the OP goes the brass filling and rethreadeing route could he have the damaged section English threaded? That would eliminate the need for an almost unobtanium French threading die and make future headset choices a lot easier.

Oldguyonoldbike

There’s plenty of threading for the locknut to engage with. I’m pretty sure the problem is damaged threads.

reconnaissance

I have a similar issue where the threads are worn just where the locknut needs purchase on the threads and thought of doing this too. Do you recall which brazing rod you used? I’ll probably use MAPP gas to provide the heat.