Got a project bike a few weeks ago that needed to be greased throughout before use. It's a mid 80's Fuji Sand Blaster youth MTB. Appears to have a 68mm BB tube .

After cleaning/lubing the BB, the bike doesn't want to hold a BB adjustment, loosens quickly in use. After the second failure, I disassembled it, cleaned the threads with alcohol, applied blue Loctite and let dry overnight.....no luck as it went loose very quickly. threads in the frame, cup and lock ring seem good (albeit somewhat easy to turn). Try again with red Loctite? Just replace it with a cartridge, but won't those threads loosen, too?

Suggestions appreciated, thanks for your time!

I'd stick a cartridge BB in it. The natural precession will tend to tighten it as you ride.

Well an experiment as to whether the loss of adjustment is a slowly rotating cup is easy. Reassemble all with whatever methods you want. Then mark the BB shell and each cup with a paint/sharpie to indicate the cups' rotational position. Return after play develops and see if one of the cup's mark has moved off of that side of the shell's mark. But I also agree that a cartridge BB unit will likely solve many issues in one fell swoop. Andy

How are you tightening the lock ring? The only time when I have had this issue is when I have failed to tighten the lockring sufficiently. Usually this is user error but sometimes the lockring threads are damaged so you just can't tighten enough (but you try anyhow and hope it works)

You may not be getting it tight enough, Drive side,
and not getting the lock ring tight on the other..
lacking the proper tools? (I cannot see your work..)

Yea , go with a Cartridge BB.. UN 26 are cheap!,
you need to buy a different tool with those..






...

I wonder if the loosening might be the worn threads "jumping" to the next looser thread under pedaling stress, rather than slowly backing out. A new BB will presumably have its threads unworn so a tighter fit may ensue. Red Loctite is a one time fix; when the BB wears it will be difficult if not impossible to replace or adjust it.

Does it even have a lock ring?

I think we have a couple winners in the contest!!

Marked the cup, lock ring and shell per Andrew's suggestion and rode the cranks off of the bike. All the marks are still aligned, so dsbrandtjr's theory must be correct as the BB is too loose to continue riding. Must be the threads on the cup. I'm hoping the UN-55 with both ends thread supported will solve the issue. Banging my head against a wall instead of spending $25, duh!

Thanks everyone!

Possibly someone used an French or Italian lock ring on a British cup?
If you have a "spare" lock ring from another bike, maybe try it as a test?
Maybe a few wraps of teflon tape for a "shim"?

Glad I could help the OP find the path. I do wonder at the opinion that the cup's threads might be worn. It's been my experience that the shell is made of softer steel then cups typically are. So it's my concern that the shell's threads are worn. Andy

Having had to saw through BB cups frozen to the shell, I can attest to the fact that they are incredibly hard. Certainly check the shell threads. If worn, there are threadless BBs that can be used.