I have an oddball problem with the bb of my '90 Trek 420 frame. I bought the bare frame (it's REALLY bare, not even paint!) at the local co-op and it had a piece of an aluminum bb retainer or cup (couldn't tell which but it did say Shimano 600 on it) on the non-drive side that I had to chisel and saw out of the shell. Those threads are OK in that side but on the drive side the threads on the outer half of the shell (facing the crank) are simply worn away. I inserted a fixed cup in the shell and it just slides in until I get down the the last 1/8 inch from the shoulder of the cup and then it "catches" the good threads an just tightens up like normal.

My question is since the fixed cup is snug and tight like normal I'm still afraid to actually ride the bike like that. There doesn't seem to be enough threads to keep in place under load. I don't know if it will hold or not since I didn't install the crank yet. It might be just fine for all I know.

I was thinking of getting the threadless bb that Velo-Orange sells but that's kind of pricy. Should I go this route or just use a conventional bb?

if you are a belt and suspenders type of guy go for the velo orange right off (although i have absolutely zero first hand experience with them, just what i'd read) or chance an inexpensive shimano un-25 and if and when it breaks, THEN go orange.

Sunlite also offers a threadless bottom bracket. It's cheap ($20-30), but I think you might need to chamfer one or both edges of the shell for it to seat properly.

Another option is to re-tap the bottom bracket to Italian threading, which has a slightly larger diameter. After tapping to Italian threading, just install any ol' Italian threaded bottom bracket. The small catch to this is that Italian bottom bracket shells are a little wider than English -- 70 mm versus 68 mm -- so you may need to use a thin spacer between the drive side cup and shell.

THANK YOU SKYDOG! You might just have given me a better option for my old 80's vintage Canondale frame. The threads are completely gone other than some stubble. A regular BB slips in with NO thread bite at all. But this could be the cure it needs.

Smurfy, there's another thread right now about using Loctite Form-A-Thread. The use the other member wants to put it to is simply not going to work due to the torque issue.

In your case the lack of support from the worn outer portion will seriously strain the last 1/8 worth of good thread. So it too would tear out pretty soon. BUT! If you were to use the Form-A-Thread goop it would provide the support needed to let the last few threads do their job and stabilize the BB so those last good threads don't see a cyclical load that would tear 'em out at the roots.

The Loctite stuff is basically just epoxy. A GOOD epoxy but still just epoxy. You would get similar good results with something like JBWeld for an application such as this.

Someone else does a similar thing for around $30. I have the much lighter VO one in my parts bin...

Although maybe you could affect a thread repair, since you have some thread to locate a cup: you could spray one with something to work as a release film (WD40? That leaves a residue), smear epoxy into the worn threads, screw it up snug and hope it can come undone when it's cured...

The 2 or 3 metal threads would do most of the axial work holding the cup in the BB shell, while the epoxy filling the space out to the edge would prevent any radial movement. Could work... and better if it's a traditional loose BB; a cartridge one puts more load on the threads IMO. A loose one only has to cop the axial load from bearing preload, while a cartridge one puts axial load on that thread from the left cup tightening with way more torque than you preload bearings with.

Or if you like, you could have my used VO BB for $30 plus shipping. It's 110.5mm, 250g, and from the first batch with the oddball anodising with one side pink and the other blue. Naturally, I sanded the visible anodising off and painted it black. IIRC it's only done a couple thousand km, but the ally cups got slightly mashed a couple of times from using worn tools on it. I'll throw in a couple of allen head crank bolts, with no rust. If you're interested, PM me and I'll shoot a couple of pics.

I don't do square-taper anymore

Thanks for the info and offer, Kimmo!

Unfortunately 110 isn't near wide enough as I'll probably need at least 117 for the crank I'm using. I will use your advice and either use a conventional non-sealed bb or save my pennies and get the VO one. There's no hurry to get this bike done since I can't ride it anyway due to tendon surgery.