Cup and cone BB help


Fat Chance Fan
Guys - am in dire need of technical assistance here. Probably a no brainer to a lot of you but I am completely flummoxed and I need help.

Long story but am doing a replica build (kind of) for my bro, as a surprise at Christmas. He’s more of an enduro rider and he is not into retro. Yet. Anyway, this is a special effort by me because my bro shouldered a huge amount of burden over the last year, dealing with my terminally ill Dad, whilst I was comfortably away from the horror of it all in London.

The fat man brought my bro an inferno red splatter GT Outpost bitd.. but I thought he deserved an upgrade this Christmas to an inferno Karakoram, which is visually the same, just better, as we know. I have already nearly eaten a bullet because the Kara frame (which I have had in storage of years) transpired to be a 16” rather than an 18”, which is what I thought I had bought a decade ago.. but I think I can just about get away with it. Here she is to date:


Now I know what everyone is going to say, so let’s address it upfront. Yes, that is red & black cork tape on the bar ends. What can I say? It’s a replica build and that’s how his 13 yr old self had the Outpost. Plus it looks all matchy matchy, which is nice.

My issue is this. I have never built with anything as old as ‘91 / M550 / 650. My sweet spot is ‘93 onwards. Naturally I therefore picked up a new sealed shimano bb, as below, only to discover the damn thing won’t screw in. Evidently the Kara’s bb thread pitch is some arcane, dinosaur era format.


This dismayed me, but I thought I had a solution. By happy chance I have a couple of precious NOS M550 chainsets.. complete with NOS BBs. I am fitting a mint m550 c/s to the Kara, so I figured I’d use one of these NOS BBs. However, these things are cup and cone and try as I might, I cannot figure them out. When I install the non-drive cup, it needs to be screwed so far into the frame, to remove play, that the lock ring has no thread left. I thought I had got round this by using masses of thread locker.. but now I have an axle issue. Basically the non-drive chainstay crank clearance is about 3mm, whilst the drive side clearance is about 13mm!

Something ain’t right - but I have no clue what I am doing wrong. I know the axle length is correct. Can someone please take pity on me and explain what stupid thing I’m doing before I sack off the whole project and burn the damn thing?



Fat Chance Fan
I thought they were 68mm until ‘91, then 73mm thereafter. Naturally I could have measured the shell but didn’t bother. Will a modern un300 in 73mm work or is the thread pitch wrong?