Cup and cone BB help

BlackCat

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:

3E561480-6DAE-41C1-A718-33ADB5A8BED6.jpeg

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.

AC0554A8-8E56-4454-B36C-7CA2F8FBE6CF.jpeg

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?

Thanks
 

BlackCat

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?
 
Top