Getting the right Bottom Bracket I’ve recently got my hands on an old steel frame which I’ve decided to turn into a bit of a build project. I’ll be honest though, whilst I’ve ridden for many years, beyond basic maintenance my knowledge of bike mechanics is almost zero :( 

The main issue i that I have been unable to identify the make of the Frame. It’s had several re-paints and when I finally got it back to bare steel the only ID number (On the down tube) doesn’t seem to adhere to any of the manufacturers standards. This makes getting the correct parts a little difficult

My first task is to source and fit a bottom bracket. The shell itself is 34 MM Diameter by 68MM wide and is threaded so will, in theory, take a standard threaded BB i guess. But how do I know which thread do go for? Is there an easy way to distinguish between British and Standard thread?

Next up is, what’s the optimum spindle length? I’m planning to make the bike Single Speed and have replaced the Rear drops with Track Fork ends (I adjusted the spacing to 120MM between the frame to take a standard SS or maybe a flip flop hub) but haven't fitted anything yet so not sure how far this will offset from the frame.

Any (low tech) advice people are willing to offer would be gratefully received 

Cheers
If you have an idea of what model of frame it is, it would help. Otherwise, your best bet is to take it to a bike shop and try an ISO bottom bracket (gently) to see if it fits, or measure the threads and their orientation to match to this chart. 

There are also universal threadless BB's like this one from Velo-Orange which skirt the issue by expanding to fit. In this case, the thread condition doesn't matter and you can reuse the threads for the proper bottom bracket provided they're in good enough condition. These are a good bit more expensive than your typical Shimano ISO bottom bracket, though. 

The spindle length needs to be selected in conjunction with the crankset to achieve the appropriate chainline. I'd suggest going to a shop to pick up a proper bb and crankset at once.