Have a british frame with 24tpi BB threads. The bottom bearing casing is 66mm across flats, may have been machined down from 68mm from what I have read online. Existing cottered loose bearing is 135mm end to end. Rear dropouts are 100mm.
Looks like it will be difficult to get a non-cottered 68mm x 135mm bearing, so I assume I will have to go to something like 68mm x 127mm which is readily available. But if I do that how do I spec the crank so that the chain line is correct??

I would pick the crank first. There are a lot of choices with BBs (and you could possibly use the same BB for multiple cranks). Have you seen the Sugino RD2? That one looks really nice:

Sugino RD2 Single Speed Crankset > Components > Drivetrain > Cranksets | Jenson USA

Dave

I think something is wrong. The rear dropouts of a British 3-speed frame should be more like 114mm (4.5"), not 100 -- is this a junky old frame that you're trying to fix up?

Hi. Sorry the rear dropouts measure exactly 107mm so I guess they will go to 100mm or 110mm.

20170716_123026 (800x600).jpg

20170716_123036 (600x800) (2).jpg

Your frame has nothing to do with determining what length bottom bracket spindle you need. Spindle length is dictated by cranks choice.

Select whatever single speed or track crank set you like. The manufacturer of that crank will indicate in their literature what length spindle to use in order to achieve a ~42mm chain line.

SquidPuppet is generally correct about matching the bottom bracket spindle length with what the maker of that crankset's specs for it but most, if not all, of us have heard "horror stories", especially on unusual conversions, about guys buying a BB with the "right" length spindle for a particular crank and for whatever reason...the assembly doesn't work right with their bike.

It may not be part of choosing a BB for your bike but I find your weird rear spacing interesting. My recollection is that 110mm was for for older track, coaster brake and other single-speed hubs, while 114 was 3-4-speed and some IG hubs...

Interesting. If it were my frame, the first order of business would be to have the frame respaced to 120mm, and the dropouts aligned. Modern single-speed/fixed hubs are generally 120mm.

Yeah respacing a rear triangle isn't exactly rocket surgery.

Thanks to all.
The cold setting of the dropouts to 120mm sounds likes a simple solution.
I will put a threaded rod and nuts on it today and see how easy it goes.

You can't do it that way. There is no guarantee that both stays will move an equal amount. You'll end up with a misaligned rear triangle.

Read this before you start.

https://www.sheldonbrown.com/frame-spacing.html

Thanks SquidPuppet.
I'll use the Sheldon String Method for checking as I go.

I have never seen it mentioned anywhere, but would it be a good idea to clamp across the rear brake bridge when resetting the rear dropout width, to prevent the brazing of the brake bridge to give way??

Yes, it's a very good idea and it's done all the time. Some people put rounded blocks of wood on the outside to prevent the clamps from denting the frame tubes. Try to position things as uniformly as possible so that the left and right seat stays "Bend" in the same locations.

I highly recommend Sheldon's wood 2x4 method. I've done it a couple times and it's surprisingly easy. In fact, be careful as you go. I had to go backwards a few mm after being too aggressive on my first frame.

When you are done your drop outs might be misaligned a little. That's easy to fix with a couple long bolts, a few nuts, washers, and a little brute force to tweak them toward one another.

Let us know how it goes.