cottered crank swap for square taper
 

hey guys
so I'm tryng to do a bit of a restoration on an old Hercules commuter 3 speed and I'd like to swap the bottom bracket for a sealed new square taper one as opposed to the cottered one currently on it.
the one issue I'm having is that I will be replacing the rear wheel for another sturmy archer hubbed one and the crank set too but I'm unsure on how to get my chain line correct with a new bottom bracket.
I understand the shell size of the bracket is a 68mm but I don't know what spindle length to get in order to keep the correct chain line.
my first bet would have been to go the same length as the old spindle but I haven't got a clue as to how to measure a cottered shaft in relation to spindle length.
I don't have my new rear wheel yet either so I don't know the whereabouts of my rear sprocket in relation to chain line yet.
so I was wondering if there is a measurement I can take when I get a rear wheel or right now to find what bottom bracket would have the right spindle to save me buying a range of brackets with different lengths until I get it right :s

any help would be really appreciated!
regards Tom :)
http://i1186.photobucket.com/albums/...psa7cc162e.jpg
http://i1186.photobucket.com/albums/...ps34cd7ca1.jpg

SB listed a lot of options in this link. Unless you simply re-tapping the threads, it is not going to be cheap with special BB cartridges and crank sets.

Threading/interchangeability Issues for Older Raleigh Bicycles

I personally like cottered cranks. I never had any trouble with them and the American one piece crank. They are a littler heavy. With square taper cranks, I have seen stripped pedal threads, enlarged holes, clicks and creaks, etc.

Everything on a bicycle works together. Rarely can you change just one part without having to change something else to make it work.

Chain line basically means having your front chainring and rear cog on the same plane. Since you're changing your rear hub, that end might have changed. If that's the case, you'll want to change the front to match.

Of the factors that determine front chainline, the design of the crank arms is the most significant. They can vary by a LOT and the newer ones generally take shorter BB spindles. Generally when you buy a new crankset the manufacturer will recommend a spindle length to yield a common chainline. If I don't have that data, I just test fit any bottom bracket I have on hand that will fit the frame (I wasn't aware of the thread pitch issue, that's going to make it hard to do). Then I simply measure how far in or out I need to go.

Good luck!

Are you assuming that the BB width is 68mm because that's the ostensible English "standard," or have you actually measured it to be 68mm? Make no assumptions with British 3-speeds! My Raleigh "Superbe" and my daughter's "Sports" are 72mm wide, and require correspondingly longer BB spindles for cotterless conversion. Hercules became a Raleigh subsidiary over half a century ago, and your frame may be more like my Raleigh frames than it is like other English "standard" frames.

thats not a Raliegh BB cup. At least all the ones Ive seen dont have a hex fitting on them.
Might be old enough to be pre raliegh