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The bottom bracket (BB) is the interface between your frame and the cranks. But I guess you know this already. :)
As such, it needs to suit both.
The "68" refers to the width (left to right) of the frame's bottom bracket shell (where the BB fits) in millimetres : a 68mm shell is English/British/BSc standard and one of the most common there is.
The "103" refers to the width (left to right) of the spindle in millimetres: different cranks need different width spindles to achieve the same chainline: the distance from the centre of the rear hub to the centre of the rear sprocket. With a single-speed/fixed wheel drivetrain, the front and rear chainlines need to match as close as possible and is usually 42mm.
Typically, a road double has a 41mm and 46mm chainline (inner and outer rings respectively) and a triple, 40/45/50mm. You could get away with the 1mm of the inner ring of a road double, but any of the others will require you adjust the chainline via the BB.
Particularly with a triple and possibly with a double, and depending on your frame, if you bring the crank in too far the crank arm or spider will strike the chainstay.
TL/DR
A 68mm shell gives you lots of options, but a "track"/single ring crank will give you the least grief, especially if it's intended for a 103mm BB, as you'll be able to use the BB you already have.
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Scilly.Suffolk
Hi,
I have a 2007 Langster which has a Sugino Messenger crankset - well, half of one anyway as it's missing the non-drive side crank arm!
Since not being able to find a replacement arm (170mm, black) I'm considering replacing the whole lot!
So... from the original specs I believe the bike has a 'sport cartridge square taper bottom bracket' which is 68 x 103mm.
What are my options with regards to replacing this? Can I use a triple (or double) and convert to a single? Can I use an alternative bottom-bracket??
Thank you,
Mike