+1. The shell width is determined by your frame. It affects nothing else, and there are no substitutes. If you have a 68mm shell, english threaded, you will NEED a 68mm BB with english threading. Simple as that.

Spindle length determines how wide the spindle is, from tip to tip. The cranks press onto the spindle, and the BB comes with the bolts/nuts that hold the cranks on, so these threadings aren't something you have to worry about. What you do have to worry about is the TAPER of the cranks and the CHAINLINE that the cranks will give you.

TAPER is determined by the Crank/BB manufacturer. Shimanos, Suginos, and Stronglights use a JIS taper, Campy and Miche use ISO taper. Phil Wood makes BB's for both tapers.

CHAINLINE is determined by your rear hub. Most track hubs have between a 42 and 44mm chainline. You will want a BB that pushes your crank out far enough (or pulls your crank in far enough) to get a similar, if not the exact same, chainline.

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