Originally Posted by
oldbobcat
73-68=5. You need 5 mm of spacer. If you put them on the non--drive side, they will bind on the non-machined surface of the spindle, but if you put them on the drive side, they might give you a chainline problem. Fool around.and find out.
That's an erroneous assumption.
The BB makes the width of the outer bearings +/- a few millimeters of what the crank requires. Then you put whatever spacers along with a wave washer to use up the additional play the crank has to deal with that +/- from the BB.
You can also put a spacer under the cups, and many BBs designed to work on either 68 or 73mm BB shells for 73mm compatible cranks will come with two 2.5mm spacers for use with 68mm bikes. But that isn't always the issue - especially if the play is much less than 5mm.
Basic plastic crank spacers are usually have either 24mm hole for Shimano or GXP type spindles or 30mm holes for BB30 style. Bike shops always have them. The wave washer is so you don't have to have the perfect spacer stack height do a fraction of a millimeter.