When installing a new or replacement threaded external bottom bracket onto your bike, it's important to get the right number of spacers on either side. Whether you're running a SRAM GXP, Shimano Hollowtech II or any similar design, read on to find out how many spacers you should run.
The first thing you'll need to do is to measure your bottom bracket shell of your frame. As in the picture below, the best way to do this is with a set of vernier callipers, but you can also use a steel ruler or tape measure. Obviously, the old bottom bracket will need to be removed to do this and you should measure from the edge to edge, so if you're not using callipers, stick your ruler inside the shell so that any chain guide mounts or similar don't skew the measurement.
You should now know how wide your bottom bracket shell is. For a threaded bottom bracket, there are only three common sizes; 68mm, 73mm and 83mm. If your bottom bracket shell measurement isn't very close to one of those, you've probably measured it incorrectly - or something very odd is going on with your frame.
The shell width determines how many spacers you need on each side.
For 68mm and 83mm bottom bracket shells
A 68mm or 83mm bottom bracket shell needs two of the 2.5mm spacers on the drive (chainring) side and one 2.5mm spacer on the non-drive side.
For 73mm bottom bracket shells
If your bottom bracket shell is 73mm wide, you only need one 2.5mm spacer fitted to the drive side.
If you need to use a bottom bracket mounted chain guide or derailleur, then you'll also need to remove the innermost 2.5mm spacer on the drive side - so a 68mm or 83mm shell will have one drive side spacer and one non-drive side spacer, while a 73mm shell will need no spacers at all.
You might also like: