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, SRAM DUB, Shimano Hollowtech II or any similar design, read on to find out how many spacers you should run.
[Updated 13th January 2020 with SRAM DUB]
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, but it also depends on which brand you're using.
For 68mm and 83mm Shimano HTII bottom brackets
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 Shimano HTII bottom brackets
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.
For SRAM GXP 73mm bottom brackets
You don't need to install any spacers onto the bottom bracket if you have a 73mm shell.
For SRAM GXP 68mm bottom brackets
You'll need to use both the provided 2.5mm spacers for a 68mm shell, placing one on either side of the bottom bracket.
For SRAM DUB 73mm bottom brackets
Much like GXP, you don't need to add any spacers to the bottom bracket if you have a 73mm shell, but you will need to add a 4.5mm spacer to the right-hand side of the crank axle, unless you're using a chain guide, in which case a 2mm spacer is required instead, along with a 2.5mm spacer on the right-hand side of the bottom bracket.
If your bike has Super Boost hub spacing (157x12mm) and a 73mm shell then you'll need a 4.5mm spacer on the left-hand side of the crank axle, a 9mm spacer on the right-hand side of the crank axle and no spacers on the bottom bracket.
If you're running a chain device with your Super Boost setup then you'll need a 2.5mm spacer on the right-hand side of the bottom bracket and a 4.5mm spacer on left-hand side of the crank axle and a 6.5mm spacer on the right.
For SRAM DUB 68mm bottom brackets
Once again, DUB 68mm is just like GXP, with a 2.5mm spacer needed on either side of the bottom bracket shell. You'll also need a 4.5mm spacer on the right-hand side of the crank axle.
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