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How to measure your bottom bracket shell

Two caliper readings, internal diameter and shell width, are enough to identify almost any bottom bracket standard.

What you need

A vernier or digital caliper with jaws long enough to reach across the shell, and a way to remove the fitted bottom bracket so you are measuring bare frame. Measuring over installed cups gives false numbers, so take the old bottom bracket out first.

Clean the shell so paint chips or old grease do not skew the reading, and have the manufacturer geometry chart handy to cross-check what you find.

Measure the internal diameter

Set the caliper inside the shell and open the jaws until they touch the bore. For a threaded shell read across the thread crests: 1.37 inch means BSA, 36mm means Italian, 47mm means T47. For a pressfit shell read the smooth bore: 41mm points to BB86 or BB92, 46mm to PF30, 42mm to BB30.

Take the reading in two positions ninety degrees apart to catch an out-of-round shell, which is more common on pressfit carbon frames.

Measure the shell width

Measure face to face across the frame, not over any seals or cups. Common widths are 68mm and 73mm for BSA and PF30, 70mm for Italian, 86.5mm for BB86 and 91.5mm for BB92. Width is what separates a road shell from a mountain shell that share the same diameter.

If the number sits oddly between two standards, suspect a previously installed adapter or an unfaced shell and measure again on bare metal.

Turn the numbers into a standard

Pair the diameter and width and the standard usually falls out: 1.37in by 68mm is BSA road, 36mm by 70mm is Italian, 41mm by 86.5mm is BB86, 46mm by 68mm is PF30. Then check whether the bore is threaded or smooth to confirm the family before you buy any cups.

When in doubt, note both readings plus threaded-or-pressfit and match them against a reference chart rather than guessing from the fitted part.

Related

Sources: Park Tool bottom bracket reference · Wheels Manufacturing