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How to identify your bottom bracket standard

The bottom bracket standard is decided by the frame shell, not the crank, so identifying it starts with what the frame will accept.

Start at the frame, not the crank

Cranks and bottom brackets get swapped over a bike's life, so the part currently fitted is not proof of the standard. The frame shell is the fixed reference. Two numbers describe almost every shell ever made: the internal diameter the bearings sit in, and the width of the shell measured across the frame. Get those two right and the standard usually names itself.

First decide whether the shell is threaded or pressfit. Threaded shells have machined threads cut into the metal and the cups screw in. Pressfit shells are smooth bores, sometimes with a plastic or alloy sleeve, and the bearings or cups are pushed in under force. If you can see thread crests inside the shell, it is threaded.

Measure the internal diameter

For a threaded shell, the common diameters are 1.37 inch by 24 tpi (BSA, also called English), 36mm by 24 tpi (Italian), and 47mm by 1mm pitch (T47). Measure across the crests of the thread with a caliper.

For a pressfit shell, the bore diameter is the tell. A 41mm bore with a smooth shell is BB86 (road) or BB92 (MTB). A 46mm bore is PF30. A 42mm bore where bearings press straight into the frame with no cups is BB30. Trek BB90 and BB95, Specialized OSBB, BBRight and BB386EVO are all pressfit families with their own bore and width combinations.

Measure the shell width

Width separates variants that share a diameter. A BSA road shell is 68mm wide; a BSA MTB shell is 73mm. Italian is 70mm. PF30 comes in 68mm road and 73mm MTB. BB86 is 86.5mm and BB92 is 91.5mm, which is where those numbers come from.

Measure the true shell face to face, not over any installed cups or seals, and take the reading in two or three places to catch a shell that has been faced unevenly.

Read any markings

Many frames have the standard etched or printed near the shell or listed in the manufacturer geometry chart. If the bike is complete, the fitted bottom bracket cups are usually laser-marked with a code you can cross-reference. Treat markings as a strong hint but still confirm with the caliper, because a previous owner may have fitted an adapter.

Related

Sources: Park Tool bottom bracket reference · Wheels Manufacturing