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Bottom bracket thread direction: which way to turn
More bottom brackets are damaged by turning them the wrong way than by wear, because the drive-side cup on the most common standard is reverse threaded. Here is the rule for each threaded shell.
BSA / English: drive side is reverse threaded
On a BSA, also called English, threaded shell, the non-drive (left) cup has a normal right-hand thread: it tightens clockwise and loosens anticlockwise. The drive-side (right) cup is reverse threaded, so it tightens anticlockwise and loosens clockwise. This reverse thread exists so that the forward pedalling load tends to tighten the cup rather than unwind it.
The practical rule at the drive side is the opposite of everything else on the bike: to remove the drive-side cup, turn it as if you were tightening a normal bolt.
Italian: right-hand both sides
An Italian threaded shell, 36 mm by 24 tpi, is right-hand threaded on both sides. Both cups tighten clockwise and loosen anticlockwise. This is simpler to remember but is the reason Italian bottom brackets have a reputation for working loose under load, because the drive-side cup is not self-tightening the way a BSA drive cup is.
Because the two standards look similar at a glance but thread differently, confirm whether a shell is BSA or Italian before applying force to the drive-side cup.
T47: right-hand both sides, wider thread
T47 uses a 47 mm thread at 1 mm pitch and, like Italian, is right-hand threaded on both sides in its common form. It was created to give a threaded shell wide enough to house 30 mm spindle bearings, combining the serviceability of threads with the stiffness of a pressfit-width shell.
Always identify the standard first, then apply the direction rule for that standard, and use a proper cup tool so the load stays square to the threads.
| Standard | Non-drive (left) | Drive (right) |
|---|---|---|
| BSA / English | right-hand (loosen anticlockwise) | left-hand (loosen clockwise) |
| Italian | right-hand | right-hand |
| T47 | right-hand | right-hand (common form) |
Thread directions from Park Tool and Sheldon Brown references.
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Sources: Park Tool: Bottom Bracket Standards ยท Sheldon Brown bicycle glossary (bottom bracket, cranks)