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BSA vs Italian threading, and why Italian creaks
BSA and Italian shells look similar but thread differently, and the difference in thread direction is exactly why Italian has a reputation for coming loose.
The two thread specifications
BSA, also called English, is 1.37 inch by 24 tpi with a shell width of 68mm on road frames and 73mm on mountain frames. Italian is 36mm by 24 tpi with a 70mm shell. The pitch is the same 24 tpi, but the larger Italian diameter and different width mean the cups are not interchangeable.
Both are threaded interfaces installed with a splined cup tool, so at first glance servicing feels identical. The critical difference is which way each side threads.
Thread direction is the whole story
On a BSA shell the drive-side cup is a left-hand, or reverse, thread. Because the drive side sees the load of pedalling, a reverse thread means normal pedalling forces tend to tighten the cup into the shell. The non-drive side is a conventional right-hand thread.
Italian threads right-hand on both sides. That means on the drive side, the direction of pedalling can work to unscrew the cup rather than tighten it. Over many miles this is why Italian bottom brackets are more likely to loosen slightly, and a cup that is not held firmly against the shell face is a classic source of creaking.
Living with an Italian shell
Italian shells are perfectly usable and were common on classic Italian steel frames, but they reward good preparation. Facing the shell so the cups seat flat, using a quality thread preparation, and torquing to spec all help keep the drive-side cup put.
If a shell keeps loosening or creaking despite good fitting, that is the design at work rather than a fault, and it is one reason later standards moved toward reverse drive-side threads or a fully symmetric large-diameter interface such as T47.
Choosing between them on a new build
If you are commissioning a frame and have the choice, BSA is the safer bet for a quiet, low-fuss build thanks to that reverse drive-side thread. Italian belongs on frames designed around it or on restorations where the original standard matters. Always confirm which you have before buying cups, because the wrong diameter will not start in the shell.
| Standard | Thread | Shell width | Drive-side |
|---|---|---|---|
| BSA / English | 1.37in x 24 tpi | 68 or 73mm | Left-hand (self-tightening) |
| Italian | 36mm x 24 tpi | 70mm | Right-hand (can loosen) |
BSA and Italian threaded shells compared
Confirm your threaded standard before buying
Open the BB compatibility toolTell it your shell and crank and it names the exact bottom bracket.Find italian threaded bottom bracket cups on eBayaffiliateRelated
Sources: Park Tool bottom bracket reference ยท BikeGremlin