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T47 vs BSA: why frames moved to the wider thread
T47 is the answer to a specific problem: riders wanted the quiet, serviceable feel of a threaded bottom bracket but frame makers wanted a shell wide enough for a 30 mm spindle. T47 threads a pressfit-width shell.
BSA is narrow but proven
A BSA shell threads at 1.37 inch, about 34.8 mm, by 24 tpi. That bore is comfortable for a 24 mm spindle running in external cups, which is why Shimano Hollowtech II and SRAM GXP live happily on BSA. It is not wide enough to house a 30 mm spindle bearing inside the shell, so 30 mm cranks on BSA rely on external cups that carry smaller bearings.
BSA's strength is service: the cups screw in and out, and most creaks are cured by re-greasing the threads rather than pressing anything.
T47 widens the thread to 47 mm
T47 keeps threads but opens the thread diameter to 47 mm at a 1 mm pitch. That extra width means a 30 mm spindle bearing can sit inside the threaded shell, giving a threaded frame the stiffness advantages of a pressfit-width shell without the pressfit creak. There are internal-bearing and external-bearing versions of T47, so the exact bottom bracket depends on which the frame expects.
In practice T47 gives a frame designer the wide, stiff bottom bracket area of BB386EVO or PF30 while giving the owner threads that screw in and out.
Which should you want
If your frame is already BSA, there is no upgrade to make: BSA is reliable and every mainstream crank has a BSA option. T47 matters when you are choosing a frame or when a frame was designed for a 30 mm spindle and the maker wanted threads instead of a pressfit shell. Both are serviceable, and both avoid the pressed-into-bare-carbon interface that causes most creak complaints.
Confirm the frame's actual standard and whether it is internal or external T47 before buying, because a T47 bottom bracket must match that.
| Standard | Thread diameter | Pitch | Houses 30 mm bearing | Serviceable |
|---|---|---|---|---|
| BSA / English | 1.37 in (34.8 mm) | 24 tpi | no (external cups) | yes, screw-in |
| T47 | 47 mm | 1 mm | yes, inside shell | yes, screw-in |
Figures from Park Tool and Wheels Manufacturing references.
Fitting a bottom bracket to a T47 or BSA frame?
Open the BB compatibility toolPick your shell and crank spindle to see the exact bottom bracket you need.Find T47 bottom bracket on eBayaffiliateRelated
Sources: Park Tool: Bottom Bracket Standards ยท Wheels Manufacturing bottom bracket fitment reference