A threaded bottom bracket shell has internal threads that accept screw-in bottom bracket cups.
In many cases, measuring the shell width will narrow down the standard. If the shell measures 68 mm wide, you should also check the approximate diameter and thread specification, as both BSA and some T47 systems can use a 68 mm shell.
Common Threaded Frame Standards
BSA
BSA is one of the most established threaded bottom bracket standards.
Typical shell widths:
• Road: 68 mm
• Mountain bike: 73 mm
Thread specification:
• 1.37 inches x 24 threads per inch
The non-drive cup uses a conventional right-hand thread. The drive-side cup uses a left-hand thread, which means it tightens anticlockwise and loosens clockwise.
This reverse drive-side thread is important to remember when removing or installing a BSA bottom bracket.
ITA
The Italian threaded standard is generally found on road bikes and uses:
• 70 mm shell width
• 36 mm x 24 TPI threading
Unlike BSA, both cups use conventional right-hand threads. Italian threading is most commonly associated with Italian-made frames, particularly traditional road models.
T45
T45 is a less common threaded road standard with:
• 82.5 mm shell width
• M45 x 1.0 threading
It has appeared on selected Colnago models. The non-drive side uses a conventional right-hand thread, while the drive side uses a left-hand thread.
T47 External "Outboard"
T47 combines a large threaded shell with the installation security of a screw-in bottom bracket.
Typical shell widths:
• Road: 68 mm
• Mountain bike: 73 mm
Thread specification:
• M47 x 1.0
In the outboard configuration, the bearings sit outside the frame. This allows the system to use the familiar 68 or 73 mm shell widths while accommodating a larger threaded interface.
The non-drive cup uses a right-hand thread, while the drive-side cup uses a left-hand thread.
T47 Internal "Inboard"
The Original Trek version of T47 generally uses:
• 85.5 mm shell width
• M47 x 1.0 threading
The bearings sit inside the bottom bracket shell, which is why this configuration is described as inboard.
Although the thread specification is shared with other T47 systems, the shell width and bearing position mean that the correct T47 configuration must still be selected.
T47A
T47A is an asymmetric road standard with:
• 77 mm shell width
• M47 x 1.0 threading
As with BB30A and PF30A, the letter “A” indicates an asymmetric design. The cups use a right-hand thread on the non-drive side and a left-hand thread on the drive side.