The BSA and Italian thread bottom bracket shells are predominantly something you find on bikes made before 2010. These two types of bottom brackets are the so-called threaded bottom brackets, which feature a thread on both sides of the shell.
A frame with a BSA fit is incredibly commonplace. There are 4 widths available with BSA bottom brackets; 68mm, 73mm, 83mm or 100mm.
For road bikes, a BSA bottom bracket is always 68 mm wide. Other sizes, and particularly the 73 mm width, is commonplace with mountain bikes. The 83 mm and 100 mm dimensions have become rare these days.
A BSA bottom bracket can also be recognised by the writing on the bracket itself. The bearings denote: 1.37 * 24. This means that the diameter of the thread is 1.37 inch and that it turns 24 times per inch. The diameter of a BSA bracket is between 33.6 and 33.9 mm.
The thread on the left side (non-drive side) is a regular thread. You turn it clockwise to lock it. The right part (drive side) is a reverse thread and has to be done up counter clockwise. A BSA equipped frame can be fitted with almost all types of cranks.
