Other terms for this press fit standard include BB86, BB89.5, BB92, BB107, BB121, BB132, PF24 and Shimano® Press-Fit. Shells of the PF41 standard have a smooth bore with a nominal 41mm inside diameter. Because the frame shell bore diameter is the only consistent and most important part of the press fit system, the bore size is the preferred system and will be used here. Some standards were named for the spindle diameter, others were named for the shell width, and still other terms came from the size of the bore where the press fit occurs. The terminology of press fit bottom brackets has become especially confusing. The bottom bracket is held in by the pressure created by the interference. This creates an interference fit, also known as a press fit.
The bearing or bearing cup will be slightly larger than the bore of the frame and is pressed inside the shell. Press fit bottom bracket frame shells use a simple bore with no internal threading. Other less common threading standards such as ISIS Overdrive, Italian, French, Swiss and others are discussed later in this article. The term ISO is refers to the International Standard Organization, an international standards organization, which also adopted this same standard to be the ISO standard. The term BSC stands for British Standard Cycle, which adopted the same standard. The term BSA stands for Birmingham Small Arms, a British company that produced bicycles, cars and other durable goods. These are: BSA, BSC, BC, ISO, British, and Euro, the name used in BMX bike circles. This standard also goes by other names, but they all mean the same thing. Directional arrows on English threaded cups indicating direction of tightening
The shell inside diameter will be approximately 33.7mm. The drive side (right side of the bike) uses a left hand thread, tightening counterclockwise.
The non-drive side (left side) on the English standard is a right hand threading, tightening clockwise. The most commonly used term for this standard is “ English” threading, sometimes abbreviated as ENG. The common threaded bottom bracket uses cups or adapters with the thread specification of 1.37″ x 24 threads per inch (approximately 34.8mm diameter). Threaded bottom brackets shells have been made over time in various thread standards that are not interchangeable. For a review of tool selection for bottom bracket service, see Bottom Bracket Tool Selection: Threaded and Thread-Thru. The frame shell may be threaded directly at the shell, or there may be an insert installed that has internal threads. Threaded bottom brackets use a shell with internal threads to hold threaded parts.
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