The humble bottom bracket BB gets subjected to some of the most intense and punishing conditions of any system on the bike. High loads are continuously channelled through rotational motion onto the bearings, which take the full brunt of both bodyweight and muscle power. Servicing will rarely involve actually repairing the BB itself. Diagnosis involves listening for rough sounding noises, and feeling for play or lack of smoothness when spinning the cranks. Remove the chain, spin, feel and listen. Quality aftermarket replacements such those by SKF or Crank Brothers can be worthy alternatives, but generic econo-brand BBs are a bit hit and miss.
Octalink or square taper bottom bracket/crank?
Bottom brackets best to worst? — BikeRadar
This system gradually spread to the lower Ultegra and component sets, but has now been replaced by the new Hollowtech II system introduced first on the speed Dura-Ace in However, there are still many older bikes around fitted with the original Octalink splined system. At the Interbike show, Truvativ, Chris King, and Race Face sat down and began collaborating on a spline interface for bottom brackets and cranks. The standard consists of two parts: one governing the interface between the crank and bottom bracket spindle, the other describing a set of standard spindle lengths. The spindle length section defines the relationship between the bottom bracket and the frame centre line for different standard spindle lengths, thus ensuring the correct position of the crankset relative to the frame centreline.
Compatibility  Bottom brackets
This post will explain bottom bracket compatibility. Which bottom brackets BBs can be combined with which crankset standards. Posts that explain in detail the history and construction of each BB standard can be found here: 1 History and threaded BB standards 2 Threadless press fit bb standards T47 — a new threaded BB standard.
Once upon a time, all bikes were made of steel. And the bottom bracket axle was a solid steel rod. There were cups in the bearings into which loose ball bearings fitted, with the axles having a curved surface which pushed against the ball bearings. A threaded bottom bracket shell used to be the norm when bikes were made of steel.