The 20-tooth splined cartridge bottom bracket pre-dates the modern external-bearing convention by a decade. Square-taper, Shimano Octalink, ISIS Drive, and most other internal-cartridge BBs from the late-1990s through the mid-2000s all use the same 20-tooth splined interface for removal. The 1671.1/4 is the dedicated socket for that interface, sized to drive cleanly with a 1/2″ ratchet or an adjustable wrench. For any bike still running an internal-cartridge BB rather than an outboard-bearing BB, this is the socket the work calls for. How it's used Insert the 1671.1/4 into the BB's splined recess until all 20 splines mesh fully. Press home; partial seating skips and rounds the cup splines on the first hard turn. Drive with a 1/2″ ratchet, an adjustable wrench against the body, or a torque wrench when the manufacturer's spec calls for it. The socket has wrench flats on its body for an adjustable-wrench drive. For benchtop use where a torque-controlled install matters, the Cartridge Bottom Bracket Wrench (1671.8/2BI-US) combines the same 20-tooth socket with an integrated 350 mm handle and a spring-loaded retaining screw that prevents the socket from cam-out during high-torque pulls. Drive side on most cartridge BBs is right-hand thread (loosens counter-clockwise). A few older Italian-thread frames are reverse threaded; check the BB markings before applying force. Compatibility Square-taper cartridge BBs (Shimano UN-series, FSA Square-Taper, Race Face Square-Taper, generic JIS / ISO) Shimano Octalink V1 and V2 cartridge BBs ISIS Drive cartridge BBs (Truvativ Howitzer, Race Face X-Type ISIS, FSA ISIS-drive) Most other 20-tooth splined cartridge BBs from the 1995–2007 era Specs Material: premium flex plus carbon steel, hardened Surface finish: chrome-plated to ISO 1456:2009 Drive interface: 1/2″ square drive on the back face; wrench flats on the body for adjustable-wrench drive Spline count: 20-tooth (industry standard for cartridge BBs) Built in Zreče Unior has been forging hand tools in Zreče since 1919, and sponsors professional road and mountain-bike race teams. The 1671.1/4 is hardened tool steel because the splines have to hold their shape across the full life of an older bike's BB-service history; the spline geometry on a worn cartridge can already be marginal, and a tool that adds wear to the cup is the wrong tool for the call. Pro tip from our mechanics The most common cartridge-BB-removal failure is cam-out under load. The socket lifts a few millimetres out of the splined recess; the splines disengage; the next half-turn rounds both the cup and the tool. Either hold the socket pressed firmly into the cup with one hand while turning with the other, or use the Cartridge Bottom Bracket Wrench (1671.8/2BI-US), which has a spring-loaded retaining screw built into the body to prevent exactly this failure mode. For the cartridge-era BB removal procedure plus the cleanup-and-regrease step that prevents the next install from creaking: Replace or clean your bottom bracket →.
