Shimano Octalink cartridge bottom brackets with outside notches, along with Bontrager and Truvativ ISIS Drive types, use an 8-notch splined recess rather than the 20-tooth pattern that became the cartridge-era standard. The two interfaces are visually similar but mechanically different; a 20-tooth socket skates across an 8-tooth cup, and the wider tooth pitch on the 8-tooth pattern needs the dedicated tool for clean engagement. If a customer's bike has an 8-notch cartridge BB and a 20-tooth socket isn't gripping, this is the tool the work calls for. Square-taper, Shimano Octalink, ISIS, and other internal-cartridge BBs from the same era that happen to use the 8-tooth pattern all take the same 1671.2/4. The same 8-notch pattern also secures the front chainring lockring on Bosch Generation 3 and 4 e-bike drive units, so this doubles as the lockring tool for those motors. How it's used Insert the 1671.2/4 into the BB's splined recess until all 8 splines mesh fully with the cup's pattern. Press home; partial seating skips on the first turn and rounds the cup. Drive with a 1/2″ ratchet, an adjustable wrench against the body, or a torque wrench when the manufacturer's install spec calls for it. On English-threaded frames, which covers most bikes, the drive-side cup is left-hand thread; it loosens clockwise. Italian-thread frames from the same era are the exception and reverse this; check the BB cup's marking before applying force. Compatibility Shimano Octalink cartridge bottom brackets with outside notches (the 8-notch interface) Bontrager and Truvativ ISIS Drive type bottom brackets, plus other internal cartridges of the era with the same 8-notch pattern; also the Bosch Generation 3 and 4 e-bike front chainring lockring Not compatible with the more-common 20-tooth cartridge BBs; for those, use the 20 Tooth Cartridge Bottom Bracket Tool (1671.1/4). Specs Material: premium flex plus carbon steel Weight: 211 g Drive interface: 1/2″ square drive on the back face; wrench flats on the body for adjustable-wrench drive Spline count: 8-notch (Shimano Octalink outside-notch cartridge interface; Bosch Gen 3/4 lockring) Built in Zreče Unior has been forging hand tools in Zreče since 1919, and sponsors professional road and mountain-bike race teams. The 1671.2/4 is made from Unior's premium flex plus carbon steel because the 8-notch BB interface lives mostly on bikes that are decades old by now; the splines on the cup are often already marginal, and a tool that's softer than the cup adds wear instead of releasing the BB cleanly. Pro tip from our mechanics A cartridge BB that has been seized in place for two decades won't yield to force alone. Two recoveries: penetrating fluid at the thread interface for fifteen minutes before applying force, or, if the cup is genuinely stuck, gentle warming of the BB shell with a heat gun before the next attempt. Patience and chemistry beat brute torque on cups that have been in place since the bike was new. For the cartridge-era BB removal procedure and the warning signs that indicate “ready to come out” versus “still seized”: Replace or clean your bottom bracket →. FAQ Which bottom brackets does the 1671.2/4 fit? It drives cartridge bottom brackets that use the 8-notch pattern: Shimano Octalink units with outside notches, Bontrager and Truvativ ISIS Drive types, and other internal cartridges of that era built on the same interface. It does not fit the far more common 20-tooth pattern; that job belongs to the 20 Tooth Cartridge Bottom Bracket Tool (1671.1/4). Does it work on Bosch e-bike motors? Yes. The front chainring lockring on Bosch Generation 3 and Generation 4 drive units uses the same 8-notch pattern, so the 1671.2/4 installs and removes those lockrings. Its 1/2-inch square drive takes a torque wrench directly, useful when the motor maker specifies an install torque. How can I tell the 8-notch recess from the standard 20-tooth one? Count the castellations on the cup face. The 8-notch pattern has eight wide, widely spaced teeth; the 20-tooth pattern is finer, with twenty narrow splines. They look similar at a glance, but the two tools do not interchange, so a socket that spins without gripping is the usual giveaway that you have the other pattern. Which way does a cartridge bottom bracket unscrew? On English-threaded frames, which is most bikes, the drive-side cup is left-hand thread and comes out clockwise; the non-drive cup is conventional right-hand thread. Italian-threaded frames use right-hand thread on both cups. Check the cup markings first, and seat the tool fully in the notches before applying real force.
