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Square taper, Octalink and ISIS: the older crank interfaces
A huge number of bikes still run a cartridge bottom bracket whose spindle is part of the sealed unit, and the crank bolts onto it. Three interfaces dominate that world: square taper, Octalink and ISIS.
Square taper: the universal old standard
Square taper cranks clamp onto a tapered square spindle, most commonly to the JIS or ISO taper standard. The crank is drawn onto the taper by a bolt, and the interface has been used for decades across almost every kind of bike. It is simple, cheap and serviceable, and cartridge square taper bottom brackets remain widely available in many spindle lengths.
The two sub-standards, JIS and ISO, differ slightly in taper dimensions, so a JIS crank sits a fraction further onto an ISO spindle. For most riders the practical point is to match crank and bottom bracket to the same taper family.
Shimano Octalink: a proprietary spline
Octalink replaced square taper on many Shimano groups with a splined hollow spindle for greater stiffness and easier crank removal. There are two versions: Octalink V1, used on road and older mountain groups, and Octalink V2, used on later mountain groups, with different spline depths. An Octalink crank needs an Octalink bottom bracket of the matching version, and does not fit ISIS or square taper.
Octalink is a closed Shimano standard, which is part of why the open ISIS standard appeared alongside it.
ISIS: the open splined standard
ISIS, the International Splined Interface Standard, is an open ten-spline interface created so that any maker could build compatible cranks and bottom brackets without licensing Octalink. ISIS cranks fit ISIS bottom brackets from any brand that follows the standard. The larger spindle diameter can shorten bearing life in the smallest cartridge units, which is one reason external-bearing systems eventually replaced all of these.
If you are servicing an older bike, identify which of these three the crank uses before buying a cartridge bottom bracket, because none of the three interchange.
| Interface | Type | Open or proprietary | Interchanges with |
|---|---|---|---|
| Square taper | tapered square | open (JIS / ISO) | nothing splined |
| Octalink V1 / V2 | 8-spline | Shimano proprietary | same version only |
| ISIS | 10-spline | open standard | any ISIS part |
Figures from Sheldon Brown and Park Tool crank references.
Servicing an older cartridge bottom bracket?
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Sources: Sheldon Brown bicycle glossary (bottom bracket, cranks) · Park Tool: Bottom Bracket Standards · Crankset (Wikipedia, CC BY-SA)