Making sense of bottom brackets, part 1: Introduction and threaded types

If there’s a bicycle component that has the most potential for aggravation, it has to be the bottom bracket.

As I’ve mentioned before, the bottom bracket is one of the four main sets of bearings a bicycle relies on for its function. Its purpose is to allow the cranks and their connecting spindle to spin freely, in order to accept the rider’s pedaling power.

So why the potential for aggravation? It’s primarily because bicycle parts manufacturers can’t seem to stick with a tried and true concept, proliferating new bottom bracket standards and frame interfaces in the name of increased frame stiffness and pedaling stiffness. The plethora of standards can be an overwhelming minefield for those not in the know, which is why I’ve decided to help out.

We’ll begin with what I consider the elder statesman of the standards: bottom brackets for frames with threaded shells and three-piece cranks.


A bottom bracket shell with threads being tapped into it.

Once upon a time, all bicycle frames had bottom bracket shells with threads tapped into them. After the French retired their idiosyncratic screw thread standards and got along with the rest of the world sometime in 1985, two major standards were left: English (also called “BSA”) and Italian (or “ITA”).

Both standards refer to a bottom bracket shell that is 68 mm wide. The only real difference between BSA and ITA is the threading of the non-drive side. Frames made to fit ITA bottom brackets are right-hand-threaded (turn clockwise to tighten) on the non-drive side. For BSA bottom brackets, the non-drive side is left-hand-threaded (turn counter-clockwise to tighten). It could be argued that BSA is the more common threading of the two, these days.

A Shimano Hollowtech I three-piece crankset. The Octalink-spindled bottom bracket serves as the third piece.

Many bikes of old also used three-piece cranksets. These consist of:

  • The drive side crank arm with the chainring(s)
  • The non-drive side crank arm
  • A separate spindle inside the bottom bracket connecting the two crank arms

On a three-piece crank, the spindle and bottom bracket are a single unit. There are variations of these, but they tend to differ only on how the crank arms attach to the bottom bracket spindle.


A Shimano UN55 square taper bottom bracket. The “113” refers to the total spindle length from end to end, in millimeters.

These are so called because of the shape of the spindle. The spindle has a square end which tapers (narrows) as it extends from the bottom bracket. The crank arms attach purely by friction due to the tapering of the spindle. A square-taper bottom bracket usually has a spindle with a 17 mm nominal diameter – at least in the center where it’s actually round.

Square-taper cranks and bottom brackets are remarkably long-lived. They’re still around today, serving duty in lower-cost parts and applications, such as fixed-gear and track bikes. Bino’s stock crank is based on a square-taper bottom bracket. Many square-taper bottom brackets these days use cartridge bearings and aren’t really designed to be serviceable. Once it goes bad, it’s usually cheaper to replace with a new one instead of having a mechanic dismantle it.


A Shimano ES25 Octalink bottom bracket. Note that this is “Version 2” with the spline teeth cut deeper into the ends of the spindle.

Shimano introduced Octalink with its Dura-Ace 7700 groupset. Splines are basically gear teeth cut on a cylinder or the end of a shaft, instead of on a flat wheel like a normal gear, cog or chainring. In the case of Octalink, there are eight splines on the end of the bottom bracket spindle. The thinking went that a splined interface between crank arm and spindle would improve power transfer compared to a square-taper system.

Octalink still exists today, mainly in the form of Shimano’s entry-level Claris 2400 eight-speed groupset and its Dura-Ace 7710 cranks for velodrome track racing. They are nowhere near as populous as square-taper parts though.


An ISIS splined bottom bracket sold by Nashbar.

Shimano kept its Octalink system proprietary. In response, an alternative, open standard called ISIS (International Splined Interface Standard) was born, used by companies such as FSA and Truvativ.

ISIS made use of ten splines on the spindle, as well as adopting the tapering spindle concept of the square taper bottom bracket. In theory it should have been sound, but problems in the design meant that in most ISIS bottom brackets, the bearings themselves were too small to be reliable. Here’s a good read on the subject.

Exploded-view anatomy of an ISIS splined bottom bracket.

For the most part, ISIS is an orphaned standard now, one that stopped making sense because Shimano decided to fuse the spindle with the drive side crank arm – creating the two-piece crankset. When other manufacturers followed this design, it effectively meant that Octalink got “discontinued” as well, too.



A Shimano Hollowtech II bottom bracket installed on a bike, characterized by the notched outboard (external) bearing cups

As Shimano fused the spindle with the drive-side crank, it also learned from the difficulties ISIS had and thought of thinking outside the BSA/ITA bottom bracket shell width (68 mm for road bikes and 73 mm for mountain bikes). By moving the bottom bracket bearings outside the bottom bracket shell, it could use larger, more efficient bearings while effectively widening the shell at the same time.

The Shimano Hollowtech II system was born. It has many defining characteristics:

  • The bottom bracket bearings are mounted in outboard cups that screw into the bottom bracket shell’s threads. Once the cups are mounted, the shell effectively widens from 68 mm to 86.5 mm for road bikes, and from 73 mm to 91.5 mm for mountain bikes.
  • All Shimano Hollowtech II cranks have a hollow 24 mm steel spindle fused to the drive-side crank arm. With two-piece cranks, this spindle diameter now becomes a very important detail.
  • The spindle is splined to accept the non-drive side crank arm.
  • The non-drive side crank arm is the “compression slotted” type, according to Park Tool’s classification. It has a slot and a compression cap that pre-loads the bearings by squeezing the crank arms together, and is tightened with a proprietary eight-point star tool (Shimano’s TL-FC16). Once that is set, two opposing pinch bolts secure the non-drive side crank to the spindle.
Drive side crank of a Shimano 105 FC-5800 two-piece crankset. Note how the spindle is permanently fused to the crank arm and is now independent of the bottom bracket.

Chances are, if you have a Shimano crank, you use at least part of Hollowtech II. If your frame has threaded-in bottom bracket cups, you have the full system.

The Hollowtech II design proved so successful that many other manufacturers provided their own versions on the formula – which are generally very similar. I’ll mention a few standouts.


The main difference of GXP compared to Hollowtech II is the “stepped” spindle diameter. The drive side is 24 mm as before, but this tapers down to 22 mm on the non-drive side. This supposedly allows for larger bearings to be installed on the non-drive side, and it is also inherited from the ISIS design.

A Truvativ GXP crank. Note the ISIS-like splines on the spindle, and “step-down” in spindle diameter on the non-drive side.

GXP is still around. If you have a SRAM crank, you’re on GXP unless your frame requires a BB30 or PF30 bottom bracket. SRAM makes specific versions of its cranks for those, which have straight spindles (no taper).


A Campagnolo Ultra Torque crank. Note that both crank arms have half the spindle. Also note the bottom bracket bearings which are threaded onto both arms.

While Hollowtech II cranks fuse the whole length of the spindle onto one crank arm, Ultra-Torque splits this straight in the middle. Both arms have half the total spindle length fused to them. They meet halfway and mesh in the middle of the bottom bracket shell via a Hirth joint made up by meshing teeth on each half, then a bolt joins the two crank arms in the middle of the spindle.

While Ultra-Torque uses external bearing cups, the bearings themselves aren’t integral to the cups as they are on Hollowtech II. They just slide onto each crank arm.


Hyro came stock with an Omega crank. These also use outboard “MegaExo” bearing cups, similar to Hollowtech II and GXP in general design, but the cranks have a strange 19 mm spindle diameter which is hard to get replacement bottom bracket bearings for. The non-drive side crank arm is the “self-extracting” type, as well.

This takes care of bottom brackets for frames with threaded shells. In the next installment I’ll discuss the messy minefield that is the bottom bracket situation for frames with press-fit shells.

3 thoughts on “Making sense of bottom brackets, part 1: Introduction and threaded types

  1. Thanks so much for your article. The splines on my bottom bracket are a little worn and I’m doing a lot of research and learning to uncover my options for fixing it. This article really helped clear up different terminology (“spindle”, “spline”, “bottom bracket”)


  2. You forgot POWER SPLINE. As the late great Sheldon Brown once said “the problem with standards is that there are so many of them.”


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