Bottom Bracket Standards Explained: Shells, Spindles & Threading

The BB is a bridge, not a single spec

The bottom bracket (BB) is the part that lets the crank spindle spin inside the frame's BB shell. It's easy to think of a bottom bracket as one spec, but it's really two independent interfaces meeting in one part: the shell side, which has to match the frame, and the spindle side, which has to match the crankset. A frame and a crankset can each be perfectly correct in isolation and still need a specific bottom bracket to actually join them — the BB is the bridge, not a leftover accessory.

Threaded shells name the shell; press-fit shells name the bore

Bottom bracket standards split into two families that are identified completely differently. Threaded shells are named by their threading and shell width — English/BSA (the long-standing mountain bike default), Italian, and the newer T47 (a modern threaded-oversized standard). Press-fit shells, by contrast, are named by their bore diameter, not the shell width or the spindle size — PF41 (a nominal 41mm bore, covering the BB86/BB92/BB89.5 family), PF42 (42mm, covering BB30-family shells), and PF46 (46mm, covering the PF30/BB386EVO family) are the common ones. A couple of proprietary designs, like Trek's BB90/BB95, skip the bore-and-cup idea entirely and build the bearing seat directly into the shell. Knowing which naming convention you're looking at is the first step to reading a BB spec sheet correctly — a shell width number and a bore number are not interchangeable facts.

BSA68 and BSA73 are the same standard at two widths

Within the English/BSA threaded family, you'll see two common shell widths: 68mm and 73mm. These are not two different threading standards — they're the same thread and bearing interface at two different shell widths, and a BB built for that thread family works across both once the correct spindle length is chosen for the width. Treating 68mm and 73mm as genuinely incompatible standards is a common data-entry trap, not a real mechanical fact.

A threading quirk worth knowing: not every shell tightens the same direction

Threaded bottom brackets have a subtle handedness detail that catches people out. On English/BSA and T47 shells, the drive-side (right) cup is deliberately reverse-threaded — it tightens counterclockwise — while the non-drive (left) cup threads and tightens the normal, clockwise way. This isn't arbitrary: pedaling load tends to make a rotating interface unscrew itself in the direction a normal right-hand thread would loosen, so reversing the drive-side thread makes pedaling load tighten it instead of working it loose over time. Italian-threaded shells break this pattern — both cups thread the standard, right-hand way — which is part of why Italian threaded bottom brackets have a longer-standing reputation for the drive-side cup working itself loose in service if it isn't kept properly torqued. If you're servicing an unfamiliar bike, don't assume both cups thread the same direction; check which standard you're actually working on first.

The tightest common pairing (30mm spindle in a 41mm-bore shell) is real, but it's a bearing-life tradeoff

Some spindle-and-shell combinations leave very little room for the bearings themselves. A 30mm-diameter crank spindle inside a PF41-family shell (BB86/BB92/PF24) is a tight fit — there's a documented product for exactly this pairing, using a dual-row bearing design specifically because the available radial space is so thin. It's a genuinely servable combination, not a fabricated one, but it's worth understanding that a tight spindle-in-shell pairing like this one generally trades some bearing life for fitting at all, compared to a roomier 24mm spindle in the same shell.

A vocabulary trap: DUB is not the same as "30mm"

SRAM's DUB spindle standard is commonly described in casual conversation as "30mm," but its actual spindle diameter is 28.99mm — genuinely undersized relative to a true 30mm spindle (used by cranks like eeWings, Cinch aluminum, and Hope). The two are not interchangeable despite the casual naming, and mixing them up is one of the more common spec-sheet misreadings in bottom bracket shopping.

How BuildMyMTB checks this

The BB is its own single-slot part in the builder — it isn't bundled into the frame or the crankset the way you might expect. Once you've picked a bottom bracket, its shell spec is compared exactly against the frame's shell standard, and its spindle spec is compared exactly against the crankset's spindle interface; a mismatch on either side is an error. Before you've picked a BB at all, the tool shows an informational note naming the spindle and shell your frame and crank require, since a bottom bracket is typically sold separately and isn't assumed to be bundled in. Nothing about your build is blocked by an unfilled BB slot — it's an optional pick that becomes an exact-match check the moment you fill it.

Practical takeaways

• Read your frame's shell spec and your crank's spindle spec separately — the BB you need is determined by both together, not either alone.
• Don't assume a 68mm and 73mm BSA shell need different families of bottom bracket — they're the same thread standard at two widths.
• If you're servicing a threaded BB and something won't budge, check which direction that side is actually supposed to turn before forcing it — the drive-side cup on English/BSA and T47 shells is reverse-threaded on purpose.
• Don't read "DUB" and "30mm" as the same spindle diameter — they're close, not identical, and not interchangeable.