The ultimate T47 bottom bracket guide

T47 was conceived in 2015 to solve one specific, maddening problem: press-fit creak. Press-fit bottom brackets push the bearings directly into the frame shell, and over time that interface can loosen, move and creak — the bane of many a modern bike. T47 was a collaboration between several builders — including Chris King, Argonaut, White Industries and Engin — to combine the easy serviceability of a threaded shell with the size advantages of the wider press-fit shells.

The idea is neat: take the 46mm internal diameter of a PF30 press-fit shell and cut a thread into it (officially M47 × 1.0mm), so the bearing cups thread in rather than press in. You get the big shell and the reliable threaded interface.

BSA (also called English or ISO threaded) is the long-standing threaded standard — a 68/73mm shell, a 1.37-inch internal diameter, and 1.37″ × 24 TPI threads. It’s reliable and serviceable, which is why it never really went away. T47 keeps the threaded principle but builds it around the much larger 46mm shell.

The single most important comparison is wall thickness with a fat 30mm crank spindle — it’s what makes the “bigger is better” argument concrete rather than hand-wavy.

Why that one row matters: BSA’s geometry is ideal for a 24mm axle (around 5.5mm of wall) but chokes a 30mm axle, leaving only ~2.5mm of wall. T47 gives a 30mm spindle about 6mm of wall — almost exactly what BSA only ever managed with a skinny 24mm spindle. That’s the real “why”: T47 lets a frame run fat, stiff 30mm spindles and keep the structural margin BSA couldn’t.

The benefits, plainly

This is where T47 gets genuinely confusing in the wild — but it comes down to just two independent variables: shell width (a property of your frame) and spindle diameter (a property of your cranks). Get those two right and everything else follows.

Internal vs external — and how shell width decides it. This is the single most misunderstood point, so here it is plainly: internal and external refer to where the bearings sit, and your frame’s shell width forces the choice.

External — bearings sit outboard of a narrow 68/73mm shell. Internal — bearings sit inboard, inside a wide 85.5/86/86.5mm shell.

So the rule of thumb: a 68/73mm shell needs an external-type T47 (unless you’re running BB30/PF30-style 30mm cranks, in which case internal); an 85.5–86.5mm shell needs an internal-type T47. The frame decides this, not the cranks — which is why “just measure your shell” is the right first step.

Spacers do two different jobs, and conflating them is a common source of confusion, so keep them separate.

You can also adapt across sizes: a 24mm adapter (30mm outer diameter, 24mm inner) can be added to a 30mm BB to run a narrower crank.

T47’s weakest spot isn’t the standard itself — it’s that the tools aren’t standardised across brands. The threaded concept is simple to service, but notch counts and diameters vary, so the tool that fits one brand’s T47 may not fit another’s.

The practical upside: some brands deliberately design for standard spline tools (for example, the common 44-16 and 46-12 spline interfaces) so no proprietary tool is needed.

Two quick things round out the picture:

On an off-the-shelf bike, the bottom-bracket standard is whatever the frame came with, good or bad. On a custom titanium build, it’s a deliberate choice made at the design stage. For most builds the answer is T47: threaded so it won’t creak, large enough to run any modern crank and route cables cleanly, serviceable anywhere in the world, and built into a stiff, generous BB junction. It’s the standard that quietly makes the rest of the bike better, and exactly the kind of decision Will talks through with every rider.