Home / Guides / Fitting a 24mm crank in a 30mm shell (adapters)
Fitting a 24mm crank in a 30mm shell (adapters)
A 24mm crank drops into a 30mm-bore shell only with a reducer bottom bracket that shrinks the effective bearing diameter down to 24mm.
Why a plain bearing will not do
A PF30 shell has a 46mm bore and a BB30 shell a 42mm bore, both designed so a 30mm spindle passes through with bearings sized for 30mm. Push a 24mm Shimano spindle through those bearings and it rattles around with a large gap, because the bearing inner is 6mm too big.
The fix is a reducer, sometimes called an adapter bottom bracket, that carries a 24mm bearing inside a body sized for the wider shell. It bridges the gap so the smaller spindle is properly supported.
How reducer bottom brackets work
A reducer for a pressfit shell presses cups into the 46mm or 42mm bore that in turn hold 24mm bearings. For a threaded 30mm-capable shell like T47, threaded cups do the same job. Either way the result is a 24mm bearing correctly positioned for a Hollowtech II crank, with the extra shell width taken up by the cup body and spacers.
Good reducers include the spacers and seals needed to set the crank at the right width and keep grit out. Cheap or ill-fitting adapters are a frequent source of the very creak these wide shells are already prone to, so quality matters here.
The DUB and 30mm alternatives
If you would rather run a bigger spindle, the same shell can usually take a DUB bottom bracket at 28.99mm or, for PF30 and BB30, a 30mm bottom bracket that uses the shell's full bore. The choice comes down to the crank you own: fit the bottom bracket to the crank spindle, then to the shell.
There is nothing wrong with running a 24mm crank in a wide shell; it is a very common combination. It simply needs the right reducer rather than a standard 30mm bearing.
Getting it quiet
Because a reducer adds another set of press or thread interfaces, careful installation is what keeps it silent. Clean the bore, press square with the right tools, respect the torque spec on threaded cups, and set the spacers so there is no side play. Done well, a 24mm crank in a 30mm shell runs as quietly as any other setup.
Need a reducer for your wide shell?
Open the BB compatibility toolEnter your 30mm shell and 24mm crank to get the exact adapter BB.Find PF30 to 24mm shimano adapter bottom bracket on eBayaffiliateRelated
Sources: Wheels Manufacturing ยท Park Tool bottom bracket reference