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Hollowtech II vs GXP: two 24 mm spindle systems
Hollowtech II and GXP both put an external-cup, oversized aluminium spindle through the frame, and both suit the same threaded and 41 mm pressfit shells, but the spindle is not the same at both ends, and that decides the bearing.
Shimano Hollowtech II: 24 mm throughout
Hollowtech II uses a hollow 24 mm aluminium spindle that is 24 mm at both bearings. The bottom bracket is a pair of external cups each carrying a 24 mm inner-diameter bearing. Because the spindle is a constant 24 mm, the two bearings are identical, which keeps service simple.
This is the most widely supported crank interface in cycling, with a bottom bracket option for BSA, Italian, T47, BB86, BB92 and, via reducers, the 30 mm pressfit shells.
SRAM GXP: 24 mm drive, 22 mm non-drive
GXP, SRAM's Giga X Pipe, uses a spindle that is 24 mm at the drive-side bearing but steps down to 22 mm at the non-drive bearing, where the spindle also seats hard against the bearing to set the bearing preload. This means a GXP bottom bracket has a 24 mm drive bearing and a 22 mm non-drive bearing, and the non-drive cup is not interchangeable with a Hollowtech II cup.
The step and the hard seat are why you cannot simply run a GXP crank in Hollowtech II cups or the reverse: the non-drive bearing size and the preload method differ.
What this means for a swap
The shells that accept a 24 mm system accept either crank, but the bottom bracket must match the crank. Fitting a Shimano crank means a Hollowtech II bottom bracket for that shell. Fitting a SRAM GXP crank means a GXP bottom bracket for that shell. The frame does not change, the bottom bracket does.
Confirm your crank's spindle before buying, because a 24 mm Hollowtech II cup and a GXP cup look similar but are not the same part.
| System | Drive bearing | Non-drive bearing | Preload set by |
|---|---|---|---|
| Hollowtech II | 24 mm | 24 mm | adjuster cap |
| SRAM GXP | 24 mm | 22 mm | spindle seats on bearing |
Figures from SRAM and Shimano technical documents and Park Tool.
Swapping between Shimano and SRAM cranks?
Open the BB compatibility toolPick your shell and crank spindle to see the exact bottom bracket you need.Find GXP Hollowtech II bottom bracket on eBayaffiliateRelated
Sources: Park Tool: Bottom Bracket Standards · Sheldon Brown bicycle glossary (bottom bracket, cranks) · Crankset (Wikipedia, CC BY-SA)