From Hollowtech II to GXP

LimitedGarry

Ey, so it took me more than five minutes to find a solid answer on this on Google, so I'm just going to ask.
If I have 73mm BB with Hollowtech II, can I just unscrew the Hollowtech II cups and install the GXP ones?
In other words, are the threads identical?

I'm asking because the selection of 1x specific cranks seems better on the SRAM side of fence and lots of them have 94bcd, which is compatible with the chainrings I currently have an excess of.

BTW I'm thinking that yes, because it always just says "68/73mm BB" and nothing about different kinds of threads or anything.

cooldad

Yes.

rockmonkeysc

Yep

LimitedGarry

Thanks for answers.

supersonic

You will not need any spacers though - the Shimano should have 1x2.5mm driveside spacer for 73mm (unless you have a BB mounted chain device or other unit there ie E type front mech). The GXP for 73mm requires nothing.

LimitedGarry

Thanks again. Didn't know that about GXP. How does it work on 68mm? Is there a tolerance or would one need some special cups?

supersonic

With 68mm shells, GXP requires a pair of 2.5mm spacers, one each side.

The reason why Shimano chose to use a single 2.5mm driveside spacer for 73mm, and 2x2.5 driveside and 1x2.5 nondriveside for 68mm, is that it allows fitting of ancillaries. For example an e-type front mech or bb mounted chain guide can be used as they are usually 2.5mm wide and are just substituted in for the driveside spacer. SRAM's setup means that on 73mm, you can't do this.

Shimano MTB external threaded bottom brackets are plenty deep enough to provide support in all these scenarios, but many road designated bottom brackets are not and are best not avoided, especially on 73mm BB widths.

LimitedGarry

I see. Thanks for clearing that up