I'm considering upgrading to an Ultegra groupset, found a very good price online, but I do not know what bottom bracket is in my bike. I have a 2015 Giant Defy 2, aluminum frame, and to me, this looks like a press fit bottom bracket. However, the technical documentation that I got when I purchased the bike from a local bike shop says it has a Shimano BB4600, which is a threaded bottom bracket and looks nothing like what is on my bike so far as I can see. The only press-fit option with this groupset available is the BB71-41B. Can someone tell me if this will possibly work with my bike, or how I would go about determining compatibility? Additionally, since the crankset on the bike now is a Tiagra crankset, would it be possible to just swap it out with the ultegra crankset and leave the BB alone? Thanks.
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Ultegra is still hollowtech II. It seems like the Defy 2 in the UK has a pressfit bottom bracket for hollowtech II cranks, while in the US it has an usual threaded hollowtech II bottom bracket. In any case, you need a hollowtech II compatible bottom bracket, which you already have (Tiagra is Hollowtech II only).
Just remove the current crankset and put in the ultegra crankset.
Aside: This guide from BikeRadar summarizes bottom bracket standards.
