Replacing BB, but found mountain + road cranks and am confused Last year I bought a Norco Search 105 Hydro (https://www.norco.com/bikes/2018/dirt-dropbar/gravel/search-aluminum/search-a-105-hydro/) and from the beginning had bottom bracket creaking. Brought it back and they put in a 2mm spacer and creaks went away for the season. About half way through this season, they came back.

I tried to remedy myself, pulled off the cranks, pedals, bb and cleaned it all up and re-lubed and assembled. Was good for a ride or two and creaks again.

So I thought maybe it was the bb itself so went to purchase a new one but this is where I am confused...


Frame is 68mm
Original BB is a Shimano threaded mountain SM-BB52 (68mm)
But cranks are a road pair Shimano FC-RS510


I can't seem to find a 68mm mountain BB, everything is 68/73 which doesn't work with the road cranks because I need the spacers but then the road spindle isn't long enough. I bought a BBR60 (road BB) but the threads where resisting and I didn't want to force it so I returned it.

Any advice on how to move forward? I'm confused why they chose to a put a mountain BB with road cranks? Also should a road BB work in this setup? Did I just need to give threading it a little more muscle?
To use a 68/73mm bottom bracket in a 68mm bottom bracket shell, you use more spacers than the 73mm shell (normally 2 x 2.5mm spacers extra). Or in other words, you leave some spacers out when using it in a 73mm shell.

If you have a 68mm shell and road cranks, your best bet is to use a 68mm road BB. The dimensions are just a little different on the cups, as explored in this question here. The differences are slight so plenty of bikes could have a mountain HII bb with road cranks, normally without issue, but if you're wondering which to buy, I'd go with the proper road one.

Don't forget that creaks can come from lots of places, don't forget to check things like your headset, QR skewers if fitted, saddle rails etc.

If the threads were resisting, and you didn't put the wrong cup in the wrong side(!) then the threads in the frame may need 'chasing' with the proper tool. Doesn't take very long in the LBS if you don't have the tool (expensive). A clean thread typically accepts a new BB cup being threaded all the way in by hand. A rough thread however will feel tight and need some force. 

I like installing a BB into a cleaned thread, threaded by hand, so that I'm confident it is not cross-threaded.