Bottom bracket help please

timbo_tim Posts: 199
edited May 2016 in Workshop

I realise this is at the lower end of the biking scale, but I am running this as my winter / commuter bike and I think the BB could need some work as its creaking when out the saddle. I get easily confused by the various BB's (other than knowing square taper is basic / cheap) so wanted to canvas replacement options and what tools I would need. I don't really want to change the cranks or chainset but they are showing signs of two full winters commute so I could do it if it makes sense.

Given the budget of the bike would it just be sensible to replace with another square taper one and does this need anything more than a socket set and hex keys?

Sorry for the open ended question, but i am a clueless.



  • keef66
    keef66 Posts: 13,123
    Yes, just stick another square taper cartridge BB in there. Shimano UN 26 ... prod38569/ is cheap as chips, or you can spend a bit more for the UN 55 which has metal as opposed to plastic cups. You do need to order the right axle length. Easiest is to take out the old one and measure it, or often it's labelled up. The UN 55 seems to be available in a wider range of sizes...
    You'll need to take out the crank retaining bolts, and importantly any washers they leave behind. Then you'll need a crank extractor to pull the cranks off the square taper axle. Then depending on the make of the old BB you'll need a splined bottom bracket tool and a hefty adjustable wrench or socket handle to unscrew it. (Most are Shimano pattern splines) Remember that for a BS bottom bracket the drive side unscrews clockwise and the non drive side anticlockwise. ie in each case with the wrench / handle horizontal and pointing to the front of the bike, push down to undo. They can take a bit of shifting; don't be shy.

    Then clean up the threads in the BB, apply grease or copperslip liberally, and using the BB tool, screw in the new BB making sure it's the right way round, and remembering the drive side is reverse threaded. Nice and tight but don't go mad. Then pop the cranks back on with retaining bolts and washers. Make sure they are pointing in opposite directions!
    Check the tightness of the crank bolts after the first couple of rides.

    Shopping list:
    BB tool
    Crank extractor
    New BB
    Grease or copperslip
  • keezx
    keezx Posts: 1,322
    Creaking of a square taper crankset can happen in 2 locations:
    1-Crankbolt not tight enough, so movement between crank and axle, most likely the LH crank.
    If this situation lastst long, the taper form of the alloy crank is ruined and you need a new crank.
    Pull the left crank off and inspect the inside for damage.
    No visible damage yet?> put it back on with some threadlock on the bolt and tighten properly.
    2-Sometimes the cups of the BB itself are not tight enough in the frame and need tightening.
    I would take it apart anyway because in 2 winters it can be seized.
    Put it back with grease to prevent this in the future.
    Tools: you need a 8 mm hex key, BB key (likely Shimano) and crankpuller, neither are expensive.
  • timbo_tim
    timbo_tim Posts: 199
    Thanks both, exactly what I needed. There are some BB tools (shimano I think) in the cheapo Halfords small tool kit I bought, so looks like I might have 90% of what I need. Just missing the brute force element!