Replaced External BB - wrong size?

JimboPlob
JimboPlob Posts: 397
edited March 2013 in Workshop
My CX bike's bottom bracket had developed some play.

I confirmed it was a English thread BB that I needed. The old one was a Shimano extranl one, so I purchased an English thread tiagra BB.

Unscrewed the old one, and compared the new to the old. The plastic bit in the middle is longer on the new one than it is on the old one. The only way to make it work is to use the new BB cups with the old plastic bit (else its simply too wide).

Do the plastic bits come in different widths? I thought they were standard?

Thanks

Comments

  • nicklouse
    nicklouse Posts: 50,675
    yes they are standard.

    use the old one or none at all. all it is is a crud protector.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • JimboPlob
    JimboPlob Posts: 397
    nicklouse wrote:
    yes they are standard.

    use the old one or none at all. all it is is a crud protector.

    So why was the old one shorter? I dont get it?
  • nicklouse
    nicklouse Posts: 50,675
    crushed. the new one came from a different set up?

    who knows ask the seller.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • JimboPlob
    JimboPlob Posts: 397
    Well I put the new one in, and when I tightened up the cups, it crushed the new sleeve.

    I had no choice but to use the old one.

    Still baffled.
  • keef66
    keef66 Posts: 13,123
    Maybe you got one for an MTB?
  • keef66
    keef66 Posts: 13,123
    Looking at the pics on eg CRC's website it looks like the road BB's don't have those 2 lines around the plastic sleeve but the MTB ones do. I think you had a road BB with the central sleeve for an oversized 73mm BB shell.

    You'll be fine with the old one which I suspect lacks the 2 lines.
  • nicklouse
    nicklouse Posts: 50,675
    but road bikes do not use 73mm BBs.

    MTB effective BBs are the same when 68 or 73mm as they use spacers to make the width the same.

    road HT2 do not use any spacers.

    sounds like the wrong one was put in the bag.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • JimboPlob
    JimboPlob Posts: 397
    The pack I received came with spacers... which baffled me at first. I quickly realised these were not needed!
  • Ouija
    Ouija Posts: 1,386
    JimboPlob wrote:
    The pack I received came with spacers... which baffled me at first. I quickly realised these were not needed!

    Sounds like you got a MTB BB (hence the inclusion of spacers and a longer sleeve). The cups are slightly narrower and the sleeve is much longer, to space the outside edges of the cups at 73mm. It is possible to use MTB on road bikes and road bike BB's on mountain bikes if you swap sleeves for the correct size. You just have to be aware that the cups themselves are different sizes too (from the start of the thread to the outside where the crank arm makes contact). Fitting road bike BB's to a MTB with a 68mm shell requires the longer sleeve and a need to swap some of the 1.5mm spacers for 1.0mm spacers to put the outside edges of both cups the correct distance apart. Fitting a MTB BB to a road bike requires the shorter sleeve and the use of a spacer (either 1.5 or 1.0... i forget) to compensate for the slightly narrower cups (it'll move your chainrings in by about 2mm if you don't, which can mess up the front derailleur)..
  • JimboPlob
    JimboPlob Posts: 397
    I feel like a right idiot now!

    I looked down at the BB cups this morning, only to see the word "mountain" staring at me in the face. Looks like I got sent the wrong BB (Tiagra can only be road right?)

    It fits fine, and the spacing doesn't cause any issues with shifting... (its a cx bike)

    If anything, I prefer it, I think its closer to my BB30 widths?

    Thanks
  • nicklouse
    nicklouse Posts: 50,675
    mountain BBs have more thread on the drive side so will not suit all frames. also the actual bearings are narrower so you should really fit 2 1mm spacers. 1 each side to give the same external width between bearing faces.

    but if you can preload the bearings fine then all that has happened is you have reduced the Q factor.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • JimboPlob
    JimboPlob Posts: 397
    nicklouse wrote:
    but if you can preload the bearings fine then all that has happened is you have reduced the Q factor.

    Happy days, I seem to go better with a narrower q-factor - I am used to BB30 bikes.
  • keef66
    keef66 Posts: 13,123
    Any chance the MTB bearings are better sealed than their road equivalents?

    And have you left appropriate feedback for your Ebay seller?