I'm trying to rebuild an old Univega MTB. Primary goal is to build a good or decent bike from used parts.

The crankset+bb that was on the bike when I got it was really worn out. I unscrewed it and cleaned the frame carefully. I asked around and found a friend that had a Shimano FC-M391 crankset with its original BB I could get.

When I installed it I just noticed that the new BB axle was ~10mm longer than the one I removed from the Univega. Unfortunately I didn't measure it but I guess it's 123mm. The original from the Univega was 110,6mm.

The new crankset doesn't come close enough to the frame. From what I can see I would like to have it ~10mm closer? I think I got it to come up all the way on the BB-axle.

I thought the BB axle length was specified with the crankset? I looked in the Shimano docs where it says 123mm.

What width should I use for this bike?

Univega MTB with FC-M391 crankset Chain line?

3 Answers 3


The spindle length depends on your frame and on the used crank. The combination of those two will dictate your chainline which should be 47.5mm. In some cases it can be 47.5-50mm ( see https://www.sheldonbrown.com/chainline.html ) There are also chainlines for other types of bike, the 47.5mm are for Mountain Bikes.

You can measure your current chainline and then calculate the spindle length you need based on the current chainline and spindle lenghth. So for example if you now got a chainline of 52.5mm and need 47.5mm your BB spindle should be 10mm shorter to achieve this.


As noted in @nollak's answer, spindle length required is dependent on the frame and the offset of the chainrings on the crank.

If the old crankset was a MTB triple, you can start by assuming the chainring offset is the same as the Acera unit. So you then need a BB unit that has the same spindle length as the original.

  • @nollak ‘s suggestion of measuring the chain line is preferable, given that the crank is installed already.
    – Swifty
    Commented Aug 6, 2018 at 14:58
  • Yeah but how do you measure the chain line? Best way is to re-install the original crank and measure to middle ring. Commented Aug 6, 2018 at 15:00
  • I think it’s better to assume nothing. Measuring the old one and assuming the new one matches isn’t as good as simply measuring the new one.
    – Swifty
    Commented Aug 6, 2018 at 19:29
  • 1
    @Argenti Apparatus: Chainline can be measured pretty easily with a caliper. Just measure the seattube width and divide in half, afterwards measure from seattube to middle of second chainring. Then just add up those values.
    – nollak
    Commented Aug 7, 2018 at 8:49
  • @Swifty That's my point, your have to re-install the old BB and crankset to do so Commented Aug 7, 2018 at 11:25

Thanks for all your help.

I bought a 113mm BB and it seems to work. I couldn't measure the original crank + bb combo so I measured how much space I had between the cogs and the frame and bought a bb that was shorter than the one I got before.

  • Nice to hear how you got on👍
    – Swifty
    Commented Aug 20, 2018 at 10:24

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