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Today I noticed that there's a left-right play at the bottom bracket of my bike. You can see a video below. On the video, it looks like I am moving it with force, but I am not: it slides with ease, even if pushed only with two fingers.

Questions:

  • Is this normal? I assume not.
  • If not, can you point me to instructions on fixing it?

I happen to have a box of tools borrowed for the weekend, so it's the perfect time to try to fix it.

This photo shows the drive side:

enter image description here

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This is actually an easy fix, but requires some special tools.

On Shimano cranks, the left hand crank arm clamps the axle. When this is installed the crank and axle are pulled together by tightening the plastic cap that threads into the axle through the crank arm. This sets the bearing pre-load and removes any axial play.

From the video it looks like you have lost the cap and that has allowed your crank to slip off the axle a little resulting in that axial movement.

Once you have a replacement cap. all you need to do is loosen the crank pinch bolts, finger tighten the cap to set the pre-load (the cap should not be over tightened) and re-tighten in pinch bolts.

However, you need a special tool such as the Park Tool BBT 1 -2 to tighten the cap. Also you really need a torque wrench to tighten the pinch bolts properly to avoid over-torquing them and stripping the threads. Conversely, under tightening the pinch bolts allows the crank arm to walk around on the steel axle which deforms the splines in the arm. Both are rather emotionally traumatic experiences as you realize you now need to buy a whole new crankset.

This is a good video showing how the crank is removed and re-installed, including setting the bearing pre-load.

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  • It seems the toolbox I borrowed has such a tool. But can you clarify what you mean by the plastic cap? I don't think there ever was one on this bike. I can see through the (hollow) axle. It was like this when I bought the bike. – Szabolcs Aug 7 '20 at 14:03
  • You mean something like this, right? bike24.com/… (I have to figure out if it's the exact one I need, though) – Szabolcs Aug 7 '20 at 14:15
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    @Szabolcs yes that's the right type of thing. You'll need to figure out what crank you have and find the specific cap required. Local bike shops may have some lying around. – Argenti Apparatus Aug 7 '20 at 14:24
  • Local bike shop had a used one lying around, and it happened to fit. All fixed now, thanks for the advice! – Szabolcs Aug 7 '20 at 16:39
  • Is the plastic cap always supposed to be present? Do you think it was missing because of sloppy assembly? It definitely was not on the bike when I got it. – Szabolcs Aug 11 '20 at 13:02

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