After replacing a bottom bracket with square taper I repeatedly have to tighten the screws that fix the crankset (on each side) to the bottom bracket axle or otherwise I can feel sometimes a slight movement of the crankset on the axle. I understand that this might be needed once or twice for parts to really settle in but I wonder how common that is or wether there are tricks to avoid it. This is an old Shimano crankset and a new Shimano bottom bracket. Since the screws are not very long (about 10mm of thread) I also fear I could damage them in the process.
asked Jun 25, 2017 at 8:20
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Could be your crankset is getting old too? And the change of BB is showing up wear in the square interface? Did you over-grease the spindle surfaces before fitting the cranks?– Criggie ♦Commented Jun 25, 2017 at 9:16
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1I had originally greased the spindle surfaces but since the problem started removed the grease both on the spindle and the screws. I believe it reduced the problem. I am surprised that the screws would loosen at all.– Christian LindigCommented Jun 25, 2017 at 9:37
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4The problem with square taper cranks is that the holes widen over time. Up to the day where they go to 'deep' over the spindle and the bolt can't push them in because the head sits on the spindle.That's when the time has come to replace the cranks.– CarelCommented Jun 25, 2017 at 10:36
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1I wouldn't worry too much about the threads if both the screws and the spindle are steel. But I wonder if the wrong thing is bottoming out so it never does up tight. What happens if you screw a screw in without the crank arm on that side? It should go all the way home just turning it with your fingers.– Chris HCommented Jun 25, 2017 at 10:39
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@Carel that's a good point but only if you keep taking them off. Most square taper cranks these days are riveted to the chain rings so the cranks get replaced and hardly ever refitted. One of my bikes with square taper is on its third BB and second crankset, for example.– Chris HCommented Jun 25, 2017 at 10:42
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1 Answer
As some commenters have indicated, your crank arms are worn out.
We can speculate all day about what was the cause, but since you indicated that you felt some wobbling, that means they are now because that is the fast-track to ruining the square taper interface.
You'll have to replace your crank arms.* When replacing them, make certain that your crank bolt is tightened to spec. It needs to be pretty dang tight. Also, when you get the new crank, replace the crank bolts with the snazzy allen headed crank bolts.
*If you are short on cash or like to gamble, you could also just replace the crank bolts with the snazzy ones, then tighten the absolute snot out of the bolts. While a low chance of success, it might work. The BB spindle is hardened steel, and the cranks are aluminum, so you don't pose much risk to the BB spindle if you choose this option. But still abide by the no riding when the crank is wobbling rule.
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I have speculated that one might alleviate the problem with wobble-worn crank arms using shims cut from beer cans, taking advantage of the fact that the thickness of the cans sides vary across their height, so a wedge shape could be obtained. Would take some cleverness, however. Commented Jun 25, 2017 at 20:42
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Indeed, I don't have the "snazzy allen headed crank bolts" but the regular machine screws with M14 head. Why are the allen-headed ones superior? Commented Jun 25, 2017 at 20:56
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@DanielRHicks: I've tried the beer can shims to address an ISO<->JIS mismatch and it didn't work. I think if it didn't work even for that it probably wouldn't work for this. Worth a shot (beer) though. Commented Jun 25, 2017 at 22:30
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1@ChristianLindig: one of their main advantages in my opinion is the 8mm key, an 8mm wrench is usually on multi-tools so you can do repairs in the wild, which is not usually workable with the often non-standard socket sizes needed otherwise. Commented Jun 25, 2017 at 23:00
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1The 8mm allen bolts will be so tight that they can hardly be unscrewed with an on-board multi-tool. The leverage is way too small. Even if you take along a standard 8mm allen wrench it will be hard to work.– CarelCommented Jun 26, 2017 at 12:02