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Currently, I'm little bit worried about the compatibility of my crankset and the bottom bracket. The crankset is Truvativ Isoflow, the BB is BB-UN55 (Square taper). Yesterday, after the bike hit +1400 km mark, I scheduled maintenance for the crankset. I disassemble the crank arms and the BB, cleaned them, apply grease (I use Phill Wood), and reassemble.

However, when I removed the crank arms, I didn't need a crank puller at all. This made me worried that the crank arms might be loose. So what I might want is tigthen the crank bolts. However, on the drive side, the right crank arm already hit "the limit" (see an image below) so it wouldn't go any further in.

To the best of my knowledge and according to this specification document, the BB should be compatible with the crankset. Its spindle width is 113mm to achieve 48.5mm chainline (The bike is a single speed), see a table on the bottomleft of the pdf document.

Another solution I found on the web is not applying grease on the spindle and the crankarm holes to increase friction. This is what I did. So think the crankarms are tighter right now. So far I have no problem, no creaking noise, and I doubt the crank arms will be fall off anytime soon. However, I'm not sure whether I'll have problems in the future.

edit 1:

I put some more information:

  1. according to the pdf document, the crankset is compatible with truvativ LE square taper BB which has JIS interface. The UN55 also have JIS. So the crankarms and the BB should be compatible, right?
  2. When I built the bike in April, I was noob so I put too much grease on the spindle. I think this was the reason why the crankarms fell
    off easily. That's why I didn't grease the spindle yesterday.

enter image description here

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    Not applying grease will not solve the problem. If the crank arm is not new with the cartridge then likely it was worn before and needs to be replaced. Or the two are simply incompatible. A possible hack, though, would be to grind down the back of the arm a millimeter or two.
    – Daniel R Hicks
    Commented Jun 20, 2015 at 12:02
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    I would use a shim. Coke can, maybe. That way it wouldn't bottom out, and you'd be able to really tighten it down.
    – BSO rider
    Commented Jun 20, 2015 at 22:49
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    With crank bottomed out is the end of the shaft even with the seating surface for the bolt? This is the determining factor for a shimming the shaft or grinding the back of the crank. decision.
    – mikes
    Commented Jun 21, 2015 at 0:53
  • @mikes - Good point. If the shaft end is projecting out of the front of the crank then grinding the back of the crank will do no good. In such a case the crank arn is likely toasted, though OverOver's shim idea is maybe worth a try.
    – Daniel R Hicks
    Commented Jun 21, 2015 at 2:42

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If you are able to seat the crank arm onto the axle such that it hits the green area you marked (i.e., it will slide all the way down the taper), then the crank arm is "worn out." The taper on the crank has been enlarged by wear or over tightening to the point where it can no longer form a solid connection to the axle. The "right" fix would be a new crank arm, but it seems like you could try the ideas suggested in the comments above. Do check the condition described by @mikes, if the axle is sticking out beyond the crank arm then grinding will not work, but then you are also likely to have quite a bit of room for a shim :-).

If you're game to experiment, my biggest concern would be that there might be cracks starting to develop in the corners of the crank arm socket from the stresses of the arm trying to rotate on the axle. Eventually those could lead to the crank arm failing at the socket.

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  • The crank arms are quite new, although they already hit +1600km mark. Since I reassembled the crankset (with very minimal grease on the spinde) a week ago, I've managed to ride the bike for 100km in local trails, hit curbs, climb hills, etc. The bike is well abused but I haven't had any issue yet. I do hope this will last for another 1000km. I also agree with your suggestion, I should get a better crankset in the future since I begin to hate square taper.
    – azer89
    Commented Jun 26, 2015 at 11:32

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