The left side crank arm keeps getting loose. I tighten the crank bolt and the arm can hold tight for a while. Then it's loose again. The bolt doesn't seem to get loose. What could be wrong? Thanks.

Once you've ridden with it loose, the interface is ruined and you won't get it to stay tight. Time for a new crank arm.

As CACycling says, it's most likely ruined, particularly if this is a square-taper crank. They require a LOT of torque to seat correctly, and if you're not using a torque wrench or have never installed one before, you're probably using nowhere near enough force. And once you have a ( new ) one on, don't try to go back periodically and tighten things up again, because that's also another way of ensuring you'll end up needing a new crank sooner rather than later.

Hi,

(square taper BB)

I had that problem with my new bike, thinking it was pedalling technique,
when in fact it was a loose crank, and by the time I had realised, the left
crank would not stay tight no matter how much I tightened the nut.

(Precession undo's a left crank, which has to handle far more torque
than the right, the right handles next to no torque, though both have
to handle the leverage of the pedal force on the crank. I think its
the leverage though that causes the precession and its the torque
that exacerbates the effect for the left hand side, undoing it.)

Trashed crank apparently, new one needed apparently, but I did fix it.
No idea how easily I'd now get it off but this is what I did :

1) Cleaned up the tapers in the crank with a flat file as best as I could.

2) Cleaned up the BB shaft with a file, but doubt it made any real difference.

The filing on the crank might have been enough, but I went
further without trying the cleaned up crankarm (I should of) :

3) Oiled the crank and shaft and then applied a small quantity of two
part epoxy to each face of the shaft and then fitted an tightened the
crank, then I undid the nut, and with it set vertical filled the gaps
with more small quantities of epoxy.

4) Inevitably the cranknut ended up with some epoxy on its castelled
surface. Initially I planned to leave it for a few hours before refitting
the nut, but then thought what the hell, and refitted it and tightened.

It worked. Still does. I didn't need to buy a new crank.
Cheap bike though, that I was prepared to try stuff on.

rgds, sreten.

This approach isn't really for everyone though. You need to have some degree of understanding the shape/taper of of the spindle in order to even try this method.

Hi,

A square taper is a square taper and not difficult to understand.

The distortion of said taper on a loose crank is not hard to understand.

Even if you you don't understand either give someone a suitable
file and a knackered crank most people (if they know or are told
its knackered) IMO would know what to do with the tool and part.

rgds, sreten

Time for a new left arm. If it loosened up once and you didn't ride it much you might be able to use some loctite (on the taper interface) and tighten the hell out of the bolt to keep it in place for a little longer. But since this has happened multiple times the arm is likely completely ruined.

This is one of the few places where you absolutely need a torque wrench if you've never installed anything that requires ~40ft-lbs. It's a lot and you'll likely not get even close just guessing. Once you have the feel for it you may be able to go without the torque wrench.

i managed to create the same problem on a vintage Dura-Ace. crank got loose while riding, rode it about 10 miles till i got home, fixed it (i thought) then it kept getting loose and creaking on subsequent rides. so, as some have suggested, i thought about getting a new crank...

but i thought, what the hell, i'll wrench it on really tight, and if it gets loose, or creaks again, i'll just repeat until it either bottoms out or the arm splits at the taper. as far as i was concerned i had nothing to lose from trying. guess what? it eventually never got loose again or creaked. i think i did a "DIY cold forging" job on it.

If you really don't want to replace the crankarm, take the WMD approach and hit it with everything in your tool-arsenal:

1. red Loctite on the bolt-threads
2. star-lockwasher, bites into both sides much better than smooth washer
3. torque it to 30-35 lb*ft torque using a real wrench with at least 12" handle