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I am trying to fix the problem with my bicycle where the left crank keeps unscrewing itself. The thread on which a serrated lock nut should keep the pedal in place is conventional (clockwise to tighten).

When looking closely at both sides. There is some sort of thread on the inside of the crank cavity itself and it's not used by anything since this cycle has been bought. May anyone know what this may be for?

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    The threads are for a crank puller. They also incidentally hold the screw-on caps that cover the openings. – Daniel R Hicks Jun 28 '15 at 21:56
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    (And if the crank arm comes off without resorting to a puller it usually means that the arm (and possibly shaft) is toast.) – Daniel R Hicks Jun 28 '15 at 21:58
  • The arm with the pedal on just comes off by pulling it with my hands. In fact this is the problem, the nut works itself loose and the pedal starts wobbling. What would be the reason for this not happening? (I am inexperienced). The tightness/pressure caused by the arm going up the tapered edges of the square shaft? – midnightBlue Jun 28 '15 at 22:52
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    The problem is almost certainly that the crank arm came loose and was allowed to stay that way while the bike was ridden. Once a crank arm is loose it only takes a small amount of riding to cause the socket in the arm to be deformed such that it no longer tightly fits on the shaft. Sometimes a skilled and creative bike mechanic can supply a "fix", but most likely your arm needs replacing. – Daniel R Hicks Jun 29 '15 at 0:47
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    If the crank arm wobbles on the tapered square end it is best replaced. Shims are a bad idea anyway! I had the situation once and it resulted in a suddenly broken shaft with a razor sharp edge. Think that it is very close to your calf. – Carel Jun 29 '15 at 7:42
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The lock nut is used to wedge to crank on the bottom bracket spindle. If you open it, you should still not be able to get the crank off, at least not easily.

For square tapered bottom brackets, usually you have to use a tool called a "crank puller", which is hold in place with the outer thread.

See this article over at Sheldon Browns for details and very interesting pictures showing whats going on inside the cranks.

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