Why does my non-drive side crank arm come loose?

Question

I tighten the bolt super hard. Ride for a while. Then it comes loose again. I don't know if its my BB or the bolts. How can this be fixed??

Answer 1

By "non-drive" I assume you mean the left side. This is more apt to come loose than the right because of "precession" -- most crank bolts are right-hand thread on both sides, but the motion of the crank arm relative to the shaft tends to loosen the bolt on the left side, whereas it tends to tighten the bolt on the right side.

But if this is occurring it's because there IS motion between the crank arm and the shaft. This should not be, as the two should fit together so tightly as to prevent any motion. What has no doubt happened is that the bolt was installed too loosely at one point, the crank arm worked loose, and now the socket of the arm is misshapened so the two tend to move relative to each other all the time.

Using some Loctite (the "removable" variety) on the bolt will help temporarily, but ultimately something must be done about the misfit between the two parts. Basically either the arm must be replaced (and the shaft, if the situation goes on for too long), or (if this is a square shaft) you must carefully fashion a shim (perhaps out of aluminum pop can material) to create a better fit. (If it's a splined shaft you're probably SOL.) And regularly re-tighten the bolt -- don't wait for it to get loose.