Crank bolt came loose

I replaced my cranks last week. This was a first for me, but it felt like it went fine. I used a crank extractor, was careful to match L and R sides, and applied grease to the new cranks.

Today I noticed my left crank bolt had come way loose and the crank had slid out a few mm. I tightened it back down and tried to get it maybe a bit more snug than the first time.

Did I just not tighten it down quite hard enough the first time, or is there some other mistake that might cause this?

Pics (after tightening it back down):


Probably.

It can happen if the spacer is too short, so the crank is not sitting on the spacer. From the second picture, that doesn’t seem to be the case, but if it comes loose again, that is what I would check.

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Thanks for the reassurance! So far so good since re-tightening.

In hindsight I wonder if it’d have been better to apply grease to the base of the bolt instead of to the inside of the crank, so that the part of the bolt that the nut is trying to fasten didn’t get greased.

You don’t need any grease on the bolt/threads. The grease should be on the splines of the axle and crank.

I was thinking the grease would have transferred from the splines of the crank to the bolt as it slid down, but yeah I suppose it shouldn’t really make contact with the bolt itself unless I happened to bump it as I was putting it on (which maybe I did).