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I have already asked a question related to this but this is different.

I have a self extracting crankset that uses a 10mm Allen key to tighten and for extra info the cap like thing around it you believe uses a pin spanner and that always feels a bit loose but I don't think that is suppose to be tight but my question is my crankset keeps coming loose and I don't know why.

I don't have a torque wrench but I do tighten it pretty damn tight but it still comes loose when I go mountain biking this bike hasn't been ridden for more than probably 50 miles, please help, thanks

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    Aside - you seem to be creating new profiles/usernames a bit. Please stick with one username. stackoverflow.com/help/merging-accounts to get them all merged.
    – Criggie ♦
    Commented Sep 11, 2021 at 22:27
  • Are you using a 10mm hex driver, or a 3/8" hex driver? They are close but not the same.
    – Criggie ♦
    Commented Sep 11, 2021 at 22:30
  • In the photo of the previous post you can see 38-41Nm marked on the crankarm. This is a lot of torque. Are you using a normal hex key? They are usually only ~15cm long which limits the amount of torque you can easily create, especially compared to a real wrench with ~30cm length. I guess you could try threadlocker (or glue) on the threads and a retaining compound (e.g. LOCTITE 648) on the mating surfaces. If nothing helps I guess the mating surfaces have been irreparably destroyed from riding with a loose crankarm.
    – Michael
    Commented Sep 12, 2021 at 5:47

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Looking it up, this appears to be a 24mm FSA Mega Exo crank that presumably uses square-edged splines of some sort.

If so, the first thing to look at is the spline interface on the drive side cranks. Cranks of this sort have very little tolerance to damage to the splined interface. Any rounding, mashing, distortion, etc, and the crank is toast. Usually, repeat issues with loosening mean that's exactly what you're going to find there. The system relies on a press fit in the splines such that there's no movement in them when riding. If you do find that kind of damage, and it's really only been barely ridden, it's likely the crank bolt was under-torqued initially and/or the crank bolt wasn't lubricated properly.

Limping along a crank with these issues by using threadlocker and/or extra torque on the crank bolt doesn't usually succeed in addressing the movement happening in the splined interface. It may help for the very short term but the usual pattern is the problem will just get worse. Warranty or replacement are what you're really looking at if the splines are damaged.

If by some chance the splines are not damaged, then clean and grease them thoroughly, grease the crank bolt shoulder and threads, give it it's 41Nm or just reef down on it with something long like a breaker bar. Re-apply medium loctite to the extractor threads, put a thin layer of grease on any intermediary washer between the crank bolt and extraction cap, and install the extraction cap tightly.

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  • I really think the key here is this: "I don't have a torque wrench but I do tighten it pretty damn tight". 41 N-m may or may not be "pretty damn tight". I suspect that tightening the bolt to the specified torque will resolve the problem here.
    – jwh20
    Commented Sep 12, 2021 at 11:06
  • Thanks for all the answers I haven't really ridden it when it was loose mabye 1 mile max, how do I check if the splines are done for, should I go to a bike shop and ask them to check it out?
    – Asher
    Commented Sep 12, 2021 at 13:12
  • Also the retaining ring is a bit loose too.
    – Asher
    Commented Sep 12, 2021 at 13:27
  • @Asher going to a bike shop or even a car mechanic you are friendly with is probably your best shot at getting the correct torque on the one bolt. You may or may not be charged for this 25-second job depending on your timing and what's going on. You can work from that point yourself through Knutson's answer.
    – Noise
    Commented Sep 13, 2021 at 19:30

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