Crank arm or spindle?

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Before Christmas I had the group set replaced on my trusty Trek 3500 work bike, since then the LH crank has worked itself loose and it's had it. I'm mildly sceptical that it's due to bad fitting/lack of correct tightening when the work was done but that's a battle I can't be bothered to fight.

The arm in question appears to be a Shimano M311 170mm from the markings, the trouble is I no nothing about replacing them apart from you can buy them on Amazon with very high delivery costs. Could anyone tell me what I'm actually doing and if I can easily replace it please?

 

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https://www.swinnertoncycles.co.uk/...-m131-left-hand-crank-arm-170-mm-black-p48394

Stick that on it. Same length, same square taper pattern and from the same sort of chainset.

The exact part number Y1J498010 for yours only brings up Amazon rip off as you say.

It should be held on by a 14mm nut or 8mm bolt.

Needs to be about 40nm which is pretty much as tight as you can make it with hand tools.

Often worth putting some blue loctite on the bolt for extra security

Pedal will come off the old arm turning it clockwise.

That's great, thanks a lot for the detailed help (I was wondering how to know how tight it should be).

I'll get that on order now.