Left Crank Arm Failure

[mmcd87]

Hi,

I was out today on the MTB and lightly clipped my left pedal off what I can only imagine was a rock/boulder or similar. The left crank arm came loose (or the bolt did...) and I had to walk/freewheel the rest of the way!

Upon closer inspection the socket that holds it to the crank is slightly out of shape but the arm still seems to go on and the bolt threads in ok.

The problem I have is that I cannot seem to tighten the bolt as the socket I am using (standard M14 size) will not thread into the arm hole with the bolt - is there a special tool for this normally?

Thanks in advance.

[DRB]

Is it a square taper crank you have? It was probably loose already and the impact finished it off. Much more common for left crank arms to come loose as pedalling motion loosens the bolt on the that side. Once a square taper crank is ridden loose and the crank taper is deformed it will never stay tight, you could try tightening it up once more but if it falls off again just get a new one, not worth wasting time.

Not really a special tool for tightening, just a thin walled socket, or a ratchet spanner with a socket on one end. Most cranks use an 8mm allen key bolt instead to make it easier. Needs to be about 40nm, overtightening can deform the taper.

Hope that helps!

[Rocksteadyeddie]

You could try a bit of plumbers tape on the threads. Might hold it a little longer.

[mmcd87]

Thanks for the replys! Yes square taper socket - I suspect I will need a new arm now as the square hole looks a touch out of shape from cranking on it loose (might have been done by the previous owner too. Might give tightening a go though and see if it holds or not...

The 8mm allen key sounds less hassle but would make it harder to torque as suggested I guess.

Thanks again.

[CC07 PEU]

The only way I found to fit this type of crank arm successfully was to use a torque wrench when tightening the bolt.

If you don't have a torque wrench it could be worth buying one as they come in handy for many other things too as you can imagine.

[mmcd87]

I already have a torque wrench so will use it. Would I be better with an aluminum arm or cheaper steel (which would probably be stronger?). Thanks again.