Bike Help - crank arm working it's way loose

PinkFloyd · 30 Jun 2013 at 17:00

I've got an issue with my mountain bike - wondering if someone can point me in the right direction!

The left crank arm (non-gear side) is working it's way loose when riding my bike. I've bought a new crank arm as the old one was knackered, Presumably rounded off from riding it with it not fitted properly before.

So I've put the new one on, tightened the bolt in as best as I can do with the tools I have (4" long allen key) however after about 10 minutes riding I can feel the arm coming lose. By this point I can move the bolt with my fingers even.

I've got some pictures of what I have below. I'm wondering whether the shaft itself is too worn, whether I'm missing a washer/spacer or similar, or whether I'm just not tightening the bolt enough.

I can get more pictures if needed.

Thanks in advance!

[DOD]Asprilla · 30 Jun 2013 at 17:03

Did you replace like for like with the cranks?

PinkFloyd · 30 Jun 2013 at 17:18

The replacement arm wasn't identical to the old one, but it's the same style fitting and length. I would have thought they'd be a standard shape?

Jonny ///M · 30 Jun 2013 at 18:06

Some tapers taper slightly differently from others. You also need about 40nm torque on that bolt so it may well not be tight enough.

It does look like the axle doesn't come into the arm enough to be honest.

PinkFloyd · 30 Jun 2013 at 18:12

Worth lubing it up a bit so I can get the arm on a bit further, before tightening it up with the assistance of a breaker bar?
I'll compare it to the old pedal - see how far that one used to slide on.

[DOD]Asprilla · 30 Jun 2013 at 18:30

There are two different typed of square taper axels; ISO and JIS they can be use together but they do fit differently.

http://sheldonbrown.com/bbtaper.html

Looks to me like you are putting an ISO crank on a JIS spindle.

PinkFloyd · 2 Jul 2013 at 19:08

Cheers guys. Tried tightening it up tight but just works its way lose in no time.
If I haven't already ruined the pedal I'll be taking it back and getting them to find the right one. This time I'll take the bike with me to check. Won't get a chance for that until maybe thursday night though if not later.

marl · 2 Jul 2013 at 20:23

If it has worked its way loose is it possible this movement has caused the fitting to widen slightly? If its a soft metal it might not take much movement to wear it!

When I cycled I would gently use a piece of wood and hammer to gently, and I mean very gently, tap the crank onto the axle before tightening it. I used this method many times and never had one come loose.