Left crank arm problem

CON3

Hey guys and gals,

I have an issue and I'm really kicking myself about it. I have spent about 6 weeks building a bike from scratch, spent just over a grand. Finally get it done and take it for a ride and the left crank arms falls off. I believe I tightened it correctly but when I look inside the crank arm half the spline has bloody rounded off.

My bike:




The problem





Is there anyway I can replace the spine parts, it's the m7000 crankset BTW 175mm

02gf74

In theory any item can be repaired so unless you have access to a machine shop, in short the answer is no.

Your only hope is to pick up a left hand crank, on ebay or elsewhere but a new chainset in £81 or less, and if you took it for repair, it would cost much more than that.

There should be a large plastic nut that screws into the axle on the non drive sidewhich is to set the preload and should stop the crank from falling off, did you fit it? The fact that 1/2 the spline length is damaged could be because the axle is too short either due to wrong part for the BB width else spacers fitted when not needed.

BTW drinking whilst building may account for your woes.

CON3

Is this the nut your on about




And it is held on by by these either side. So just clamped on basically



And it shouldn't be too short as I bought the drive train set, bb, cassette and all that.

And the drink was a celebratory drink not a construction drink lol

CON3

as for the spacers only 1 was used to make sure it was aligned with the centre of the cassette, I did have a look at replacement arms and seen one knocking around for £60 and the set £80, seems a shame the spline isn't available as it looks like can be rather easily knocked out from the other side and replaced

cooldad

The plastic nut thingy is just to pre tension it, not to hold it on, the pinch bolts do that.

And the spacers are not to change the chain line, they are used depending on the BB shell width. So looks like you only had the crank partially on.

Did you read the instructions?

As for knocking out the spline - seriously?

CON3

Yeah as far as I was aware. It was a far on as it would go and made sure other side was as far in as it could go. I think I need to accept the inevitable and buy a new crank set/arm, as even looking through Shimano's technical documents to the crank set that part has no number accept the whole arm.

As for knocking out the spline I gather that is not an easy/right thing to do then?

To be honest I did expect some things to go wrong as I am a complete novice in building bikes, and to be fair this has been the only problem so far. Just a shame have to spend more money.

The Rookie

The spline is machined into the arm, it’s not an insert, so no, you can’t ‘knock it out’!

The arm was clearly not on the axle fully when the splines failed, whether that was because it was incorrectly inserted or had come loose it’s impossible to say based on the information provided.

02gf74

cooldad wrote:

The plastic nut thingy is just to pre tension it, not to hold it on, the pinch bolts do that.ly?

Yes, agreed the bolts clamp it in place but the plastic thingy should stop it coming off, there should not be huge amount of sideways force.

In addition there is a plastic tab with two holes for the bolts and a steel pin which locates in a slot in the axle, my understanding is that it is this prevents the crank from coming off.

In summary, you need a new LH crank and figure out what went wrong.

cooldad

No the plastic thingy does nothing except preload. The tab between the bolts makes sure the arm is in the right place.

The bolts hold it on.

BikerGroveish

cooldad wrote:

No the plastic thingy does nothing except preload. The tab between the bolts makes sure the arm is in the right place.

The bolts hold it on.

Exactly. Think of it as the plastic thingy loading the tension, then the bolts freezing the tension in. A lot like the way the headset works. Preload with the top cap bolt and then freeze the load with the stem bolts. Once the pre load is frozen in, you can take the top cap bolt or plastic thing on the crank arm off if you want. It’s done it’s job so you can take advantage of the marginal gain achieved by chucking the dead weight away.

steve_sordy

Are you sure that the male spline is undamaged?

thistle_

steve_sordy wrote:

Are you sure that the male spline is undamaged?

I was wondering the same. If the male splines are not damaged then the OP is luck and only needs a new LH crank, if the splines are damaged it's a new chainset (although you could reuse the chainring).

The Rookie

It's unlikely the steel splines were damaged by the aluminium ones failing.