New crankset, screw unscrews

[Lighttower]

I have bought a new (shimano) crankset (I believe it is called a diamond crank) for my MTB. I've installed it myself (which is probably the problem :-p), rode with it a couple hundred KMs and all of a sudden the non drive side crank becomes wobbly and after a couple of meters the screw that holds it unscrews and falls off.
I tried to tighten it really hard, I applied loctite at the end of the screw, but nothing helped.
On my last ride, the crank started to creak before it became loose. I think the screw cap is really tight and that is what's creaking and that is what unscrews the screw, but I am definitely not sure.

Do you have any ideas or suggestions?
Right now it is ok for 1 km before the screw comes undone.

[Bill]

EDIT: EDIT BELOW....

INSTALLING YOUSELF WHILE NOT SURE OF WHAT YOU ARE DOING SOMETIMES IS NOT A GOOD POLICY! Asked before you install and take matters into your own hands! I know this and well is why I am here!

Just my opinion.

(01-08-2016, 07:30 AM)Lighttower Wrote:  I have bought a new (shimano) crankset (I believe it is called a diamond crank) for my MTB. I've installed it myself (which is probably the problem :-p), rode with it a couple hundred KMs and all of a sudden the non drive side crank becomes wobbly and after a couple of meters the screw that holds it unscrews and falls off.
I tried to tighten it really hard, I applied loctite at the end of the screw, but nothing helped.
On my last ride, the crank started to creak before it became loose. I think the screw cap is really tight and that is what's creaking and that is what unscrews the screw, but I am definitely not sure.

Do you have any ideas or suggestions?
Right now it is ok for 1 km before the screw comes undone.

---

Now I say is it "cross threaded"? Did it go in smoothly or did you have to force it into the hole when you first installed it? If you did then you cross threaded it!

[Lighttower]

(01-09-2016, 12:58 AM)Bill Wrote:  Now I say is it "cross threaded"? Did it go in smoothly or did you have to force it into the hole when you first installed it? If you did then you cross threaded it!

It was easy to screw it in, I just unscreewd it to double check, but no, it is not cross threaded, and the screw looks like brand new (well except the loctite residue).
I don't see any obvious problems (like messing up the screw) and the drive side works just fine.

[Lighttower]

I have an idea, the loctite changed color from blue to black, might it be that some grease found it's way into the screw hole and that's causing the problem?

I will try to clean and degrease it tonight, I hope that should fix the problem.

[Lighttower]

(01-09-2016, 08:32 AM)Lighttower Wrote:  I have an idea, the loctite changed color from blue to black, might it be that some grease found it's way into the screw hole and that's causing the problem?

I will try to clean and degrease it tonight, I hope that should fix the problem.

Update: nothing worked. I bought a new screw and that works just fine. I guess that screw was a bad cast.
Thanks for your help.

[DaveM]

If it is a "square taper" fitting (the hole in the crank is square) then the screw itself isn't really what holds the crank on so much. The screw is there to tighten/wedge the crank arm onto the axle tightly enough. Creaking is a sign that the crank arm is not tight enough on the axle. It will start to move around a little and this will loosen the bolt. If it gets ridden too long while loose, it can permanently damage the crank arm by distorting the square hole so it doesn't sit tightly enough on the axle anymore.

It's actually good to put a very light coat of grease on the axle where it meets the crank and on the threads of the bolt to. Locktite is fine, but it won't stop a damaged crank from coming loose.

Anyway, make sure you have a good bolt, and it really needs to be tightened down. If it creaks, tighten immediately, don't ride it loose. If that keeps happening, you may have to replace crank unfortunately. But hopefully you just weren't getting it tight enough at first.

[Lighttower]

(02-05-2016, 07:46 PM)DaveM Wrote:  If it is a "square taper" fitting (the hole in the crank is square) then the screw itself isn't really what holds the crank on so much. The screw is there to tighten/wedge the crank arm onto the axle tightly enough. Creaking is a sign that the crank arm is not tight enough on the axle. It will start to move around a little and this will loosen the bolt. If it gets ridden too long while loose, it can permanently damage the crank arm by distorting the square hole so it doesn't sit tightly enough on the axle anymore.

It's actually good to put a very light coat of grease on the axle where it meets the crank and on the threads of the bolt to. Locktite is fine, but it won't stop a damaged crank from coming loose.

Anyway, make sure you have a good bolt, and it really needs to be tightened down. If it creaks, tighten immediately, don't ride it loose. If that keeps happening, you may have to replace crank unfortunately. But hopefully you just weren't getting it tight enough at first.

Thank you for the reply, that makes sense. o.O
Sorry for the late reply, I didn't receive an email for some reason.
With the new bolt I managed to tighten the crank good enough and it doesn't move at all. I think everything is fine now (I already have around 100k with the new bolt).