Locktite on crank arm bolts?

[Christopher Brunsdon]

Had a stunning day, besides snapping my shifting cable I discovered my one crank arm bolt was loose (again). Its a square taper. Now could one use Locktite and if so, which one?

 

Dont feel like doing a ride and then getting stuck with a loose crank arm.

[Johan Bornman]

Mr Brunsdon, you've been here long enough not to be forgiven for calling a crank, a crank arm. There is no such thing as a crank arm. Consider yourself warned.

 

Now, back to square taper and loose bolts. It's a very interesting phenomena that one.

 

The bolt didn't actually come loose. The crank got tighter. A square taper is one of the few things that tighten itself on a bike - other than that stupid DT Swiss hub with the rubbish clutch system, but that's another story. AS you know, a square taper crank is pushed onto the axle with the bolt. This strains the aluminium in the crank (and of course the steel, but to a lesser extent) and it pushes against the bolt in an effort to escape the strain. However, now you pedal and the trailing edges on the crank are now relaxed and the leading edges compressed even further. As the trailing edges relex, the aluminium squirms further up the axle and so relieves the axial stress.

 

Should you now stand on your cranks on a downhill, the opposite happens. The trailing edges are compressed and the leading edges relaxes, causing the aluminium there to squirm further up the axle. The reason they squirm further up and not down, is 'cause the crank bolt prevents it from squirming down. The net result is that the crank has moved further up the crank and the bolt is now loose. It often falls out, but the crank remains in position. If the original torque was correct, the crank will now not come loose by itself and always remain tight.

 

Therefore to answerl your question regarding loctite. No. Just torque the crank to the proper torque and this won't happen. Always torque a square taper wet - that is with grease, not without grease. My guess is that your bolt was not tight enough to start off with or, you didn't grease the axle before putting on the crank.

[Tiny K]

Cool! Now I know this too..... but what of an octo-link BB? My non-drive side came loose over time quite recently. Was it just that it wasn't tightened enough in the first place?

[Christopher Brunsdon]

Thanks JB - the crank did come loose though as I tightened it with an allen key. I have other bolts that can take a socket and will rather use those instead. Need to get the proper tools still.

 

(PS: Yes, I see now my mistake of calling it a crank arm, was tired last night and broke my golden rule of not posting when tired)

[Christopher Brunsdon]

Cool! Now I know this too..... but what of an octo-link BB? My non-drive side came loose over time quite recently. Was it just that it wasn't tightened enough in the first place?

Was my left side.

[Mojoman]

Last year I tightened the bolt a bit too much and could not get the crank off! Had to take a dremel and cut it off the square taper BB!

Tried cycling around to loosen it, hitting it with a rubber mallet but it was futile.

Thankfully I had bought a new Deore crank and the old one had done many K's but I was not happy, I had planned on using the crank on my other bike but ja, the biskit kan like to brake that way four me!

[javadude]

How much torque are we talking about? I have always been a bit wary of over tightening the crank, but it seems as though its a pointless concern if the crank is going to tighten up itself anyway.

[SCHWINN LOVER]

Ok so after all that the answer to you question is:

Use Loctite thread lock but the non permanent one ,with the permanent one the nut is never coming loose again but the non permanent one will loosen if you use a spanner and a bit of force.

[Christopher Brunsdon]

Thanks Schwinn Lover, I could not remember which loctite to get, have been warned before about the different types.

[AndreZA]

How much torque are we talking about? I have always been a bit wary of over tightening the crank, but it seems as though its a pointless concern if the crank is going to tighten up itself anyway.

 

It is tightening itself because it is not tight enough. But too tight might damage the thread or the crank as you trying to force it to go further then designed for.

[Just Keep Pedaling]

"How much torque are we talking about?"

 

Until your veins bulge and your eyes want to pop out of their sockets - then half a turn more.

[Mackie]

Turn it until the bolt cracks, then loosen slightly.

[Big H*]

"How much torque are we talking about?"

 

Until your veins bulge and your eyes want to pop out of their sockets - then half a turn more.

 

Go and look on the Park Tool website, I got a table with torque values here:-

 

http://www.parktool.com/uploads/files/blog/torque.pdf

 

Make sure what the values are they give in the table. There is a conversion table on the last page.

 

Buy yourself the Park Big Blue Book of Cycling Repair.

 

IF IN DOUBT CONSULT THE MANUFACTURER FOR SPECIFICATIONS

[AndreZA]

35-50nm

 

[TNT1]

Many moons ago, my left crank (square taper, hell, it was all we got back then, if you don't count cotter pin jobs) came loose going up Red Hill, my dad had to push me to hout bay (i think, some sh1tty little coastal town anyway...) and we stopped at a car garage to tighten the bolt.

Edited May 11, 2011 by TNT1

[Bat-ass]

Ok so after all that the answer to you question is:

Use Loctite thread lock but the non permanent one ,with the permanent one the nut is never coming loose again but the non permanent one will loosen if you use a spanner and a bit of force.

 

Loctite 243.... it medium strength.... developed for threadlocking in applications that need to be disasembled with hand tools!!!! otherwise JB's advice is normally sound!!!