Crank Arm bolts

Loctite (medium or similar) is the answer.
Degrease the threads before application.

This article

http://sheldonbrown.com/brandt/installing-cranks.html

gives a technical overview of the whole assembly, for the interested. The square-taper crank interface is interesting and unusual, and it isn't intuitively obvious quite how it works.

The reason the bolt keeps falling out is that the crank hole is already enlarged, from some earlier time. It's possible that loctite on the threads will keep everything together (I hope so!), but it may be too late, and the crank is effectively destroyed. (On a well-matched and properly assembled arrangement, loctite certainly isn't necessary)

Either way, the only way to retighten a crank bolt is with the crank already off. Repeatedly tightening a loosened bolt against a fitted crankwill eventually result in a split crank. In your case, I'd remove the crank, and reassemble with (the less hardcore) loctite on the threads, using a torque wrench to spec (if you have one).

If it still loosens after this, write the crank off and get another.

timbo_tim wrote:

Thanks for the info above, however I am confused, I thought the crank bolt screws into the "arm" that is part of the square taper BB. Therefore if it keeps coming loose (which it does despite sparingly and liberally applying loctite (243) could it imply the hole in the BB it's being screwed into has become enlarged? The crank, as far as I can tell, doesn't do any screwing, it's just the bolt into the spindle? In which case is it time for a new BB rather than crank?? Am new to this so am prepared to be massively wrong!

No problem Tim, this isn't an easy interface to visualise. The crank bolt screws into the BB axle as you say. Its purpose is to drive the crank onto the BB axle, up the taper. You'll notice in a disassembled unit that the taper on the BB axle is matched by a corresponding taper in the square hole on the crank.

The interface works by preload in the crank itself (and to a lesser degree, the crank bolt). The bolt drives the crank onto the axle, expanding (preloading) the crank hole just enough that pedalling loads are as well supported as possible, but not so much that the crank is damaged (this is a fairly wide target, but most manufacturers give a torque spec. It's more than you might think). If you're interested, you can read a far better description of the entire assembly than I can write, here:

http://sheldonbrown.com/brandt/installing-cranks.html

When things go wrong, which usually means the bolt fell out somewhere back there, the casualty is the crank itself, which- constantly mobile on the taper- has no limit on the outside, and falls off. By this point, the crank has been ridden "floppy" and the tapered hole is now rounded and a poor fit on the BB axle. I suspect this is the scenario here, even if the crank never actually made it off the bike.

If you still think the crank has a chance, refit it tight and see what happens. I suspect you need a new crank, however, which practically speaking means a new chainset.

timbo_tim wrote:

Thanks for the in depth explanation that was very helpful. I am surprised the crank is defunct though as its a fairly sturdy single speed (Pearson touché) and relatively new but I guess it is what it is. Thankfully the crank has yet to come off, but it does wobble once the bolt comes loose so I tighten it each ride.

So would you say a "rounded" crank would cause the bolt to loosen of its own accord?

Yes, because the bolt is never preloaded enough to maintain tension through the pedalling cycle. The only other cause is that you aren't reattaching it tightly enough. That's hard to judge without watching you do it up but the torque spec is about 45NM from memory, which is "pretty bloody tight". If you suspect this, take it to a bike shop and ask them to attach it.