Originally Posted by
Wariat
Forgot to ask: is there much of a difference in quality in terms of two-piece versus three-piece cranksets for mountain bikes? Only reason I ask is that the stock two-piece (I think it might have even been one-piece?) crank on my old BMX bike was pretty cheap despite being steel. I eventually moved up to a forged three piece crank that was great and incredibly sturdy.
You probably moved to quality cranks. That's what's up here. Anything is possible but generally speaking what happens is that the left arm is more flexible (it doesn't have the additional structural design/material that contributes to stiffness on the drive-side). This slight flexibility does act on the retaining bolt which holds the crank to the axle, it stretches the bolt minutely and at the same time it acts, like a good lever, to move material around the square taper of the axle. All this is very slight and generally is not significant enough to loosen the crank arm. However, provided enough force and enough bolt stretch/movement, the crank will work continuously to enlarge the hole into which the axle is stuck.
The 2-piece design offers a few advantages. The entire structure, cranks/axle/bearings, is much stiffer because the drive-side crank has the axle spindle permanently affixed and this spindle is of a significantly larger diameter (eg stiffer) than the crank style you currently have. There are some measurements that I am not able to determine so, I'm afraid, I can't make more than general suggestions.