gyozadude

I'm a bit surprised precession got into the discussion. The crank arm attachment using a tapered spindle isn't supposed to have clearance or any rotation relative to the spindle. Precession only comes into play when you have bearings and clearance between the two axis of rotating parts that the bearings separate. These conditions do NOT apply to the crank arm attachment.

The reason the left crank arm bolt loosens is because the crank is loose on the spindle and rocks back and forth. The taper encourages the crank to continue to slide outward. Downward pedaling force adds lateral outward pressure as well on the loose crank so it contacts the nut/bolt face more so the torque on the nut/bolt face tends to loosen on the left side, thus allowing greater rocking back and forth, and the bolt eventually loosens. On the right side, outward force on the face of the nut/bolt generates a torque on the face that favours rotation that tightens the bolt. So it tends not to loosen. But this isn't precession in terms of a counter-rotation due to bearing clearance.

As for Lasco crank arms, Huskybicycles.com used to carry Lasco cranksets. Don't recall them carrying 165mm cranks. Only 170 and 175mm. You might call them to see if they have any. Price for left side arm was just $8.90 plus shipping.