Left pedal coming loose On my bike, which I just built out of spare parts, the left pedal seems to constantly be coming loose. It seems to come out about 5m every time I ride ~10km.

It's an old cotter pin crank, and the pedal is a left pedal (there's an L stamped into the end of the screw that goes into the crank). The pedal has a crack in one side of the axle compartment, but I don't think that would affect it.

I thought that these pedals had left-hand threads so that the action of pedalling would work to tighten the screw? If so, why would this be happening, and is there anything I can do about it short of replacing the crank and pedal?
What I'd do:


remove the pedal
thoroughly clean the pedal threads
thoroughly clean the crank threads
let both dry
apply 2 drops of thread locker on the pedal threads (e.g loctite blue)
fasten pedal on the crank well (e.g 40 Nm)


If it comes loose after that I'd temporarily try another left pedal just to find out whether the pedal or the crank arm is problematic.

Before step #5 you need to inspect:


try to see whether the threads of pedal or crank look worn
screw pedal in crank by hand (or tool without applying a lot of pressure) until the end
was the threading smooth until the end?
does the pedal have any serious play within the crank arm? (which can only be eliminated if you screw the pedal very hardly in?)
if yes, then the threads may be worn


Sometimes people screw pedals in crank arms with no care or patience and the pedal may not be positioned correctly to be screwed all the way in because its threads overlap badly with those of the crank arm.