Worn out square taper bottom bracket spindle? I got an used bike with a worn out left crank arm. I tried replacing with a new square taper crank arm and torqued it to 40nm; however, it started creaking soon after and the bolt had become loose (I checked with a torque wrench).

Could the bottom bracket spindle be worn out as well? I did notice a very tiny bit of wiggle when I fitted the new crank arm before tightening the bolt.

Edit: the original worn crank was steel.

The square spindle (the best pic that I took):
The spindle looks a little rusty. That may be what is causing the the creak. Try completely removing the rust with a wire brush and thoroughly cleaning the crank and spindle before re-installing.

Be aware that the bolt can loosen slightly in use as the crank 'climbs' onto the square taper. See https://www.sheldonbrown.com/brandt/installing-cranks.html