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.