Advice please re : Crank arms
 

Good afternoon all,

Ive got a GT all terra Timberline (approx 1989) and while off roading recently my left hand side crank arm came loose. I had heard that when this happened the best thing to do is to replace the crank arm, so I subsequently purchased a new one and fitted it.

After 100 miles, this one also worked loose, I subsequently tightend it right back up but again it came loose after just 10 miles!

Am I missing something here ?? any advice welcome.

Either the crank was not on tight enough to begin with, or it loosened up and then rounded out. Is it a square-taper bottom bracket? After i put new cranks on, I check them after a ride to make sure they are still tight. The problem may also lie in that the bottom bracket's square-taper has been rounded out. This would cause the cranks to wear, and eventually become loose. Check that first. Maybe take both crank arms off and see if the bottom bracket looks different on the loose side.