I built a standard a2 upper and to get the shots to hit the center of the target i had to crank the rear sight 18-20 clicks to the left. Which side of the index notch do i have to file to get the sight  reasonably centered?

did you check to see if maybe you need to rotate your front sight base?  If it's pinned I know that's no help, but not having any details on your build, that is the first thing I would worry about.

+10

Yep, sounds like a canted front sight. Barrel torked to one side? Gas block canted?

The barrel is a bushmaster 20" gov't profile with a standard pinned FSB. The upper receiver is an a2 with m4 feed ramps from pkfirearms. The FSB seemed pretty straight to me but its probably canted.

If you want to spare the time, you can pull the barrel and check if the indexing pin is aligned with the front sight post.  If not, call Bushmaster and trade it out.

If it is aligned, check the index pin cutout in the upper receiver threads...  if it's off center, that could be your problem (call PK)...  if it is centered but slightly oversize, that could be another problem (you could try to manually center it and watch it while tightening the barrel nut).

Thanks ill pull it back apart and check.