I took off my A2 flash hider to remove my FSB and install a YHM flip gas block sight and then went to put the A2 back on and no matter what position I try to to start with on the threads I cannot get it back to the original position with the ports facing up. Is there something I need to do or just keep trying to put it on right. I was talking to a friend on the phone for over an hour and twisted it on and off and couldnt get it right. HELP ME!

Twist it until it indexes properly. A barrel vise will help to keep the barrel nice & secure & also so you don't over torque your barrel. Are you using a crush washer?

If it is a flat lock washer it may not return to the same position, but a peel or crush washer will work much better for you if you cant get it.

Its a cruch washer, I also tried to put on a Yankee Muzzle Break I have and it wont go on right either, it came with a crush washer.

Get a barrel vise & put it in a table vise. Tighten the FH until it indexes & there you have it. Or, you can evenly sand down the muzzle side of the crush washer for the wecsog way until the FH lines up. I prefer the vise method as it allows you to tighten the FH on the proper way & is the easiest method.