Another possible issue is interference from the bolt that secures many BB cable guide onto the shell's underside. The bolt can protrude inside the shell and contact some BB units but not others. NE Ohio has many shops which can chase (and I suggest facing too if chasing is done) the shell to insure good thread conditions. And these same shops, hopefully, have the skill to examine the threads before blindly running a chaser through to see if there's any prior issues. A empty shell chasing often costs around $25. Priceless to keep from causing more thread damage from over enthusiastic leverage. Andy.