This might be a DCC question, but since it's n-scale, I'll put it here. I installed a drop in decoder (NCE N12A2) in an Atlas GP7. Motor runs fine, very happy in fact - a nice change as the drops in don't always work so well getting those stupid motor tabs to make good contact with the decoder. So all seems well, a quick install, runs great. Uh oh - now the shell won't go on. I can BARELY squeeze the thing on, and it won't go all the way down. It looks like there is a slight bulge in the middle now - the area on the shell above where the fuel tank would be. The motor frame does not look like it could possibly go together any tighter. There are no gaps between the decoder board and the frame. Nothing obvious that I can see. Any suggestions? Interestingly, this has happened to me before, and always with the GP7/9 models. I've not been able to solve this.