RC Monster Electric Motor Mount Revo Question

Ok this maybe a stupid question I just got the Revo RC Monster Electric motor mount and I am using a Neu 1515/1y finned motor and the rear support does not seem to fit because of the fins on the can, I see there are 2 rear supports in the kit for bigger and smaller cans and the bigger one almost fits, if the fins on the can were not there it would...or am I being an idiot and not doing something right?