Chris here is the deal the best way I know it after a learning curve when I first started doing these. The early motors up to some where around 81 used a can that was 4" diameter and bottom bolted. At about that time they then went to a 3.5" diameter can that was bottom bolted and then at some point in time, I don't know when, switched to the top bolted cans. I am also only referring to the bottom mounted cans. A good example is my 81 very original Euro 635 with an M90 that has a 3.5" can that is bottom bolted