It seems like the early 1950's 25hp Johnson RD-10 through RD-17 series motors (and the Evinrude equivalent), evolved into 35 hp motors around 1957 (RD-19 through RD-21), then became 40 hp motors in the 1960 or so (RD-22 through RD-30). I think I read that the RD series were around 38 cu. in. displacement, although I don't know if that is correct. That seems large for a 25 hp, about right for a 35 to 40, perhaps.I have three questions:1. Am I right about the evolution of this motor?2. How did they do it? Did the displacement increase by changing bore and stroke on the same block, or did they change other things like carbs, ports and exhaust--or all of the above?3. How much parts interchange is there among the various generations of these motors?I'm picking up a 1954 RD-16 this weekend, so I'm trying to education myself about them.Thanks!