I don't have all the differentiating details down, like some here do, but I do know a Carrera and an S90 were different in many ways, the roller crank being chief among them. I had a S90 for a while, and its main difference vs a Super included larger valves, (exhausts were sodium filled), differnt heads, different pistons (higher compression ratio) and what was called alloy cylinders, vs cast iron. Also had 40 PIIC dual Solexes, I believe. Said cylinders were nickle-chrome coated and dimpled to hold oil, and so allowed running the engine for long periods at racing conditions. Brand new, these cylinders used about a quart in 500 mi. One fine day (in '68, it was) while running up a very long incline to reach the top of the Blue Ridge mtns, the engine went funny, and soon enough I realized I had burned an exhaust valve. So fixing that gave me an opportunity to reconsider the deal. At recommendation of the shop, I replaced the valves and I put in Super pistons and cylinders and kept the S90 heads. Lost a little in compression, but gained a whole lot in terms of reliability and oil consumption. I really did not notice a difference in the scoot quotient. Barry, if it would be of interest to you, or any of the others here, I still, to this day, have the pistons and cylinders that came out of that S90 engine. I heard that one could bore out the cylinder and resleeve it with a cast iron sleeve and get the benefit of the alloy cooling fins, but not have the aggravation of the dimpled ni-chrome. Obviously, the ni-chrome could not be bored and/or honed to refurbish them. They were a one-time deal. Anybody want to have these parts? And if so, why? Could they be used in a Type I or Type IV build?? You'd have to do the resleeving, as mentioned. Whaddaya think?