It's evolution.
The MC16E design intent was sound from day one, which is why it saw a 10 year production run, and was the basis of 6 years of customer GP motors. Compare that with the next best thing, the TZR/TZ, and you soon realise Honda hit the nail on head with the very first concept [designed in 1986]. However, each model is an evolution, including the R5/6K over the R2/4J, except the MC28, which is largely the same as the MC21 for the sake of this example.
The MC16 has small exhaust ports, and consequently matching small RC Valve flaps. The MC18 ports are significantly larger, so of course the valves are too, which obviously necessitates the use of a different controlling mechanism. The MC21 ports are similarly sized [actually a little smaller], but reshaped with matching RC Valves, so again require a change to the key components to actuate them. Bear in mind each model controls the sweep electronically, as well as mechanically.
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Andy.
NSR-WORLD.COM
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