Ah ha, yes they do all start mc16, no they are not all the same.

Why the engine numbers didn't progress with the model only Honda know, mc16, mc18 and mc21 are different for sure, I can't see any reason why the mc21 and mc28 cases aren't identical, they certainly look it and the cranks are interchangeable though not identical.

mc16 and mc18 have other issues and you should be very specific when asking what fits where.

The post seemed (to me) to be asking if the an engine number starting mc16 on an engine in an mc18, mc21 or mc28 meant the engine was wrong and they answer here is no.
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