I think you are right but wasn't there a point that the Merc/Gil manifold was used on Merc engines other than the 420's?

Believe the
420 got into the cyclone group , a bit more compression,pistodome like the 502 to hp 502

Stupid phone of mine . If you are matching a late 80's 420 it has unique pistons .little bump in comp over the 454mag

Both engines take (365 and 420) merc part #737-5536T piston. They were a GM LS6 small dome piston. The 420HP 454 was not labeled ''Cyclone".

The Cyclone engines were 370/400/440/460HP versions. They all got the same 737-5536T piston, except for the 460HP version.

Alot of these questions can be answered, by looking at the parts diagrams and drawings on mercruiserparts.com

Does anyone know the cam lift on the 365. I know the 420 were 530 and 551.

Wonder where the hp420 gets its comp bump from .I'd swear they were built like " mini hp500's' been a while since the '80s though .going to have to check da books tomarrow now curious

Important question for the o.p. why do you ask ? What are you planning ? Matching a 2nd motor ?the 420 was for sure not a common unit to come across these days

Like I was saying earlier, the 420HP engines were assembled at mercury hi performance back then. One of the other things done to them, was larger intake valves. The 420's had 2.25 intakes, 365 had 2.19. You are correct that they state the static compression ratio of a 454 mag at 8.6:1 in my book, and the 420 at 8.8:1. My guess is maybe they installed larger valves, custom valve job, and a little head milling.

Most guys assume a merc rect port head is the same from a 454 mag, to a 800sc, but they were assembled with different components per application.

Could be indeed ! Don't have 420's (I still reference them to the mini hp500's ) rolling through these parts much .do know for shure the 502 versions get a little xtra dome on the hp500 . And roller rockers on the 420hp

i believe the compression ratio on the 415 hp mpi engine and the hp500 are both 8.75/1.