are the 365 and 420 long blocks essentialy the same? crank, rods, heads that stuff? I know the cams, exhaust and intakes are different but was wonering about the basic platform.

Yes, they were both based on the LS6 platform. -Rectangle port heads, forged pistons/rods w/ 7/16th bolts, 4 bolt mains, steel cranks.

Cam, intake, carb and exhaust are the only significant differences. I think the 420's were all Mark IV's and 365's were Gen V's.

I pretty much duplicated a 420 when I hopped my 365hp/454mag.

The original 454 mags that were in my boat were 365hp and were Mark IV's.

thanks guys, I had thought they shared the same internals. whats the differance in the intake manifolds and carbs? the ones on the 365's in my old Scarab III were aluminum is the 420 intake bigger or something?

looking to do the same with the cam number you gave me. thanks

365's were dual plane w/ Quadrajet and standard Mercruiser log manifolds. 420's were single plane w/ Holley carb (800?). 420 cam was quite a bit larger and it had roller rockers and aftermarket exhaust (Gil/Merc?). 420's were used in front of TRS drives and the 365 came out in 88 or 89 and was in front of the Bravo.

365HP mag, was the same as the old 370HP in front of TRS, which was out for a while I believe before the bravos.

Now, I could be wrong. But I could have swore, that the 420HP versions, were assembled at mercury performance. They werent listed as a production sterndrive engine, which sounds about right. The 365HP had ALL GM parts. The 420, had crane supplied cam/rockers. I also believe, the 420's, were treated to a high performance valve job, where the 365's we not. I have no evidence of that though. But yes, the 420/425hp 454's had a crane flat tappet, GIL exhaust, crane gold rockers. The intakes I believe are sketchy. Some seem to have had the Weiand Team G intakes, some maybe the GM aluminum dual plane. And like BGIII says, 365 had the quadrajet, 420 a holley 800. There also was a 400HP cyclone, which was the same as the 420(same cam/rockers), except quadrajet, and log manifolds (no gils).

The short blocks have the same parts, GM steel crank, 7/16 dimple rods, and GM small dome LS6 style forged pistons. All pretty good stuff.

The 370's had cast iron exhaust exhaust manifolds. In 1988 Merc introduced the 365 Bravo package which came with Merc-Gil aluminum exhaust manifolds. The 420 had a alum single plane intake (Weiland) that was not a great manifold. There are much better intake manifold performance choices today from World Products (w/brass inserts) or from Dart.

The 365 mags came with iron center rise manifolds just like the 370s. You didn't get GIL style manifolds with them. The 365 was a basic engine that was installed in many offshore boats, as well as many smaller runabouts. The popular mod was a cam swap and exhaust upgrade. The early 1987 420s actually had a huge performance cast iron manifold, that flowed well, but had leaking issues. Merc dropped them and went with GIL.

Mercruiserparts.com check out the exhaust for the 365, then the 420/425 engines.