differance between 365 and 420's
 

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.

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.

I am pretty sure they use the same pistons also

That one I can verify , different pistons 502 mpi , hp 500 the compression. Bump comes from the dome .look close and you'll see it . Bout 9cc I believe . Je forged otherwise

Still wanna know if op has one of each ,those 454's , sitting on floor somewhere . I'm a big fan of mystery motor teardowns

Gen vi 502 mag piston #737-827664

Gen vi HP500 piston #737-827664

Mercruiserparts.com

and that is a j&e forged piston.

.510/.510 lift on the 365hp.

Cranes catalog says ( http://www.cranecams.com/114-119.pdf ) the H228 is in the 420.
I used the 242 in my 365s.
I just saw the crane #s are a little confusing.
The 420 cam is the H228 aka 132561. In my 365s I used H286-2 aka134242

Had 6 of those 502s apart at around same time 2 hp500 carb 1 efi 2 502 mag gen5 efi 1 gen 6 the blue motors had that little extra dome next to the 502 mag haven't found a shred of evidence anywhere to suggest they came from merc.that way but were standard bore .I feel like that hokey chopper pilot from mad max movie .."all this time". Have a 365 just came in though ,popped 4 freeze plugs into the bilge over winter , so far no cracks ..

Actually a couple reasons. I'm pretty content with mid power engines and have a few rebuilds going at the moment with both 420's and 365's. I am rebuilding the 365's in my old ScarabIII and would like to add a few ponies using 420 spec cam and stuff, They are a very well kept pair that had a charmed life and i think they would respond well. I am also considering a pair of 420's as partial trade on the 600 Hawks i have listed in the swap. Just want to know whats in 'em and always wondered what the diffs were. Thanks all great info . . . .

Now what about the 575 that was introduced with the 420?

The 365's woke up nicely with some upgrades.

Merc also made some HP450's for a short time. Those were hyd roller, same camshaft as the HP500 I believe, single plane dart intake, and holley 800 carb.

I'm not a ''ANTI" flat tappet guy, but I definitley think going to a modern hydraulic roller setup, really is money well spent. Not just from a power standpoint, but, you can gain idle quality, overall efficiency, increase the useable RPM range if desired, and so on.

I'd like to think with a properly spec'd, hydraulic roller, with the right intake/carb/exhaust, you could be well over the ''420'' hp range, without sacrificing idle quality, or valvetrain durability. If anything, it would improve all that.

I still think you should go thru those Hawk 600's, and run those in the Scarab! :coolcowboy:

That was a completely different engine. I believe they are tall deck 540's (not to be confused with the 575SCI variant that was supercharged and FI)