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?