Looks like I may get my turn, have pair of 91 502 mags 700 hours, going to go thru them for peice of mind, and performance increase, after reading all the posts last couple years here is what I have in my mind. Blocks recon whatever they need bore etc, thinking the 420 cam, crane kit or edelbrock and matching components, probably just roller tip rockers,
air gap intake, going to newer 496 alum ex, my heads- port/valvejob should I plan on new valves?, carb thinking 800-850 should I go mech sec or vacuum? spreadbore or not?, mostly use trips, once in a great while WOT. appreciate any input Randy

Most definitely new valves and springs. use a square bore mech. holley. I would get RMBuilder to grind you a couple of cams. I wish I would have now. Get someone who knows the clearances on marine engines to do your work for you.

Good advice---do yourself a favor and call RMBuilder/Bob Madara of Marine Kinetics in New York for the right cam for your application. Do a simple bowl/pocket port short side radius job on the heads, add stainless valves, bronze wall valve guides, a 800cfm Holley carb is more than enough carb, roller rocker arms. Just remember that the first thing you must do for your engine to benefit from any hiperf upgrades is to add a easy breathing aftermarket exhaust system. A manifold exhaust system like Stainless Marine, Dana, GIL or something comparable to those----and keep in mind that all exhaust manifold systems are NOT the same when it comes to primary runner design.

And it takes a machine shop to do the correct machine work with the correct clearances to make a hi-perf offshore marine engine survive. Have fun.

By the way, I see that you're in the western Michigan area.....I would highly suggest Performance Engineering in Jension to do your machine work. They can also assemble them for you as well if you want. They have a very good reputation for doing hi-perf offshore marine engines---they know what it takes. RMBuilder has worked with them on marine projects in the past.

RMBuilder's phone number 585-654-8583

Ateco will get you to 600+ hp with some mild work done to the engines while they rebuild them. Give Bill a call. His other customers love what he has done with their 502's.