502 DIY due to carppy job
 

With out getting into the long winded discussion, I have a 502 with less then 100 hours that needs to be rebuilt, and I'm doing it my self this time. I have rebuilt many engines but never a marine engine; what are the does and don't for marine that I should pay attention to that are different then auto engines. The engine will go back into a 94 Formula PC27. I installed a Mercruiser EFI system and through hull exhaust; I plan to use aluminum/stainless exhaust systems this time. What other items would be of benefit? I’m open to using other different distributors or timing products, in short what items would be of the best for performance and longevity.

Kind of a loaded question. A good aftermarket aluminum manifold setup is a good start. Camshaft selection is critical for marine apps. You want good exhaust valves like inconel. The stock merc distributor and ignition system works good and goes hand in hand with the efi setup you have. Marine starter, marine alternator, marine flame arrestor, should be used. Always go with a new oil cooler if you thrashed the old motor.

Is your 502 a original mercruiser engine or chevy crate engine??

Mine was a crate engine which I had to pull three times. On the third time I had the heads redone with new valves and the engine started running pretty good. A season and a half later I must have cracked a ring because it's pumping oil. I have a Mercruiser 502 EFI system on it using a VST. Thought about changing it out but this is a single Formula PC 27 install.

Keep in mind that marine engines run "on load" much more than an auto engine. Plus, the blocks run a lot colder. So, the piston to cylinder wall clearance has to be greater than an auto engine. This is due to the fact that the block is cooler and the pistons are hotter. Also as mentioned earlier, you will need better exhaust valves. Constant running under a load thermally stresses the exhaust valves because they don't get a chance to rest on the seats at low rpm's to cool down.

Thanks for all the insight; Looks like at the end of the this season, I'm either going to pull the engine and rebuild my self or replace the etire unit with a 525. Kinda leaning toward the 525, I bet it would make this cruiser move along quite well. The 502 415hp has been doing well, but when I was trying to get on plane 2-yrs ago with an injured girl and all the folks it sure had a hard time getting it up.

The story in short, the Formula PC 27 weights in at almost 5 tons. It had one 454 which Formula no longer uses a single install in a PC. So I bought a "marine" 502 (that's what they called it) crate engine from Massachusetts web site to replace my 454, engine had problems from the start. Sent the engine back it came in with 454 valves in a 502 head. Pulled engine had head fix with new 502 valves, ran great for 2 1/2 seasons, then started pumping oil. Hind sight should have torn the entire engine apart then. That's the story, hope it was short.

What reed said... if the ring end gap is too small (like what you would want in a car, is too small for a boat) it can actually close the ring. If it needs to expand past completely closed, you guessed it, broken ring. Forged pistons need a little more clearance, but that's probably for boat and car?

I used to take and have my long blocks done... I had very bad luck with the last couple so I also decided I would do everything myself... even to the point of watching during cylinder and line hones. Now if I blow an engine I have no one else to blame... and I actually like it that way.

Thanks guys, I'm starting to look for a 525 EFI complete w/XR drive to replace it or I'm going to go completely through it. I like the idea of a 525 in a 27PC, I think that would be just enough to be the fast cruiser on the lake, but the cost may be prohibitive.

After almost a year I pretty sure I have a plan. The plan is to pull the engine, replace the heads with aluminum ported and polished, then rebuild the lower end. Who would you suggest I talk to about the heads, there a a few in the classifieds to choose from?

you are going about this all wrong....you need a very high torque engine...the H/P engines are going to require more RPM than you will want to cruise at....you can build a 502 fine, used a good pair of oval port heads/intake a good cam for torque, call Bob @ marine kinectics on cam selection...might even conside a stroker kit to a 540 during your rebuild, a bravo II or III or volve duo-prop drive, that 525 and drive you are talking is a waste of $$ in that boat...

I'll call Bob but going from current rectangular port to oval port doesn't sound like a good idea.