How long will these engines last with good service history? I am looking at a boat with 850+ hours showing on the hour meter. The seller does not know ecm hours. Should I go into this purchase knowing the engine will need to be completely refreshed sooner rather than later even if it runs fine and doesn’t excessively consume oil? I’m working on trying to have a shop do a leak down but I may have to make do with a compression test when I go look at it.

Should have some good life left if properly maintained however boating is expensive anyway you look at it.

that’s very true. Gotta pay to play

The 502Mag is a great engine. With just moderate care they will run and run. You can have it scanned to see hours spent at various RPM. amazing how much time gets spent at idle to 1500. Anything that happens after 1000 hrs you can't complain. The engine owes you nothing at that point , however I would not be at all surprised to see 1300 hours out of one. Oh, if yours doesn't use any oil I would be shocked. Seems like alll 502 use some.

850 hours isn’t automatically a problem if the engine was well maintained. Service history and even compression across all cylinders matter more than hours.

May not need to rebuild the engine, but you might want to have the fuel injectors serviced if they haven't been done already.

If the boat in question uses the stock VST system, you may want to alter the fuel return hose to prevent vapor lock issues. If it hasn't been done already.

Not trying to scare you. The fuel system can use some updating due to age.

In a performance boat I would plan to rebuild it sooner than later and enjoy knowing it's good!

General consensus merc black engines 400-500hr top end freshening and 800-1000hrs bottom end.

Blue motors usually are 250 and 500.

I did 1100 on my blue engines without touching anything
i would guess 1500-2000 hours on a black one should be easy !

In reality that is a pretty low HP motor. 415HP out of 502 cubic in.

That is very under stressed. I have over 700 hours on my HP500EFI motors with only a top end and they are doing just fine.

it will most likely start burning a good bit of oil before the bottom end would let go.

my buddy finally rebuilt his mags at 1500 hours, they were just starting to use a bit of oil, no knocking. He had did the top ends once.

BTW on these motors it seems the way mercury set them up they all burn a bit, i have noticed if I idle a lot i can burn a QT in two weekends pretty easy. (150-200 miles).

If we go out and I idle 20 min out the no wake zone and run south 45-55 miles and back it will not use any.

spent time at idle and sure
enough need to add a little.

if you can pull the hours and see what amount they spent at idle, between 1500-3500 and above 3500 that will tell you a lot. every one says they "run their boat hard all the time" but the reality is that most are at less than 4000 95% of their life. back in the day when we raced the hp500 and hp525 those engines saw 90% of their time at 5000+ and still we only did the top end at around 115 hours and that was because of the abuse the valve train was taking. in several years I think I only saw one bottom end let go. these things are pretty tough for what they do. maintenance has more to do with life span that anything. and yes they do use a little oil but remember, my '96 dually with a gen 6 454 the acceptable oil consumption from chevy is one quart every 1,000 miles so they expected them to use some oil.