Looking into some really nice boats but with some higher run hrs. When is it time to pull and refreshen heads with closed cooling systems. For example 496 merc. With 500 hrs.? Is this a lot of run-time hrs for a motor with closed cooling? I have heard of guys running 496 motor/motors with 700+ hrs and no problems.

I did a lot of research on this same question 2 years ago when I bought our 32 Sunsation w/496 HO's at 475hrs. I am know at 535hrs without issue, I can tell you my research here on OSO came back with 700-900 hours as long as you leave them stock, keep the oil clean and don't abuse them. Just make sure you have leak down and compression check by a qualified mechanic before you buy, if you run a search you can probably find the thread I started when I asked the same question. A lot of people with more engine knowledge than me chimed in with things to consider.

Your not going to hurt a stock 496, it will run a long time if left as it is

My 496HO had high 300's hours when I bought it. Leak down and compression were good. Almost 500hrs and recent same test showed no change. Keep them maintained and don't overheat them.

Do you know and could you share your leak down percentage and compression numbers?

Someone will jump in with the acceptable ranges....I found my report which I have below and was told they were acceptable for my engines at 475hrs. My motors burn almost no oil between changes every 50 hours which for me is a season.
Port Engine Compression ranged from 149 -156, Starboard Engine ranged from 148-155
Both engines were 10% -12% on leak down. and I am assuming both test were done with the engine cold as I bough the boat in Iowa in late winter.

no more than 15% with a variation between cylinders of 4 to 5 percent

Thank you bigredbaja, I appreciated you giving me the info. I'm aware of the ranges the motor should be in, just use to dealing with raw water cool motors.

I have seen several head gasket failures at our marina in larger, non performance applications, couple led to severe engine damage, some could have been run hot no idea, also 1 was result of a clogged fuel filter on a volvo setup, now this is a B/S deal as you cannot change the filter without replacing the whole pump, about $1400 later. This was a friends boat and sat alot...so there can be issues as with anything

IMO a 496 that was maintained and not abused should last 1000 hrs easy. I have heard of some running 2000hrs. The 496 is used in a lot of generators and water pumping stations.