All depends, a well maintained stock motor may last 1000hrs. In salt water it may last 500 hours. If the exhaust is leaking water it may not last 300 hours. I remember going through this thought process when I bought my boat with 20 hours on a rebuilt 454. I thought it would be good for 500 hours, but the guy used non-marine aluminum heads so they didn't make it 30 hours. Don't make my mistake.

TexomaPowerboater, so would a surveyor or local boat mechanic have caught this issue with non-marine heads or do you have to dis-assemble the engine somewhat ?

Please educate me, what is the difference between marine and non-marine heads ?

What else should I be looking at (in terms of parts) when the engine has been rebuilt to make it marine worthy ?

In our previous boat, we put about 800-850 engine hours (mostly salt, flushed after every use) on a stock 496 in a little less than 4 years. No compression loss and no oil consumption. Just sold boat last month.

All I had to do was keep up with all the maintenance and oil changes. Avoid overheating like the plague. Actually broke it in per manufacturer directions but this boat did spend good time at both the lower and higher RPM ranges during use.

Definitely one of the top reasons for engine failure.

LOL, I had mine surveyed AND thoroughly inspected by my most trusted merc mechanic. Even did a compression test. A surveyor would not catch that. He might if you have him do a compression AND leakdown test, but that's only assuming the heads are already leaking. My non-marine heads were pro-comp and designed for drag racing. They were made of aluminum and had no hard coating for corrosion (i.e. non-marine) so the water passages corroded to the point that the gaskets couldn't hold a seal (on one engine), the other engine dropped a valve. They were designed to use anitfreeze which doesn't corrode. There are other differences in marine vs non-marine heads, but I don't know all the specs. If it's a custom built engine or rebuild or stock with major upgrades- find out what kind of heads are on it and do your research. If the seller wants to sell the boat he should be able to find out what kind of heads are on it. If it's stock with no mods - no worries.

Exhaust is the other biggy. Lots of guys selling boats with manifolds/headers at the end of their life. When the exhaust start leaking internally; water gets into the engine and kaBOOM!. I would think you would have to pull the exhuast to find out if their leaking, but there maybe other ways.

Does anyone know if there is a way to check for exhaust leaks without pulling the manifolds/headers?

I have 500hp carbed, with about 300 hours. How do I know that I need valve springs? The boat runs just as good as it ever has.

300 hours on carb 500's I would do springs and lifters.
My 496HO's have 530 + hours on them. Good compression, no oil consumption. Hours aren't as important as maintainence and type of usage. I scanned the ecm's, 250+ hours were under 1500 rpm's, only about an hour and a half at WOT.
150 hours at WOT and you have a tired engine.
No oil changes and 150 hours, still a tired engine.
There is no crystal ball. Get a survey, have the ecm's scanned, and have a reputable tech check the compression and look them over.
Then cross your fingers and pull the trigger.
Breaking stuff is a part of high performance boating, so consider some repairs in your budget.

I had my boat surveyed when I bought it last year and things checked out well. Last month, I had my drives serviced, and my mechanic also said that everything is great. The boat is in great condition, as it is a 98 model with that low of hours. I am just curious if there are any warning signs, and also how much $$$ I am looking at, having the valve springs done? I am not a mechanic, and I don't plan on doing anything further than change the oil.

Valve springs are about the only weak point of that motor. If it were my engine, with 300hrs, I'd change the springs. An ounce of prevention.........


Darrell.

With 300hrs, you definately need to have the valve springs changed. I would bet that several of the inner springs have cracks. The carbed 500hp springs had all kinds of problems with them breaking between 200 and 300hrs.

Its not an "if" they break situation...its more of "when"

You will be much better off to have them changed now. If one breaks badly, it could cause severe engine damage.