A compression test you crank motor over with all plugs out with throttle at wot with gauge screwed in spark plug hole to see cranking cylinder pressure. A leakdown test you place motor (usually) on tdc on compression stroke,screw a leakdown gauge in,put air to leakdown tester from air compressor with gauge on tester zeroed out then you open air inlet to cylinder and measure % of air leaking thru cylinder. It is more comprehensive than a compression test.You can also pinpoint where the air is going somewhat by putting your ear to the exhaust pipe,intake and valve cover breather. Lets say you do a compression test and your cylinders are all 155 But one is only 115 psi,you would then do a leakdown test. If you had air hissing out the exhaust (most common) you would then know a leaking exhaust valve,burn't valve,bent valve,stuck valve etc is your culprit. You could then avoid pulling motor and needlessly rebuilding it by pulling the heads and fixing them. On the other hand of you had air blowing out valve cover breather and dipstick you would know something was a matter with that particular cylinders rings,piston etc and not waste your time tearing top end off motor in boat and re-installing just to have same problem,Smitty

Thanks for the explaination

When the clearances are beyond the allowable tolerances. Or when something finally finds it's way to the outside of the block.

That would mean then I would have to weld the block back together...

Yea but my point was at what time do you check clearances? I guess every other season

See I already have 4 emails, 2 of them say every 200hrs, every 400 hours, every other season

On an engine of basically stock configuration (like a 365 Mag or a 400 Cyclone)I'd probably use 400 hours as a basis for doing more frequent compression checks and oil-pressure checks (with a mechanical gauge, not the dash-mounted one). The lower-horse motors have been known to go well over 1000 hours with no issues, a lot of them going to 2000 hours with nothing more than oil and plug changes.
Most people decide to rebuild when they want more speed from the boat.

wow those are som eincrdible numbers. Are they really realistic numbers though? We must be speaking about purely stock factory motors..

The 330/340-horse 454 engines typically go well over 1000 hours without any work as long as they are maintained and no exhaust manifold issues cause their demise. Here's a hint that everyone here will echo- keep a close eye on the riser gaskets and be wary of ten-year-old exhaust manifolds.
When the 330/340 motors are in a heavier boat like a Searay 390 Express Cruiser they tend to wear out quicker because of the increase weight they need to lug around (and the owners still want to cruise at a faster speed).
There is a guy here in west Michigan with 800+hours on his stock 365 Mag motor in a 25 Baja and he's pounded on that boat for 5 years. But he maintains it like he can't afford to fix it.

I've rebuilt a lot of boat engines at my shop. I don't recall ever having one that the bottom end was worn out because of hours. It's usually water damage. I've found that most stock big blocks start loosing the exhaust valve seats around 400 hrs. So much just depends on how it's run. The valve train takes the hardest beating.

And where (or how) does this water damage take place? How is it that water makes it way to the lower half??

Water can get in the cylinders and oil several ways. Leaking risers, too high water pressure, reversion and backwash through the exhaust are the more common ways. Then there is sinking, leaving the plug in when it rains...on and on. If you boat in salt water you can throw serious rust into that equation too. It'll rust out before it wears out. Just relaying what i've seen.

Bob