My boat has twin 525's with about 300 hours. Do not burn oil, no soot on transom. One motor has a slight valve knock when its cold and disappears after about 5 minutes of warmup.

Having leakdown test done but I don't completely understand what is being tested and want to understand the results. What information should I be looking for and what kind of results suggest rebuild.

Any guidance would be appreciated.

depends on the type of leak down gauge used, the best is a twin gauge, first gauge tells you the amount of pressure being put into the cyl. the second tells you the amount of press. the cyl. is holding, 15% leakage is the bottom line, so if you put in 100psi, min. is 85psi, and nothing more than 10% between any 2 cyl. hope this helps.
larry

If there's carbon build up on the valves, then you'll have some blow-by and you can hear it coming out of the exhaust or intake port, or both. If you're at 300 hours, you really should change valve springs. That can be done without pulling the heads. That's the weak link of that engine, internally. Your bottom end should be good out to 500hrs. If they do have to pull the heads to lap the valves, you will get new head gaskets and make sure they properly seal the head gaskets. (caulking around oil and coolant passages) Also, pressure test those headers, just to be safe.

Is valve spring issue true for an HP425 454 Gen V engine also?

Not sure on that one. It can't hurt to replace every 100 hrs or so though. The 525's have some big cams in them and I think that's the main reason they need to be changed, because if they get "soft" and can't close when they're supposed to close............

Hopefully a real expert will opine on your motor.

Red flag would seem to be the knock that goes away after running for 5 minutes. I don't think a leak down test will tell you what it is but may lead you astray. Better minds may chime in but it sounds like lifters if the rest of the oilig system is OK. If it is lifters all you are out besides the cost of the lifters is the price of an intake manifold gasket. At 300 hours they are close to due if not past, fix that and go from there.

It could be you don't have a serious problem. What you describe as a "valve knock" is more than likely a hydraulic lifter tap, caused by a lifter plunger sticking when cold. Did this tap (when cold) just develop? Given that you have no significant oil consumption and state no performance issues I'd skip the leakdown test. It would tell you if you had worn rings, leaking valves, or head gasket, but, either of these would cause performance issues and/or oil consumption. Also none of the above would cause tap when cold.
You do change oil frequently, and you're using the right stuff? Change it and run it.

Thanks for all the Info !