Off to another problematic start to the boating season. Water in the oil in one of the motors, which seemed to be running fine. I pulled the headers and pressure tested. A pin hole in one, so I replaced it. The new one and old one tested perfect. Flushed oil, refilled and started it. After 5 min, changed the oil and ran it again. After 5 min, still water in oil. Changed it again and ran it for 30 minutes at idle to try and warm it up. Dipstick was halfway up with more milk. So today I pulled the oil cooler, my next guess, and pressure tested in a water tank with 150psi. Passed perfectly. Next step was a leak down test and the #6 cylinder had 100% loss. Pulled the valve cover and found a bent / stuck valve and the rocker off to the side. Piston has a slight ding in it, but nothing appears actually broken. Pulled the head and gasket seems ok. Could a bent valve damage the guide or head enough to leak coolant into oil? Dropping off the head at the machine shop tomorrow and going to pull apart the other side and go from there. Kills me that this motor only had 30 hours over 2 years since a Mercyry Racing factory overhaul.

Bent/stuck exhaust valve?

An engine runs with an exhaust valve stuck open, that cylinder pulls water right back out of the header.

yep

That makes sense now. I could not correlate the amount of extra fluid in the oil with the lack of change in the coolant. The valves have doe rust on them, thoughts on changing them all to stainless when I rebuild the heads? I can't understand why Merc would not have stainless valves from the factory.

it isn't coolant if it is pulling in exhaust valve. it is raw water. they don't use stainless valves because it is your fault for leaving the stock manifolds on until they leak, or your fault for running a cam that pulls water in from exhaust. and the stock exh usually outlasts the warranty...

Rust on the valve stem is likely why the valve seized in the guide, and is now bent. Rust, or even the presence of moisture, can cause galling of the stem. At any appreciable RPM, an exhaust valve only needs to slow down for a fraction of a second, due to galling of the stem/guide, and the piston will catch up to it in a hurry...

The valves are a stainless steel alloy... Inconel. And they will rust. There are many grades of stainless, or stainless alloys. Many of them will rust or corrode under the right conditions, at least to a certain extent.

It only takes a minor amount of rust on an exhaust valve stem to start the galling process, which ultimately results in a seized/bent valve. Also, once the valve stem is exposed to water, it drags water back into the guide when the valve closes. When the engine sits, rust can now form in the guide as well.