anybody ever had a valve just break in half, in a newly rebuilt motor

Can you post a pic of it or the remains? To answer your question no but as we all know in the hiperf world **** has a tendancy to happen. What kind of valves are they? Intake or Exh? It's extremely difficult to cut corners to save some $$$ when engine building and the valves def aren't one of them. Especially in marine apps. All it takes is a splash of water with the right temps and good bye. Hopefully you didn't have a lot of other damage. I've seen complete engines trashed due to a boken valve. They like to transfer over to the other bank and raise haveck over there to.

Good luck and hopefully you'll be back on the water soon.

exhalst, valve, this is the second time its happen, just broke in half, and the only thing my engine builder says its seems to just be a bad valve, from mfg. it cracked, the side of cylender wall , im getting it sleeved and fixed rite now, i dont see any visable head gasket leakage,

What brand are they and type. You need to use a good Inconel exhaust valve like Manleys on a boat due to extremely high exhaust temps. Their expensive but cheap insurance. Sure their breaking and not actually burning the head off the stem?

You sure you dont have an header leak?

not at all sure how would i be able to tell ita a gil stainless riser.

Myles, who down here is doing your engine building? what kind of valves are you using? Are they one piece or two piece valves? Was it the same valve each time that broke or another? You need to pressure test your manifolds to see if they are leaking. If you need a hand with that im here in Gulfport. Wouldnt mind helping you.

Second valve/same cylinder? Something else is going on causing an uneven load on valve head. Or a little moisture on a hot exhaust valve. IMO

Let me start by saying it has been a long time since i built engines. But this is the kind of problem I had when I first started.

We found there were 3 main possibilities -

The valve itself - the stuff we used in automotive did not work for marine. For dead stock motors the OEM GM was ok, but not the normal aftermarket stuff.

For anything more than a stock cam you need at least stainless, inconel better. Manley, Ferrea, REV all worked fine.

2 - valve float - very common. Look at the tip of the valve with a magnifying glass. Does it look like it has random little dings in t? That is float. With a roller tip rocker you really want just 1 imprint of the rocker. The valve will not rotate unless you are floating (does not apply to stock stuff with valve rotators)

3 - bad valve job - the seat is not concentric with the valve guide, makes the valve bend every time it closes.

Not trying to offend, but we had boats where the headers leaked so bad they would corrode the heads from the exhaust port side and never lost a valve that way. I suppose it's possible.

Again, it's been a long time - but once we fixed these items almost never lost a valve again.

Second time it happened???? I highly doubt its a problem with the actual valve, unless they are really junk. Sounds more like an assembly or clearance problem.