On an HP 500 carb version. What causes a valve head to break off and fall in to the piston chamber

Two possibilities come to mind. One, just a bad valve. If all the other one's are OK, it could have had a flaw in the metal's structure. Two, if others are showing signs of distress, you may have an exhaust restriction causing excessive heat or you may have water intrusion through a failing exhaust. Exhaust reversion can pull water back down to the valve. Splashing cold water on a glowing hot valve is bad for them.

Besides the water hitting it like Chris mentioned a detonating motor will tulip and break valves.

Yes, good point. It doesn't take much. Usually there are tell-tale signs such as broken top ring lands and deformation of the piston domes. Do you have the heads off? If so, what else are you seeing?

Yes the heads are off. All valves look fine, except the exhaust valve in the next cylinder looks like it has pitting on it about the size of a pin head (very even) but no rust (two center cylinders).


This happeded two times, first time the valve broke at the top by the keeper, just snaped off. Second time the head fell off.
The pipes are dry all the way to the transom.

Dry to the transom is good- unless there's a leak in one. Could be a gasket- could be a pinhole. Some of the Gil's would erode badly at the port area if you lost water pressure. IIRC, there was a run of Merc's with CMI's on them that had exhaust issues- they lasted a couple seasons before springing leaks.

I would have the heads re-done with Manley or Ferrea stainless intakes and Inconel exhausts. Have the manifolds pressure tested to ensure you have no internal leaks.

how old are the springs?

Everything is new

Could somebody expand on the merits of stainless steel and iconel valves ? I recently priced these and they seem quite reasonable for the SS variety- perhaps $80 for a full set.

Stainless is more durable for the intake. Inconel is much more temperature resistant on the exhaust side. In a moderate performance engine, SS will work on both sides. On a high performance engine (high rpm's, blowers, etc) the Inconel is a requirement. One dropped valve head can destroy virtually every usable part in a motor. Looking at it that way, they're dirt cheap.