[RoninLB]

not my words.

"The key elements in good valve cooling are:
Good valve face to valve seat contact (it needs to be nearly perfect); and
Good valve stem to valve guide fit; and
Cool cylinder head temperatures.

Flat statement: I believe that virtually all valve problems originate with the factory or the overhaul shop.

The hole through the cylinder that takes the valve guide must be true, straight and centered. The valve guide must be true and straight. Finally, the valve rim must match precisely the face of the valve seat, and both mating surfaces must be wide enough to provide enough surface area to conduct the heat.

This calls for some very fine machine work, and sadly, the factories haven't done it very well. The main hole will always be microscopically off-center, and it will never be perfectly straight. Close doesn't count here. The guide must also be machined or honed to very tight tolerances, and it must be straight and true.

But that's only part of the story. Once the valve is installed nice and true, there remains the task of making the correct metal-to-metal contact between the rim of the valve face and the valve seat (which is itself yet another insert that needs to be placed with great care).

The usual way of doing this is to put a fine grinding compound on the surfaces, stick the valve in, and spin it, or rotate it back and forth, so that the two surfaces grind away at each other, hitting the high spots on both surfaces, eventually leaving a perfect match. This is called "lapping," and it's an evil chore.

Ideally, that area of contact has to be some minimum width, and it must be equal all the way around, or the valve head will be cooled unevenly. Uneven cooling will cause hot spots and cold spots, and the valve head will actually warp a bit. When that happens the contact is not even all the way around, and a microscopic gap opens. At first, the pressure of the combustion event is probably enough to smash the valve head closed and correct a small warp, but eventually, the gap will allow a tiny amount of the combustion gases to leak past. Once this begins, it's only a matter of time. This is where you begin to see a loss in compression, sometimes very rapidly!

Remember, that combustion event can be upwards of 3,000-4,000ºF and that heat blowing through a small crack will cause an intense hot spot on the rim of the valve. With the rest of the valve getting cooling, and this little arc not getting any."