"There is no seam just under the head of any valves, the process of friction welding would make that very nearly impossible to join at that point.
This is partially due to the shape of the head of the valve, it would require a very serious and complicated setup to spin the valve head against the stem which is part of the two piece process. Then due to the various head sizes, it would take massive amounts of tooling to even set up.
Friction joins dissimilar metals, they actually fuse as the molecules flow into each other.
The reason valves always break under the head is this is where stresses occur, I have never seen a two piece valve ever break at the union since that is in the middle of the guide and actual measured loading at that point is really rather gentle.
There are almost no two piece valves produced in today's world, this is because modern equipment is much faster using the one piece product. Draw forging is so fast and accurate that performance valves have actually come down in price, it is the alloy used that is costly.
A typical stock alloy like HNV material is about 20% of the cost of EV8 grade alloy. Nearly ALL imports and many domestic OEM's use HNV material, then nitride over that which is a very cheap method, far cheaper than Chrome plating the stems which would otherwise be required.
So even import valves are all one piece now. There is still a plant in Argentina that produces industrial Diesel valves using the two piece method, that is the only one left that I know of. There are no USA valve producers left in operation. Even Manley and Ferrea have gone to Huaija Denyun in China, way the world is. We still use Eaton but it gets tougher every day for us.
Hope this helps,
Ted"
You can find him at engnbldr.com. As far as I am concerned, the Professor has just spoken and left the room. I always feel enlightened when he speaks.
