The “mushroom valves” are one of those components that are still irreplaceable today. Efficiency, practicality and cost are the main characteristics of valves.
For several decades, engine valves were one of the main weak points of internal combustion engines. Their rupture was a very frequent occurrence and caused, of course, immediate downtime. Today, the situation is very different. Mainly due to advances in metallurgy, even in very sporty applications where engine speeds and stresses are high. Valves do their job under conditions that are anything but easy. When they are in the closed position they must ensure a perfect seal against the gases in the combustion chamber, but they must also handle very high temperatures.
A valve consists of a “mushroom,” along the circumference of which is the truncated conical sealing surface, which normally has an inclination of 45°. The mushroom is the part that in the closed position goes against the seat. Then there is the stem, which slides in a guide implanted in the head. Implanted in the end of the stem are the half-cones, which are inserted into the cup on which the return spring acts.
Given the different temperatures involved, intake and exhaust mushroom valves are made of different materials. Simpler steels are used for the former. For the latter, more high temperature resistant steels with different binders and crystal structure are needed. In engines where specific power is high and temperatures are higher, nickel- and chromium-based alloys, which are stronger, are also used.
Freely excerpted and translated from https://www.sicurauto.it/