Classroom Manual for Automotive Engine Repair and Rebuilding
Barry Hollembeak
Delmar Publishers, 1997
ISBN 0-8273-6187-4
Most valves are constructed of high-strength steel; however, some manufacturers are experimenting with ceramics. Most valve heads are constructed of 21-2N and 21-4N stainless steel alloys. 21-2N alloy is commonly used in original equipment exhaust valves. 21-4N is a higher grade of stainless steel containing more nickel. One of the primary concerns in valve construction and selection of alloys is the operating temperatures to which they are subjected. Combustion chamber temperatures can range from 1,500 to 4,000F (82 to 220C). The types of material used to construct the valve must be capable of withstanding these temperatures and dissipating the heat rapidly. For every 25F reduction in valve temperature, the valve's burning durability is doubled. There are several alloys available for valve construction that will increase the valve's burning durability. An alloy that is being used by many manufacturers is Inconel. Inconel is constructed from a nickel base with 15% to 16% chromium and 2.4% to 3.0% titanium.
SAE has devised a code method to classify valve alloys. The code uses different letter and number designations corresponding to the types of materials used. For example, an NV prefix designates a low-alloy structural steel, while an HNV prefix designates a high-alloy steel. Both of these designations are generally used for intake valves. Common prefix designations include EV, which denotes an austenitic steel. HEV denotes a high-strength alloy.
The commonly used 21-4N stainless steel alloy is SAE classified as EV8. This valve contains 21% chromium and 3.75% nickel. Inconel is classified as an HEV3 alloy.
Ideally, when replacing valves, match the original equipment alloy. Sometimes the only assurance you have concerning the grade of alloy used in a valve is the reputation of the valve manufacturer. A magnet can be used to separate stainless steel from carbon steels. Stainless steel is nonmagnetic, while carbon is magnetic.
Valve construction design is also an important aspect for controlling heat. A valve can be constructed as one piece or two piece. One-piece valves run cooler since the weld of a two-piece valve inhibits heat flow up the stem. Two-piece valves allow the manufacturer to use different metals for the valve head and stem.
In addition to alloy selection, manufacturers design the valve and cylinder head to dissipate heat away from the valve. Most of the heat is dissipated through the contact surface of the face and seat. The heat is then transferred to the cylinder head. About 76% of the heat is transferred in this manner. Most of the remaining heat is dissipated through the stem to the valve spring and on to the cylinder head. To increase the transfer of heat through the stem, some stems are filled with sodium.
Most manufacturers use chrome-plated stems to provide additional protection against wear from initial engine starts when oil is not present on the valve stem. In addition, chrome works to protect against galling when cast-iron guides are used. If the valve stem is reground, the chrome plating will be removed. In this case, either the valve will have to be replaced or a bronze guide must be used.