Selecting Valve Seats for Gas and Diesel Engines
By Larry Carley
Hard working diesel engines, performance engines, and engines that run on dry fuels such as propane or natural gas produce a lot of heat in the combustion chamber and often require valve seats that are harder and more heat-resistant. Stellite, chromium, cobalt, tungsten and nickel alloy valve seats are commonly used for such high heat applications as are tool-steel valve seats. Beryllium-copper or copper-nickel alloy seats are often used in racing applications, typically with lightweight titanium valves. Average combustion temperatures in a street performance engine can range from 1,400 to 1,700 degrees F. Nickel alloy cast seats can usually handle 1,400 degrees F with no problem, while cobalt is good for up to 1,650 to 1,700 degrees F. With nitrous oxide, temperatures can soar to 4,400 degrees F, which can make some seats become hard and brittle. This increases the risk of seat cracking and failure. For ordinary passenger car and light truck engines, however, temperatures are lower so iron alloy valve seats are perfectly adequate. Iron alloys are less expensive and easier to machine than hard,
high temperature alloys. But in recent years, powder metal seats have become the norm for most original equipment passenger car and light truck gasoline engine applications. Powder metal valve seats are very different from cast alloy seats. Powder metal seats are made by mixing various metal powders and then pressing the powder under extremely high pressure (up to 100 tons!) into a mold. The seat is then baked at high temperature to sinter (partially melt) the ingredients so they stick together and form a homogeneous matrix. The end result is a valve seat that has very consistent and uniform properties, and requires minimal finish machining. The neat thing about powder metal technology is that you can combine various ingredients that would not normally mix together if you were trying to create a cast alloy seat. Consequently, solid lubricants can be added to the mixture to improve machinability and wear resistance. What’s more, the powder can be blended differently for different types of applications. Infusing the mix with copper, for example, can improve the seat’s ability to conduct heat for high heat applications such as dry fuel engines, marine engines or motorcycle engines. Powder metal seats often show little wear at high mileage. Consequently, if you are rebuilding a head with powder metal seats, the seats may only need a light touch-up. But because of the work hardening that occurs with powder metal seats, they can be difficult to machine. The naturally smooth exterior surface finish of a powder metal seat also improves the metal-to-metal contact between the seat and its counterbore in the cylinder head for better thermal conduction. Adding a radius to the outside corner also makes installation easier. The powder metal matrix also has a certain amount of elasticity that helps retain the seat in the head with less interference fit. That’s why many original equipment powder metal seats are installed in
aluminum heads with only .002 to .003 of interference fit.