Powder Metal Seats
Powder metal seats are used in many late model passenger car and light truck heads because the are less expensive than cast iron alloy seats, they are very durable and different materials can be combined by sintering that normally would not mix in a cast seat. PM seats are highly wear resistant and often show little wear at high mileage.
However, they do work harden as they age, making them difficult to machine after they’ve been in service for a period of time. When new, PM seats are relatively soft (Rockwell C 25) and have to be machined without producing too much heat, otherwise they will harden as you are working on them.
Powder metal seats are made by mixing together various dry metal powders such as iron, tungsten carbide, molybdenum, chromium, vanadium, nickel, manganese, silicon, copper, etc.). The powder is pressed into a die mold, then subjected to high heat and pressure (sintering) to bond together the metals and form a solid composite matrix with very uniform and consistent properties.
PM replacement seats are available from many aftermarket valve seat suppliers, but others have gotten out of the PM seat business altogether for various reasons. Cast alloy seats can be substituted for PM seats, and vice versa. But the best advise is to not mix different types of seats in the same engine. If only one or two valve seats have to be replaced, use the same type of replacement seat as the original.
The reason for doing so is to maintain the same heat transfer characteristics as before. A replacement seat that runs the same or cooler than the original is okay, but a seat that runs somewhat hotter might not be okay depending on the application.
Seat Finishing
Valve seats may be pre-finished or not. If it’s a raw seat, you will have to cut it after it has been installed in the head. Accurate valve seat refinishing requires a valve-and-seat machine that is in good condition and can hold tight tolerances. The pilot to guide clearance should be .0002? or less for accurate machining. One way to achieve that is to use a high pressure lubricant on the pilot.
It’s important to make sure all valve guide work has been completed prior to machining the seats because the guides provide the valve centerline for machining the seats. The seat cutter must be sharp and spun at a high enough speed to produce a high quality finish on the seat. Excessive play between the pilot and valve guide can cause chatter, as can using the wrong cutter speed. If you are machining hard seats or PM seats, use a coolant to reduce chatter.
Seat angles will vary depending on the application. Most stock engines specify a seat angle of 45 degrees. Three angle cutters generally provide the best flow by tapering the gas flow as it passes over the seat. The commonly used 30-45-60 degree three angle cut may vary somewhat depending on the application with shallower or steeper angles specified for either side of the 45 degree seat.
Some performance engine builders say they see the best flow numbers using a 58 degree undercut below the primary seat, and a 70 degree cut below that. Using a steeper top angle of 33 to 37 degrees on the intake seats also helps reduce turbulence as the air enters the combustion chamber.
