Valve seats are a critical engine component because they are the foundation of the valvetrain. The seats provide a surface for the valves to seal against when they close so there’s no loss of compression or pressure from the combustion chamber. The seats also help cool the valves by conducting heat away from valves into the cylinder head.
The seats also support the valves and determine installed valve height, which affects valvetrain geometry and valve lash. The diameter of the seats and the contour of the angles that are machined into the face of the seats also limit how much air the engine can flow at any given valve lift and rpm. Because of all these factors, the valve seats deserve a lot of attention whether you are rebuilding a stock engine or building a performance engine.
What’s Hot In Seats
Though the basic metallurgy of valve seats hasn’t changed a great deal over the years, there have been some significant developments in the past year or two that is changing the way many performance engines are built. Racers love titanium valves because of their light weight. Titanium valves are typically 40 percent lighter than steel valves, but they are also very expensive. The price of titanium has soared to record highs due to rising demand and a limited supply of metal, but for serious racers there is no other alternative (except maybe hollow stem stainless steel valves).
One of the drawbacks of using titanium valves is that titanium does not conduct heat as well as steel. Consequently, the valve seats should have good thermal conductivity to pull heat away from the valves. If the exhaust valves get too hot, they can cause preignition and detonation that can destroy an engine. Excessive temperature can also cause the valves to burn, and the seats to pit and erode, either of which can cause compression loss in a cylinder.
When titanium valves first became popular, tool steel alloy valve seats were often used. The seats were durable enough but proved to be too hard for the titanium valves, and they didn’t pull heat away from the valves fast enough to provide adequate cooling at high rpm. Cast iron seats were tried and worked well enough on street engines, but they lacked the durability required for NASCAR and other high rpm, high horsepower engines.
The seat material that proved to work best with titanium valves turned out to be beryllium copper (Be-Cu).
The amount of beryllium, copper and other ingredients in the alloy also affect the thermal conductivity of the seat and how quickly it can transfer heat from the valve to the cylinder head. Some 2 percent beryllium alloys have a thermal conductivity rating of up to 140 BTUs per foot per hour per degrees F, while others that contain less beryllium are in the 60 to 68 BTUs per foot per hour per degrees F.
Most tool steel and iron alloy valve seats, by comparison, are rated much lower at 20 to 22 BTUs per foot per hour per degrees F. This is good enough to provide adequate cooling for stainless steel valves in an aluminum or cast iron cylinder head, but not for titanium valves in a high horsepower (over 600 hp) performance engine.
Many racers have been using a softer beryllium copper alloy for the intake valve seats, and a harder, more thermally conductive alloy for the exhaust seats. Intake valves run much cooler than the exhaust valves, but also tend to have more radical lift profiles and slam shut harder than the exhaust valves. Consequently, a softer seat material has more of a dampening effect when the valve closes to reduce the risk of valve bounce when the valve closes. It’s also kinder to the valve face and helps extend valve life – which is important when you’re paying up to $100 or more each for titanium valves!
Though beryllium copper seats have been the alloy of choice for racers using titanium valves, one of the drawbacks to using seats made of this material is that beryllium is a toxic metal. There’s no risk in handling the seats, but the dust that’s given off when cutting or grinding the seats can be dangerous because of the beryllium it contains. The danger is in inhaling dust that contains particles smaller than 10 microns in size. Beryllium may cause a lung disease called berylliosis or other allergic reactions. Dust can be minimized by using a liquid coolant while machining the seats. Wearing a dust mask that meets HEPA standards is also a good idea.
Occupational Safety and Health Administration (OSHA) regulations say workers should be exposed to no more than 2.0 micrograms of beryllium dust per cubic meter of air during an 8 hour shift. But these regulations are over 50 years old, and some are calling for much more stringent regulations that would reduce exposure to 0.2 micrograms per cubic meter. Because of these concerns, other valve seat alloys are now being used in place of beryllium copper.
Rule Changes Bring In New Seats
The 2007 rule change in NASCAR that finally did away with leaded gasoline forced many teams to take a second look at the valve seat alloys they were using in their engines. Tetraethyl lead is an excellent octane booster for high compression racing engines, and it also forms a protective coating on the valve seats that acts like a lubricant to extend valve and seat life. But lead is a toxic heavy metal. Because of this and the fact that lead poisons catalytic converters and oxygen sensors, tetraethyl lead was phased out of most motor fuels back in the 1970s.
Even so, NASCAR continued to use leaded fuels because there were no rule requirements for emission controls, and electronic engine controls were forbidden. NASCAR engine technology was essentially frozen in the pre-fuel injection era, so leaded racing gas lived on.
When NASCAR finally succumbed to environmental pressure to get the lead out, some teams found the seats they were using didn’t provide enough cooling for the titanium valves in their engines. Other alloys were tried, and some new beryllium-free copper-nickel based alloys were found to provide even better cooling and durability.
One of these new alloys is a product called “Moldstar 90,” a patented and proprietary copper-nickel alloy with a thermal conductivity rating of 90 BTU per foot per hour per degrees F. Tom Malaska of CV Products said many NASCAR teams are using this new alloy because it is safe to work with (no beryllium dust hazard in the machine shop), and it cools better than Be-Cu seats. Malaska said the new alloy is being used mainly by NASCAR teams, and that availability is limited. The alloy is currently only produced in bar stock rather than tube, which means there’s a lot of waste when valve seats are machined out of the bar stock.
Steve Erickson of Winsert, a supplier of unfinished valve seats to other aftermarket valve seat suppliers, said his company is introducing a brand new beryllium-free copper alloy seat called “Velocitor” at the Performance Racing Industry (PRI) Show in Orlando, Florida. The alloy is intended for racing applications that currently use beryllium copper seats. Erickson said testing has shown their new Velocitor copper alloy outperforms beryllium copper valve seats in both thermal conductivity and durability. It is also easier to machine and poses no health risks whatsoever.
Erickson also said the new alloy should give engine builders more freedom to maximize horsepower by opening up the inside diameter of the seats and changing the seat angles to optimize airflow. He said some tests have shown as much as a 10 percent gain in horsepower is possible over beryllium copper seats!
Higher Metal Prices
Another factor that is driving change in valve seat alloys is the soaring cost of nickel, cobalt and many tool steel alloys. Two years ago, nickel was selling for $6 to $7 a pound. Today, the price has soared to $32 a pound! Some blame the Chinese for driving up prices with their exploding economy. Others blame the ongoing war in Iraq. Some claim metal suppliers are creating artificial shortages to manipulate the market and drive up prices.
Whatever the causes are, the fact remains that many metals today are much more expensive than they used to be.
Brian Bender of SB International said some OEMs are now substituting seats made with new iron-based alloys for ones that were formerly made of nickel or cobalt alloys. These changes are affecting mostly heavy-duty diesel engines, and industrial engines that run on natural gas or propane. He said the new alloys are working well, and are less expensive than the nickel and cobalt alloys they replaced.
Bob McBroom of Dura-Bond said his company sells a lot of tool 70000 series powder metal high alloy tool steel seats for dry fuel engine applications. He said the price of their raw materials have doubled, but thinks the price increases have leveled off. The company also sells a 30000 Series powder metal valve seat that works well with stainless steel or titanium valves, and is used in many racing heads. Dura-Bond also has low alloy 15000 and 25000 Series seats for stock passenger car and light truck engines.
“We have a new high performance seat that contains 15 percent copper to provide higher thermal conductivity,” said McBroom.
Most late model aluminum heads are fitted with either cast iron or iron alloy seats, or powder metal (PM) seats. PM seats have become very popular with the vehicle manufacturers for a variety of reasons. PM seats are less expensive than iron seats, and they are proving to be very durable. PM seats often show little wear at high mileages. Consequently, if you are rebuilding a head with PM seats, the seats may only need a light touch-up.
Simple, right? Well, PM seats tend to work harden as they age, and can be be very hard (up to Rockwell 40 to 50) making them difficult to machine. As long as you have equipment that can cut hard powder metal seats, remachining the seats should be no problem. But if you don’t have equipment that is designed for this kind of work, you may be better off replacing the seats with new ones to get restore the seats. New powder metal seats are much softer (typically around Rockwell C 25) when they are initially installed, and easier to machine than aged seats. They also require less force to press into the cylinder head than iron or steel valve seat inserts.
One difference between cast alloy seats and powder metal seats is the way the seats are manufactured. Cast alloy seats are made by melting and mixing different metals together so they combine chemically. This molten soup is then poured into a mold and cast to shape. The rate of cooling and subsequent heat treatment of the metal determines its microstructure, hardness, strength and other physical properties.
Powder metal seats, by comparison, 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 (a process called “sintering”) to bond together the metals and form a solid composite matrix with very uniform and consistent properties.
One of the advantages of powder metal sintering is that materials that are difficult or impossible to mix together in a molten state can be blended together and bonded to create totally unique materials. Another advantage of the powder metal process is that parts can be manufactured very close to final tolerances, reducing the amount of machining that’s needed to finish the part to size. This lowers production costs and boosts manufacturing productivity.
The main reason why vehicle manufacturers have switched from cast alloy seats to powder metal seat inserts, however, is to extend engine durability. Most late model engines have to be emissions-certified to 150,000 miles or higher depending on the application and model year. If the valve seats can’t go the distance during durability testing, the vehicle manufacturer can’t certify the engine.
Powder metal seats are very good at handling thermal stress as well as impact stress, and typically show minimal wear after tens of thousands of miles of use. The homogeneous consistency of a powder metal seat also improves heat transfer, which is good for the valves, too. Powder metal seats also tend to experience less micro-welding between the seat and valve even at high combustion temperatures, which helps extend the life of both components. Yet some engine rebuilders are leery of powder metal seats, and prefer to replace PM seats with conventional iron seats.
Reconditioning The Seats
In cast iron heads with integral seats, the sealing surface on the seats is restored by cutting or grinding. How the seats are cut also affects valve height. Changes in valve height can be compensated for by grinding the tip of the valve stem to reduce its overall length. But there are limits as to how much you can grind off the hardened tip of a valve stem before you grind away the case hardened layer. Replacing the stock valves with ones that have thicker heads is one alternative.
In situations where an integral valve seat is damaged, the head can often be salvaged by cutting out the old seat and installing a seat insert – provided the casting is thick enough to accept a seat.
With aluminum heads, badly worn or damaged seats can be removed and replaced with new seats. But as one supplier said, the demand for replacement seats has been declining because of the longevity of the original equipment powder metal seats that are used in many late model engines.
When seats are replaced, the amount of interference fit is critical for proper seat retention and heat transfer. The seats in some OEM heads may have as little as .002 inch of interference fit – but keep in mind these seats were installed in brand new heads. Engine rebuilders tend to use more interference fit to compensate for any distortion in the seat recess that occurs when the old seat is removed. Some use as much as .005? of interference in cast iron heads, and up to .007? of interference fit in aluminum heads. Additional peening or staking of the seats should not be necessary if the correct amount of interference fit is used.
For best results, the valve seat inserts should be chilled in a freezer prior to installation, and the cylinder heat heated to reduce the amount of force needed to install the seats. The outside diameter of the seats can be chamfered to ease installation, and a lubricant also helps. No sealer should be needed for seat retention, and some sealers may actually inhibit heat transfer by forming a thin barrier between the seat and cylinder heat.
Latest posts by Larry Carley (see all)
- Cutting Tools and Abrasives for Engine Work - Jul 16, 2014
- Front-Mounted Oil Pumps: Good or Troublesome? - Jul 2, 2014
- Replacing Crankshafts, Connecting Rods and Bearings - Jun 27, 2014