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Valve Seat Selection and Machining
The type of seat material that’s “best” for a particular cylinder head will depend on the application, fuel used, how much power, type of valves, and longevity and durability the engine is expected to deliver.
By Larry Carley
When rebuilding a cylinder head for a stock automotive application, a performance engine, or a heavy-duty diesel, what kind of replacement seats should you use if the original valve seats need to be replaced? Should you install new seats that are the same as the original seats, or should you use a different type of seat material or alloy that provides better valve cooling or wear resistance?
The type of seat material that’s “best” for a particular cylinder head will depend on the application, the type of fuel used, how much power the engine produces, the type of valves (stainless steel or titanium), and what kind of longevity and durability the engine is expected to deliver.
A hard, wear-resistant valve seat is obviously a must for any engine application that will see a lot of miles. Stock passenger car and light truck engines as well as performance street engines need seats that are wear resistant so they will last. Heavy-duty diesel engines are even more demanding, and require seats that can go hundreds of thousands of miles. So cutting corners by installing the cheapest cast iron low alloy seats you can find is probably not the smartest move on such applications.
One valve seat supplier we interviewed said cost seems to be overriding almost every other consideration these days because of the current economic crunch. “The market has been flooded with cheap valve seats from China, India and Korea. Consequently, the average selling price of a cast iron valve seat today is about half what it used to be. The low prices may seem like a bargain for engine builders, but the quality of these cheap imported seats is all over the place. People who used to buy high quality made in USA alloy seats for $5 are now buying $2 cast iron seats from overseas and are learning the hard way that many of these seats are no bargain at all.”
If an engine comes factory- equipped with heat-resistant nickel or other alloy valve seats, substituting plain cast iron valve seats to save money is asking for trouble. Plain cast iron seats won’t hold up with unleaded gas in a late model, highly stressed engine.
Another trend that valve seat suppliers are seeing today are engine builders replacing valve seats more on a piecemeal basis than replacing an entire set. “We’re getting more calls for one or two seats instead of a complete set. We’re also getting more requests for replacement valve seats by size rather than by part number or application.”
Most of the valve seat suppliers we talked with reported the automotive market for valve seats has been flat, which comes as no surprise in light of current economic conditions. OEM valve seat suppliers, by comparison, have seen a slump of up to 40% in their sales. But several suppliers said they’ve also seen some positive movement in valve seat sales the past month for agricultural and industrial engine applications. Hopefully, we’ve turned the corner and are starting to see some light at the end of the tunnel.
PERFORMANCE VALVE SEATS
Circle track and drag racing, by comparison, are high heat applications that must have high quality seats capable of delivering good cooling performance for the valves, especially the exhaust valves. Wear resistance isn’t as important in racing because most of these engines don’t accumulate much mileage between rebuilds. What’s more, if the seats are too hard, they may accelerate valve wear. That’s something you don’t want if the engine has expensive titanium valves. Replacement seats are much cheaper than replacement valves.
Beryllium-copper (Be-Cu) alloy seats have long been the racer’s choice for lightweight titanium valves. Titanium valves are up to 40% lighter than stainless steel valves, which means high RPMs before the valves start to float. But titanium valves run hotter than stainless steel valves, so they require seats that can pull heat away from the valves quickly.
Beryllium-copper have a thermal conductivity rating that can range from 60 to 140 BTUs (British Thermal Units) per foot per hour depending on the alloy. That’s three to seven times the cooling performance of most cast iron seats! The downside with Be-Cu seats is that they are expensive, they require slightly different alloys for the intake and exhaust valves (a softer alloy for the intakes), and require special machining precautions.
Be-Cu valve seats typically contain around 2% beryllium, which isn’t a lot, but is enough to require some health precautions when you are finishing the seats. Beryllium dust is a dangerous material that can cause lung problems if inhaled. OSHA says workers should not be exposed to more than 2.0 micrograms of beryllium dust per cubic meter of air during an 8 hour work day. Using a coolant while machining the seats should keep your exposure to a minimum, but you should also wear a dust mask that meets HEPA standards for extra protection.
In recent years, various proprietary beryllium-free copper-nickel alloy cast seats have been developed that cool as good as or even better than Be-Cu seats for racing applications that demand high heat transfer rates between the valve and cylinder head. Copper-nickel alloy seats have proven to be popular with NASCAR racing teams because of the superior cooling performance and wear characteristics the new alloys deliver.
Average combustion temperatures in a street performance engine can range from 1,400° to 1,700° F. Nickel alloy cast seats can usually handle 1,400° F with no problem, while cobalt is good for up to 1,650° to 1,700° F. With nitrous oxide, temperatures can soar to 4,400° F, which can make some seats become hard and brittle. This increases the risk of seat cracking and failure. So the seat you choose has to have high temperature resistance if an engine is boosted with nitrous oxide.
DRY FUELS & DIESELS
Dry fuel engines such as large stationary diesel generators that are burning natural gas or propane typically require seats that are much harder to resist wear. Stellite, chromium, cobalt, tungsten and nickel alloy seats are typically required for these types of applications. But high tool steel content powder metal seats can also be used in dry fuel stationary engines, according to one supplier.
Hard alloy seats provide good wear resistance, but most are expensive, making it tempting to substitute less expensive seats. No one alloy is right for all heavy-duty diesel applications, so it’s important to use replacement seats that have similar wear and thermal characteristics to the OEM valve seats. Using an inferior alloy can be a very expensive mistake. Most of the cost of overhauling a big diesel engine is for the labor and machine work, not the parts that go into the engine.
In recent years, heavy-duty diesel OEMs have developed their own special proprietary cast iron alloys that perform just as well as the more expensive alloy seats, but at a lower cost.
Wear and corrosion resistance is critical on over-the-road heavy-duty diesel engines these days because of the exhaust gas recirculation (EGR) systems that are now required. EGR reduces the formation of oxides of nitrogen (NOX) in the exhaust by recirculating hot exhaust gases back into the intake manifold. But EGR also makes the intake valves run much hotter. This means high chromium, cobalt or nickel alloy seats are needed for the intake valve seats to resist corrosion and wear.
Today’s engines are also producing a lot more horsepower and torque from the same number of cubic inches. That means more heat load on the engine and valves, which requires a premium valve seat material with high thermal conductivity. At the same time, the seat alloy must also provide high temperature wear resistance for hundreds of thousands of miles of operation.
Many of the big, high output diesel engines today are running steel pistons rather than aluminum pistons. This means a catastrophic valve seat failure can cause a lot more damage inside the engine. So the last thing you want is a seat alloy that’s prone to cracking.
Though powder metal seats are used in most late model passenger car and light truck engines with aluminum cylinders, powder metal seats are not used in most heavy-duty diesel engines. Why? Because the type of powder metal seats that perform well in automotive engines generally don’t do well in a heavy-duty diesel environment. That’s why heavy-duty diesel OEMs typically use stellite, chromium, cobalt or nickel alloy seats, or the new proprietary cast iron high heat alloys that have been developed for these engines.
Another change that’s taken place in the heavy-duty market is that many OEMs have stopped selling replacement valve seats because they want to sell complete cylinder heads. Others only offer replacement seats in standard sizes (which is good for aftermarket valve seat suppliers provided they have oversized replacement seats in the right alloys for the application). Many of the OEM seats are also pre-finished, which means you don’t have to cut them after they’ve been installed. But it also means the seat counterbore had better be concentric with the valve guide, otherwise you’re going to have problems.
If a valve seat is not concentric with the center of the valve guide, it may prevent the valve from sealing tightly causing a compression leak. It can also cause the valve stem to flex slightly every time the valve opens and closes, which can lead to metal fatigue in the valve stem and valve failure down the road. A nonconcentric seat can also cause one side of the valve to run hotter than the other, increasing the risk of valve burning and seat erosion.
Interference fit is one of the main concerns when installing valve seats. You want the seat to fit tightly so it doesn’t fall out, even if the engine overheats. But you don’t want it so tight that there’s a danger of cracking either.
On passenger car and light truck engines with aluminum heads, the seats are usually factory installed with about .002˝ to .003˝ of interference fit. Some say powder metal seats require a little more interference fit than cast iron alloys, while cobalt alloy seats require a little less because of their higher coefficient of thermal expansion.
Keep in mind these numbers are for brand new heads with brand new seats. After tens of thousands of miles, seat counterbores can become distorted and eroded, requiring an increase in interference to keep the seat tight.
The most common recommendation from valve seat suppliers for cast seats being installed in aluminum heads is .003˝ to .005˝ of interference fit. If you are installing powder metal seats, use .005˝ to .007˝ of interference. If you are using beryllium-copper seats, go with .004˝ to .0045˝ of interference fit.
Many valve seats have a radius or chamfer on the outside bottom edge to make installation easier. Seats with square cut corners are more difficult to install and may damage the counterbore if they snag any metal or become cocked while they are being driven in.
Chilling the valve seats in a freezer and preheating the head are often recommended to make installation easier, especially if you are using a lot of interference fit. Using a lubricant also helps. When heating the head, don’t get carried away. You only need about 160° to 180° F. If you get the head too hot, say 200° to 250° F, things can start to move around and change the alignment between the valve guide and seat.
Unless you are installing a pre-finished seat, the seat will have to be be cut after it has been installed in the head. This, of course, requires installing or reconditioning the valve guides before you do the seat work. The center line of the valve guide will determine the location and concentricity of the valve seat.
Accurate seat refinishing requires a valve-and-seat machine that is in good condition and can hold tight tolerances. You can’t have a couple thousandths of an inch of slop and do a good valve job. The pilot to guide clearance should be down around .0002˝ or less for accurate machining. One way to achieve that is to use a high pressure lubricant on the pilot.
The seat cutter must be sharp and spun at a high enough speed to produce a high quality finish on the seat. If you’re getting chatter while cutting a seat, the problem may be too much play between the pilot and valve guide, the speed of the cutter, or the machine is out of level. Using a coolant when cutting hard seats will also reduce chatter.
The profile of the seats will depend on what you are trying to achieve. A single cut 45° seat may be all that’s needed for a low output stock engine. But on a high performance engine, a multi-angle valve job is an absolute must to optimize the breathing potential of the cylinder head.
The commonly used 30-45-60 degree three angle performance cut certainly flows better than a seat with a single 45 degree cut. But more angles breathe even better. Adding additional cuts under the seat, and using steeper angles generally helps the airflow numbers even more. Some well-known performance engine builders say they see the best flow numbers using a 58° undercut below the primary seat, and a 70° cut below that. Using a steeper top angle of 33° to 37° on the intake seats also helps reduce turbulence as the air enters the combustion chamber. Others use special cutters with an infinite radius or a CNC-controlled single point cutter to contour the seat above and below the primary 45° seat to optimize flow.
Unfortunately, there’s no tried-and-true formula that works best for every engine. It takes a lot of time on a dry flow bench and wet flow bench to figure out which angles actually produce the best results. And sometimes what looks good on a dry flow bench doesn’t always produce more power because of fuel separation problems at the valve and seat interface. Because of this, it often takes some trial-and-error experimentation to find the optimum combination of angles or seat profile that delivers the most power and throttle response.
The listings included in the directory accompanying this article are taken from the 2009 Engine Builders Buyers Guide. To search for products or services interactively, visit www.enginebuildermag.com/BuyersGuide. Manufacturers: look for the forms for the 2010 Buyers Guides coming to you soon!