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Valve Seat Installation Procedures
Valve seats are an extremely important part of a cylinder head because the seats cool and seal the valves. They also support the valve when it closes, which affects both valvetrain geometry and valve lash.
By Larry Carley
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If a valve seat is damaged, cracked, loose, receded or too badly worn to be recut or reground, it can cause a variety of problems: loss of compression, valve burning, valve failure, valvetrain wear and breakage, even head and valve damage if the seat comes loose.
For that reason, replacing valve seats is often necessary when reconditioning aluminum or cast iron cylinder heads. Another reason to replace a seat is if a valve has broken because the seat is not concentric with the guide. Misalignment between the seat and guide causes the valve stem to flex every time the valve closes. Eventually, this flexing leads to metal fatigue and valve failure. When this happens, the counterbore must be remachined (if the head is salvageable) to realign the seat with the guide.
New seats may also be required if a cylinder head has been straightened or welded, if there’s any evidence of corrosion around the outside diameter of a valve seat, or if the engine is being converted to run on a dry fuel such as propane (LPG) or natural gas.
Integral seats in cast iron heads are no less important even though the seats are part of the head itself. An integral seat may have to be cut out and replaced with a new insert if the seat has receded, is badly worn or damaged.
Some experts say when late model aluminum heads are reconditioned the valve seats should always be replaced to maintain correct valvetrain geometry. This applies to overhead cam engines as well as pushrod engines. It’s expensive but is usually necessary to restore proper installed valve height and valvetrain geometry.
Rick Emert, product and technical services manager for SB International, says most valve seat failures (more than 50 percent) are due to one of two things: misapplication (wrong valve and seat combination) or valvetrain "mismotion" (incorrect valvetrain geometry or lack of seat/guide concentricity). He also explains that preignition causes a lot of seat failures, too.
"When seats get too hot, microwelding occurs between the valves and seats. The valves are harder than the seats so microscopic particles of metal from the seats stick to the valves," says Emert. "When the engine cools, these particles are then washed into the exhaust. This causes rapid recession of the seats and is most common in dry fuel (LPG or natural gas) engines."
Emert said another reason for replacing seats in some late model heads is because the OEM powder metal seats become too hard to machine. Many late model gasoline engines with aluminum heads from Ford, GM, Chrysler and many imports are factory-equipped with sintered powder metal seats. Powder metal seats are used because they are harder and more durable.
Powder metal seats combine various materials to achieve special properties. Many powder metal formulas work-harden as the engine runs. A new powder metal seat that has a hardness of RC 25 when it is first installed will develop a hardness of RC 40 to 50 after several thousand miles. Seats that hard are difficult to refinish by cutting, so one alternative to grinding is replacing the old seats with new powder metal or alloy seats.
Emert says his company recommends alloy seats for most applications because they are easier to machine. "We have eight different alloys in our product line, and each one is engineered for a specific type of application." His company’s two most popular alloys are "Cast XB" (an iron-based alloy), and "N-Series" (a nickel-based alloy). SB International also has powder metal seats for those who want to install the same kind of seats as original equipment, Emert says.
"But we don’t recommend installing powder metal seats in diesel engines because powder metal seats can’t take the heat and compression in this kind of application. They may shatter," says Emert.
Other valve seat manufacturers offer a variety of different alloys for valve seat inserts, as well, including various powder metal formulas. But powder metal has been slow to catch on in the aftermarket. Although engine rebuilders are seeing more late model heads, many still prefer to use alloy inserts.
However, Bill Dolak of Dura-Bond says traditional cast iron seats won’t hold up well in late model engines. He recommends one of two different alloy seats depending on the application, either the "30000 Gold Series" valve seat inserts made of finely dispersed tungsten carbide in a matrix of tempered tool steel and alloy iron particles, or his company’s "70000 Diamond Series" inserts which use a higher temperature mix of tool steel and tungsten carbide. He says the 30000 Gold Series seats are easily machined and offer good wear and heat resistance for naturally aspirated and turbocharged engines. For high performance, heavy-duty and dry fuel applications he recommends the higher temperature 70000 Diamond Series inserts.
Dale McKitterick of Precision Engine Parts clarifies what alloys are appropriate for which applications. His company offers high chrome alloy seats (which are good for unleaded gasoline engines in most passenger car and light truck applications), nickel alloy seats and M2 tool steel alloy seats (for severe duty, high temperature applications), ductile iron seats (popular with many race engine builders) and beryllium/copper seats (used mostly in megabuck high end racing engines). McKitterick says he also has powder metal seats, but only a few customers have asked for them.
Qualcast, Tucker Valve Seats, Martin Wells and others all offer a variety of different alloy seats for various types of engine applications. The important point here is to choose a replacement seat that is right for the application. Higher load, higher temperature applications require harder seats. Follow the recommendations of the valve seat insert suppliers because they know what works best in each type of application.
Cast iron inserts are still used for light duty intake valve applications but should never be used on the exhaust side. The metal is just too soft to withstand the operating temperatures. For exhaust valves, a hard insert made of high chrome stainless steel, high nickel alloy or a heat resistant alloy must be used. Stellite inserts, which are made of a nonmagnetic cobalt alloy and are the hardest inserts available, are recommended for the exhaust valves in heavy-duty, high temperature engines and those that burn dry fuels such as propane or natural gas.
Tom Tucker of Tucker Valve Seat, says 440 stainless steel seats or Silicone XB (an iron seat with 18 percent chrome) are the most popular aftermarket seat materials today. But he also stressed the importance of choosing a seat that’s designed for a specific application.
"We have a tool steel tungsten carbide material for natural gas applications that holds up especially well. We also have an ‘E’ series material that provides superior hot hardness but is not as hard or abrasive as #3 Stellite."
Roger Klump of Martin Wells says his company has been selling its Well-Tite alloy for more than 30 years and he’s never seen a failure with the product. "It has the same wear characteristics as a 52 RC Stellite-type of product but with a hardness of only RC 35 to 37," explains Klump. "The Well-Tite alloy contains 42 percent nickel, which provides good heat transfer and valve cooling. It also contains 10 to 12 percent chrome for oxidation resistance, and 7 percent moly for toughness."
Seats should not be replaced until the head has been thoroughly cleaned and inspected. This includes checking for cracks (especially around and near the valve seats) and checking the deck surface and cam bore for straightness. Any welding and/or straightening that may be needed must be done before remachining the valve seats or installing new inserts.
Also, the valve guides should be replaced or reconditioned before the seats are machined. Concentricity between the seat and guide is absolutely essential for a proper alignment, good compression and long term valve durability.
The cylinder head must be dimensionally and geometrically within specifications before seat counterbores are machined. That includes cylinder head thickness, valve guide clearances, concentricity and perpendicularity. There should be no warping, twisting or any type of misalignment anywhere in the head.
The first step in seat replacement is removing the old seats. A variety of methods can be used to remove valve seat inserts from aluminum heads. Putting the head in a cleaning oven is sometimes used to loosen the seats enough to where they may fall out. Knowing the secret password necessary to keep good seats in place while allowing the damaged heads to release is critical (of course, there is no password). Using an oven in this way is a lengthy process that offers no real "predictability" regarding seat loosening.
Another method that does not involve heat is to use a cutter slightly smaller than the outside diameter of the existing valve seat insert to cut away most of the old insert (this works on softer alloy seats but not very well on powder metal seats). Stop cutting when the old seat begins to rotate in the head. What remains of the old seat can now be easily removed.
Another method of cutting out a seat is to use a die grinder to slit and weaken the seat. Just be careful not to cut all the way through the seat and into the counterbore.
Prying out valve seats also works if there is enough of a lip under the inside edge of the seat, but this technique also risks damaging the counterbore if not done carefully.
To remove hard seats, arc weld a bead all the way around on the seat. As the bead cools, it will shrink and loosen the seat. For more information on this procedure, see the March issue of Engine Builder, page 28, "The Whys and Hows of Welding Aluminum."
Another trick is to place a valve that’s somewhat smaller than the seat in the head and weld the valve to the seat. The valve stem can then be used like a driver to push out the seat.
Once the inserts are out, check for cracks or erosion damage under the seats in the counterbores a common problem on many aluminum heads. If cracked or eroded, the metal can be rebuilt by TIG (tungsten inert gas) welding, and remachining the head to a new seat.
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