Over time, erosion and corrosion can eat away at the metal resulting in compression leaks, elevated valve temperatures and sometimes valve failure.
If a valve seat insert doesn’t have the correct amount of interference fit in the cylinder head, it’s possible the seat may loosen up and fall out damaging the valve, head and piston. The potential for this happening is present in any aluminum cylinder head with iron, alloy or powder metal (PM) valve seats, especially if the heat gets too hot due to a cooling or preignition problem.
Chrysler has reportedly had problems with the exhaust valve seats loosening up and dropping out in some of its late model 4.7L and 5.7L Hemi engines. Seat failures have typically occurred between 60,000 and 80,000 miles, often after an overheating episode in which the coolant got above 250 degrees F.
Some engine builders say Chrysler didn’t use enough interference fit for the seats in these engines and recommend using an additional .002? of interference when the seats are replaced. Cutting the seat recess slightly deeper can also improve seat retention.
Some vehicle manufacturers use only a couple thousandths of interference when they install valve seats in a new cylinder head. They can get by with minimal interference because the heads are brand new castings that have been freshly and accurately machined. There’s been no wear, corrosion or motion to loosen up or distort the valve seat recess as is often the case with high mileage cylinder heads.
Many engine builders and valve seat suppliers say a few thousandths of interference may not be adequate when replacing valve seats in high mileage heads, and typically recommend using .005? to .007? of interference when installing iron alloy seats in aluminum heads.
With cast iron heads, thermal expansion is not as great, so .005? of interference is usually recommended when installing iron seats in an iron head. Powder metal seats usually require a little less interference fit compared to iron alloy seats, so the amount of interference typically recommended for PM seats is .003? to .005? in aluminum heads.
Using the recommended interference should eliminate any worries about the valve seats loosening up later and falling out. Peening or staking of the seats should not be necessary, nor should you have to use a locking compound between the seat and head.
Even so, some engine builders use locking compound or peening as added insurance to keep the seats in place. Others say the thin film of locking compound between the seat and head may inhibit heat flow from the seat into the head, increasing valve temperatures and the risk of premature valve failure.
The people who make locking compounds say this isn’t so, and, if anything, the compound will help fill any voids between the seat and heat to actually improve heat transfer. If the seat recess in the head is machined accurately and is round and smooth, however, there should be no voids and the seat should make good contact all the way around with the head.
Seat retention problems can occur if the seats are not installed properly. Cocking a seat or installing a seat with a sharp outer edge may gall the surface of the recess in the head. For best results, the valve seats should be chilled in a freezer prior to installation, and the cylinder heat heated to about 140 to 160 degrees F to reduce the amount of force needed to push the seats into place.
The lower outside diameter of the seats should also have a radius or chamfer to ease installation. Lubricating the seat can also help it slip into place without galling the head.
Valve Seat Alloys
Every valve seat supplier makes a big deal about the advantages of their proprietary alloys. And rightly so. The strength, hardness, wear resistance and thermal properties of a valve seat alloy can make or break it depending on the application.
An alloy that lacks the hardness and wear resistance for a high-heat, high-load engine application won’t hold up very well. The same goes for dry fuel applications such as natural gas and propane, which also require special alloy seats.
As a general rule, the higher the operating temperature of the engine and the higher heat loads it must endure, the higher the grade of seat alloy that will be required to handle the job. Cut corners here with a set of cheap replacement seats and you’ll probably regret it later.
Most tool steel and iron alloy valve seats have a thermal conductivity of 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.
Titanium valves do not cool as well as stainless steel valves, so they require valve seats that conduct heat at a much higher rate. For many years, the seat alloy of choice for titanium valves was beryllium copper, which conducts heat three to as much as six times faster than most iron alloys.
Beryllium copper valve seat alloys typically contain only about 1.5 to 2.5 percent beryllium. But that’s enough to create a seat material that provides high thermal conductivity along with the durability required for titanium valves in a high performance engine.
Some Be-Cu valve seats are 97% copper with 1.8 to 2.0% beryllium, and have a hardness of 38 to 41 Rockwell C. Other alloys contain less beryllium (0.2 to 0.6%) but add nickel (1.4 to 2.2%) to reduce hardness to 20 Rockwell C. By comparison, ductile iron seats typically have a hardness of around 32 Rockwell C.
But beryllium copper seats have some drawbacks. One is that the alloys are expensive. Another is that beryllium is a toxic metal. There’s no risk in handling the seats, but any dust that’s created with machining the seats can be hazardous.
In recent years, proprietary copper-nickel alloy valve seats have replaced be-cu seats in many performance and high heat applications. Some of these new copper-nickel alloys have a thermal conductivity rating of up to 90 BTU per foot per hour per degrees F., which is more than enough to provide adequate cooling for titanium valves or stainless steel valves in high output or high heat engines.
In the case of diesel engines, a couple of trends have emerged in recent years. Some automotive and light duty diesel truck engine manufacturers have started using copper impregnated powder metal valve seats. Applications include General Motors, Mercedes, BMW and others. The new copper impregnated powder metal seats are also being used in some big Class 8 truck engines.
When the seats are made, a thin copper wafer is placed on top of the powder metal seat. When the seat is sintered at high temperature, the copper melts and infiltrates the microscopic pores in the seat to increase the seat’s ability to transfer heat. It’s a more cost effective way of improving seat durability than using expensive cast alloy seats.
Some aftermarket valve seat suppliers have similar copper impregnated PM replacement seats for these applications, while others offer tool steel vanadium alloy cast seats as a lower cost replacement alternative.
Heavy-duty diesel engines have traditionally used various types of high chromium or cobalt cast alloy valve seats to handle the high combustion temperatures and loads they experience. Automotive seat alloys are usually not durable enough for diesel applications, so special alloys are required. In recent years, operating temperatures are even high due to exhaust gas re-circulation (EGR) requirements for lower oxides of nitrogen emissions from diesel engines.
This has created valve seat corrosion issues on the intake seats as well as increased wear on both intake and exhaust seats. Cobalt alloy valve seats typically require about 10 percent less interference fit because they expand about 25 percent more than other cast alloys. So this has to be taken into account if you are replacing cobalt alloy seats.
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.
According to one valve seat supplier, Caterpillar and Cummins have discontinued using high nickel alloy seats in some of their engines, and are now using a proprietary non-nickel steel alloy seat material. There’s no word on exactly what the ingredients are in these high temperature seats, but they are magnetic.