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Valve Seat Installation Procedures
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Many experts recommend recutting the counterbores to accept new oversized seats. Some engine builders will install new standard-sized inserts in the existing counterbores. It works on some large cast iron cylinder heads with thick walls, but it’s risky on most automotive applications. The recommended approach is to remachine the counterbores to accept oversized inserts. This allows you to control the interference fit between the seat and head so the seats don’t come loose.
Recutting the counterbore also allows you to control runout in the counterbore and concentricity with the valve guide. The counterbores must be smooth, round, have flat bottoms and be centered to their valve guides for proper alignment and good heat transfer between the seat and head. The final dimensions of the counterbores must be within .0005˝ for the proper fit.
If a counterbore is too rough, distorted or out of round, it won’t make good metal-to-metal contact with the seat. It can also distort the seat. This will reduce heat flow from the seat to the head and make the valve run hot. That you don’t want because it leads to valve burning and warranty problems down the road.
If you’re replacing an integral seat in a cast iron head (and the cylinder head has enough thickness to accept a new seat), the counterbore should be cut to a diameter approximately .100˝ larger than the valve head diameter. The inside diameter of the replacement seat will typically be about .100˝ smaller than the valve head diameter and require a depth of about .188˝ to .250˝ depending on the application.
Accurate cuts also require proper fixturing. Keep your tooling setup as "short and tight" as possible to assure maximum rigidity. The less deflection in the tooling, the more accurate the dimensions of the cut and the greater the concentricity of the counterbore.
Be careful not to distort or put a twist into the head when clamping it to a fixed rail holding fixture.
You’ll get the most accurate cut with correct size pilots (which must be straight), and using the correct spindle speeds and feeds. Machining recommendations vary depending on the type of equipment and tooling used, but Dura-Bond recommends using cutting oil and a spindle speed of 400 to 600 rpm when cutting valve seat counterbores in aluminum heads. When cutting cast iron heads, Dura-Bond recommends using no lubrication and a slower cutting speed of 100 to 250 rpm.
Something else to keep in mind when cutting counterbores is that the seats for many late model heads don’t go by fractional sizes anymore. Seat sizes can vary considerably so using a fixed size cutter is not the best choice. An adjustable cutter will provide the flexibility you need to properly size the counterbores.
The recommended amount of interference between the valve seat insert and head may vary depending on the size of the insert, the type of insert (alloy or powder metal) and type of head (cast iron or aluminum). The best advice is to use the amount of interference recommended by the OEM engine manufacturer.
Too much interference runs the risk of cracking the head while too little interference increases the risk of the seat coming loose or falling out. One of the leading causes of seats coming loose, however, is not the amount of interference between the seat and head but elevated operating temperatures. Anything that causes the exhaust valve to run hot may also cause the seat to loosen.
Philip Carrasco at Tucker says seats may require anywhere from .002˝ to .010˝ of interference depending on the application and the roughness of the surface in the counterbore. For aluminum heads, an interference fit of .005˝ to .007 ˝ is commonly used. For cast iron heads, .003˝ to .005˝ is about right.
Martin Wells’ Roger Klump says he recommends an interference fit of .005˝ to .006˝ for everything, aluminum and cast iron.
Rick Emert of SB International says he tells his customers to use .005˝ press fit when installing seats in cast iron heads, and .007˝ minimum in aluminum heads regardless of what type of valve seat inserts they are installing. "We do not recommend using any type of locking fluid, staking or peening when installing seats. You should be able to put a concentric seat into a concentric hole with the right amount of interference and have it stay there," says Emert.
Carrasco, on the other hand, says a lot of engine builders have had success using a locking fluid. "They tell me it helps fill any voids between the seat and head for improved heat transfer and valve cooling. You don’t see many production engine rebuilders doing this but you do see smaller shops doing it," said Carrasco.
Installing the new seats once the counterbores have been cut is a fairly simple procedure. A piloted driver is used to push the seat into position. Many aftermarket seats have a bevel or radius on the outside lower edge to make installation easier. Make sure this side faces down when installing the seat.
Some engine builders preheat the head or chill the inserts in a freezer or with nitrogen prior to installing them to make the job easier. Others say this should not be necessary if you use the normal amount of interference fit. Even so, it’s another trick that may come in handy on a problem head or application that requires something out of the ordinary.
After the seats have been installed, they can be finished as required. The guides must be reconditioned or replaced before doing this, however, because all seat work is done by centering off the guides.
Seats should be as concentric as possible for a tight compression seal and proper valve cooling. The rounder the seat, the better. Seat runout should not exceed .001˝ per inch of seat diameter. Some shops aim for .0005˝ or less of runout. The best way to check concentricity is with a runout gauge. Pulling vacuum on the valve port with the valve in place is another method for checking the mating of the seat and valve. But the ability to hold vacuum is no guarantee of concentricity. Both methods should be used to check the quality of your work.
Seat width is also important for good heat transfer, proper sealing and long valve life. If the seat is too narrow, wear resistance and heat transfer can suffer. And if the seat is too wide, there may not be enough pressure to provide a tight seal. A wide seat also tends to trap deposits that can hold the valve off its seat. This too, can reduce heat transfer as well as compression. As a rule of thumb, the ideal seat width for intake valves is usually around 1/16˝. For exhaust valves, it’s 3/32˝ or whatever the manufacturer specifies.
The point at which the valve and seat mate is also important. If the area of contact is too high on the valve face (too close to the margin), the valve may be sunken into the head. This increases installed height, upsets valvetrain geometry and restricts free breathing. If the area of contact is too low on the face (too far from the margin), the valve will ride too high on the seat. As the engine warms up and the valve expands, the contact point moves down the valve face away from the margin. The valve may lose partial contact with the seat causing it to lose compression and run hot.
Ideally, the valve should contact the seat about one third of the way down the valve face (about 1/32˝ from the margin) so there is about 1/64˝ of overhang between the margin and top of the seat.
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BACK TO BASICS Valve Seats
The most critical sealing surface in the valve train assembly is between the face of the valve and its seat in the cylinder head when the valve is closed. Leakage between these surfaces reduces the engine’s compression and power and can lead to valve burning. To ensure proper seating of the valve, the valve seat must be:
- Correct width
- Correct location on the valve face
- Concentric with the guide (less than .002˝ runout).
The ideal seat width for automotive engines is 1/16˝ for intake valves and 3/32˝ for exhaust valves. Maintaining this width is important to ensure proper sealing and heat transfer. However, when an existing seat is refinished to make it smooth and concentric, it also becomes wider. Wide seats cause the following problems:
- Seating pressure drops as seat width increases.
- Less force is available to crush carbon particles that stick to the seats.
- Valves run cooler, allowing deposits to build up on them.
The seat should contact the valve face 1/32˝ from the margin of the valve. When the engine reaches operating temperature, the valve expands slightly more than the seat. This moves the contact area down the valve face. Seats that might contact the valve face too low might lose partial contact at normal operating temperatures.
Like valve guides, there are two types of valve seats integral and insert. Integral seats are part of the casting. Insert seats are pressed into the head and are always used in aluminum cylinder heads. Most pre-1978 integral seats are soft cast iron. After 1978, most manufacturers began to produce cylinder heads with induction hardened cast-iron seats able to withstand the higher heat of exhaust applications. Insert seats are added to the cylinder head after casting, or as replacements for worn integral seats.
from the book:
"Complete Automotive Engine Rebuilding and Parts Machining"
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