"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.
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."
