When to replace your valve seats. A valve seat can only be cut so many times before they are too deep into the head. The valve will be in the wrong spot for the valvetrain when the seats have worn out.
A 911 cylinder head is made of aluminum. This is a soft metal and could not take the valve slamming into it. Some engines, (other than Porsche), use a cast iron head. Back in the day, the seat was part of the head and also made of cast iron. In some early Porsche 356 engines, we have found a bronze type seat, but this is rare. Porsche uses a hardened powdered steel alloy type material for the seats.
We got our new seats from a local supplier. They supplied a Tungsten Alloy cast seat. Then they machined them to the size we ordered. The size is determined after removing the old seats. We also had them add an angle to the exhaust exit side. Now we have a seamless line between the head and exhaust valve seat on the inside.
Replacing Valve Seat Inserts
Hardened valve seats are installed with an exceptionally large interference fit. The interference is between 0.15 mm to 0.18 mm. This is about three times the interference of the valve guides.
To remove the valve seat, it will need to be cut using a carbide end mill. Locate the end mill on the side of the seat and cut down approximately 7.75 mm. Be careful not to damage the aluminum cylinder head. Once the valve seat has been cut, carefully pry out the old seat.
With the seat removed, you will need to re-machine the pocket. To machine this pocket you use a seat counterbore tool. The reason for this step is to clean up the seat pocket. To get a gas-tight seal the pocket must be clean and square. A counterbore tool uses the valve guide pilot to keep the seat perpendicular to the valve. This is why we did the valve guides first.
Installing Valve Seats
Once you have cut the counterbore you can go ahead and size the new seats. Size the new seat so that it has an interference fit of 0.15 mm to 0.18 mm. To install the new seats, heat the cylinder head first to 200°C or 394°F. And the valve seat inserts need to be cooled in the freezer overnight. Once the cylinder head is at temperature install the valve guide pilot. Look at the seat, there is a chamfer on the down side. Then, using a valve seat driver, install the new seat. Drive the seat all the way to the bottom of the counterbore pocket.
Final Heating of the Heads
Once the new seats have been installed let the cylinder head cool. Let the heads cool back to room temperature. Then heat the heads again. Bring them back up to 200°C for 2 hours, then allow the heads to cool slowly to room temperature. To let them cool slowly leave them in the oven. This second heating will show you if you have gotten your interference correct. The seats would fall out if they were too small. The final heating will lock the new seats in.
Back to Lesson 5, Cutting the Seats
Now the inserts will just need to have the three-angle valve job applied to them. We invested in a three-angle cutting head, with adjustable size capabilities. This has been a great time saver. If you have lots of seats to cut the time saved can pay for the tooling. The tooling also allows for more consistency. When you set the depth on the first set, the others can be all the same. Because of this, there is even more time saving when setting the valve stem depth.
Conclusion on Replacing Valve Seats
You may not have access to the machines to do this job. This may mean that you do not do any of this work for yourself. This lesson will help you to understand what should be happening to your heads when you send them out.
One recommendation if you do this job at home or your shop. Buy a benchtop oven. No one likes the smell of heating an old head in the house. Your new benchtop oven is great for heating other parts. It is an asset if you want to get into the ceramic coatings that are on the market today because ceramic coatings need to be baked on.