What is valve lapping and how it’s done

Valve lapping is a process that ensures a perfect fit for the intake and exhaust valves in the engine cylinders on their seats. Usually lapping is performed after performing an engine overhaul, or when the old valves are replaced with new ones.

Lapped valves provide maximum tightness in the combustion chamber, ensuring engine normal operation and its technical characteristics, a high level of compression, and engine efficiency.

If valves are not lapped, then some energy of the burned gasses will be lost and it will not give the engine enough power.

Some modern cars are equipped with an automatic control system responsible for the valve shapes. It simply grinds the valve, so there is no need for manual lapping.

Valve lapping can be done without going to a car workshop, however, it does require some mechanic skills to do, and if you are not that person, then you should go to a mechanic.

This process requires some tools like lapping paste, a device for removing the valves, and other tools which we will discuss further in this article.

What tools are required for valve lapping?

Before this procedure, the cylinder head must be removed first, so you will also need a tool for removing the cylinder head. For this, you can use a regular screwdriver or padlock keys, but you also need a torque wrench to put the cylinder head back in its place, because you will need to tighten the cylinder head bolts, and that it’s possible using a torque wrench.

For valve lapping you will need the following things:

Drill or electric screwdriver. Because lapping is performed in both directions of rotation, you can use a drill that can rotate in both directions or an electric screwdriver that does the same thing

  • A paste for grinding valves. You can buy this ready-made paste from automotive shops. If you have some skills, you can make this composition on your own.
  • Manual valve holder. You can find this tool when buying the special paste for valve grinding, they often will be sold together or you can buy them separately
  • A hose and a spring. The spring must have a low rigidity, its diameter is 2 or 3 millimeters larger than the diameter of the valve stem. So is the hose, so it can be put on the rod. You can also use a small clip to secure it. You also need a short metal rod with a diameter similar to the piston rod so that it fits in the rubber hose.
  • Kerosene. Needed for checking the lapping and cleaning.
  • A dry cloth. To wipe the treated surfaces.
  • Solvent. To clean the work surfaces

How valve lapping is done?

Basically, there are two ways you can do valve lapping. One way is manual lapping, which requires some skills for it and the second method is the so-called “mechanized” lapping that you can do with the drill or electric screwdriver, which we will discuss later on.

Regardless of what method you choose, the first step you need to do is to remove the cylinder head, and then you should remove the valves from the cylinder head. After that, you need to clean the valves from carbon buildups

To do this, it is better to use special cleaning agents, as well as an abrasive surface to properly remove, grease, and dirt from the surface.

Lap valves using a drill

Lapping valves with a drill is the best option to save time and effort.

  • Take a prepared metal rod and put a rubber hose with the same diameter on it. To fix it better, you can use a clamp.
  • Attach to a metal rod in the chuck of the drill or electric screwdriver.
  • Put a spring on the valve’s stem and install it in its seat
  • Pull the valve out of the cylinder head and apply lapping paste around the perimeter of its plate.
  • Insert the valve stem into the rubber hose, and, if necessary, use a clamp of the same diameter for better fastening.
  • Use the drill at low speed and start grinding the valve in its seat by moving the drill back and forth, the installed spring will help here. After that, reverse the movement of the drill in the opposite direction until a matte band appears on the valve body.
  • After lapping, wipe the valve from paste residues, preferably with a solvent. It is necessary to remove the paste not only from the valve’s plate but also from its seat. Don’t swap the valves, because lapping is performed for each valve individually.

Most modern engines don’t need valve lapping because they are made out of aluminum, and, in case of damaged engine parts, the risk of valve replacement is higher.

How to check valve lapping?

After you lap the valves the most reliable way to know if they are lapped correctly is to check the passage of gases through the valves under load. For this, is necessary to position the cylinder head upside down so the valve holes are on the top and the collector well holes are on the side.

Next, add a small amount of fuel into the valve outlet cavity. Using an air compressor, add some air into the opening of both the intake and exhaust manifold. If the lapping has been successfully done, then, from under the valves, even if they are under the pressure of the compressed air, the formation of air bubbles from under the valves should not exist.

If you see air bubbles coming out from under the valves, that means there is no tightness and the lapping was done incorrectly.

The method described above is extremely effective and can be used in any engine.

Conclusion

Valve lapping can be done without going to the car workshop, but only if you have some skills and the necessary tools for it. If you are in doubt and think that is not the right thing to do, then don’t do it.

Below there is also an explanatory video to further understand.

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