How To Lap Valves
Article by Mark Trotta
Lapping valves doesn't take the place of a valve job, but it does show if the mating surfaces are compatible. Back in the day, this is one of the things that was taught in high school auto shop. Sure, it's old school, but you need to know how to lap valves if you want your engine to operate at peak efficiency.
Grinding vs Lapping Valves
The purpose of grinding valves is to have the valve and valve seat make an air-tight fit.
The purpose of lapping valves is to see if that was done correctly.
If your cylinder heads were rebuilt at a machine shop, a specialized machine put a precise bevel on the valve and seat surfaces. But a good seal is not guaranteed. Experienced engine builders will always check and recheck everything before final assembly.
If the valve seat and face were ground correctly, the lapping process should take a minute or less for each valve. It actually takes longer to clean, prepare, and round up the supplies than to actually lap the valves!
Do My Valves Need Lapping?
When closed, valves need to be air and liquid tight. With the cylinder head upside down on your workbench, prop it so the combustion chamber is flat. I use 2x4 wood blocks for this. Take a cup of water and pour it into the combustion chamber. Does it seep right through into the port? If it does, the valves are not seating properly.
What You'll Need
To lap valves, you'll need valve grinding compound and a valve lapper, which is basically a wooden dowel with a suction cup on the end.
A valve spring compressor will be needed for removing and installing the engine valves.
If you're checking the seal on a head that's just come off an old dirty engine, wire wheel the carbon and gunk off the sealing area and the stem. Make sure there is no dirt in the valve guide or on the valve stem.
Oil the Valve Stem
After taking the valves out and cleaning them, give the valve stem a light coat of engine oil before putting it in the guide. I use 5/30W, but it really shouldn't matter. Spread a bit of grinding compound onto the edge of the valve.
Stick the Rubber Cup On the Valve Face
Once the valve is in the head, the next step is to wet the suction cup on the lapper and stick it on the valve face. Once the lapper is grabbing the valve, you begin the lapping process. Place the lapper in between both hands, and using light pressure, rotate the tool back and forth at a moderate pace. The motion is similar to starting a fire with two sticks.
Lift the valve up periodically and rotate it 180 degrees to ensure the grinding compound is getting evenly spread. You'll want to watch the progress - you're looking for a consistent grey ring on the valve and valve seat with no breaks or high spots.
When you can feel (and hear) the compound losing its cutting ability, wipe off and check. Both faces should be a dull grey when you're done, with an even width across both faces.
The ring doesn't have to be in the dead center of the valve, just a nice even ring around the valve. You should see the same consistent grey line on the valve and the seat.
Although some seats may need two or three times of lapping, it is not necessary or good to over-lap the valves. It is unlikely, but possible, to remove too much, and doing so will ruin the valve seat.
If you are unable to achieve a consistent grey ring, the seats are probably worn out and will require cutting. That is a job for a machine shop or experienced DIYer.
The three-ounce Permatex 80037 is a mixture of four grits; 120, 150, 180, and 220. The compound starts off as 120 grit and as you work it, ends up as 220 grit.
Shop: Valve Grinding Compound
Shop: Valve Lapper Tool
Note: The suction lapping tool doesn't always stick on smaller valves. Those can be done slowly by hand.
Wipe Off the Compound
When done lapping valves, make sure you remove all the valve-grinding compound, as it is highly abrasive and could harm your engine.
You can test the seal by leaving the port full of water overnight with the valve springs installed. A full port the next day means the seal is good. Machine shops test with a leak-down gauge - it does the same thing more quickly.