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 run it's best.

valve lapping procedure

Is Valve Lapping Really Necessary?

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.

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 the 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 perform the lapping process!



Do My Valves Need Lapping?

When closed, valves need to be air and liquid tight. So, how can you tell if they are?

Here's a simple test: With the cylinder head upside down on a 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

The lapping procedure requires valve grinding compound and a valve lapper tool, which is basically a wooden dowel with a suction cup on the end.

best way to lap engine valves
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Clean Off Carbon

If you're checking a cylinder head that's just come off an old dirty engine, wire-brush or wire-wheel the carbon and gunk off the sealing area and on the stem. Make sure there is no dirt in the guide or on the stem.

Oil the Valve Stem

After taking the valves out and cleaning them, give the 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 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.

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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 seat, with no breaks or high spots.

lapping vs grinding

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.

valve lapping procedure

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's 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.

Permatex 80037 valve grinding compound

Shop: Valve Grinding Compound

valve lapping procedure

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 Unused Compound

When done lapping, make sure all of the grinding compound is removed from the cylinder head ports, as it is highly abrasive and could harm your engine once it gets into the oil.

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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.

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DIY vs Machine Shop

Machine shops will test valves with a leak-down gauge - it does the same thing only much quicker.

Lapping valves at home will take longer, but your only out-of-pocket expense is a valve lapper tool and a tube of valve grinding compound.

Permatex 80036 valve grinding compound


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