How To Lap Motorcycle Valves
Proper valve seating is crucial to a strong-running engine. If the valves and guides are good, lapping the valves will confirm they are seating correctly.
Grinding vs Lapping
The purpose of grinding is to have the valve and valve seat make an air-tight fit. The purpose of lapping is to see if that was done correctly.
Lapping New Valves
Valve grinding puts a precise bevel on the valve and seat surfaces, but a good seal is not guaranteed. If your heads were recently rebuilt at an engine shop, the valves were likely refaced by a machine. And if the valve seats and faces were ground correctly, the lapping process should take a minute or less for each valve.
Lapping Old Valves
Before you start, make sure there is no dirt in the valve guide or on the valve stem. Wire wheel the carbon and gunk off the head and the sealing area and the stem.
It will take longer to clean, prepare, and round up the supplies than to actually lap the valves.
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.
Shop: Valve Lapper Tool
Note: The suction lapping tool doesn't always stick on smaller valves. Those can be done slowly by hand.
Shop: Valve Grinding Compound
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. Although some seats may need two or three times of lapping, it is not necessary or good to over-lap the valves.
If possible, place the cylinder so the head of the valve is facing up. A couple of pieces of wooden 2x4's usually works.
Oil the Valve Stem
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.
Valve Lapping Procedure
Wet the suction cup of the lapping tool and stick it on the valve face. Once the lapping stick is grabbing the valve, you can begin the lapping process. Place the lapper between both hands, and using light pressure, rotate the tool back and forth at a moderate pace.
Lift the valve up periodically and rotate it 180 degrees to ensure the grinding compound is getting evenly spread. Watch the progress - look for a thin but even 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. It does not have to be in the middle of the valve, just an even ring around the valve. You will see the same consistent gray line on the valve and the seat.
Do not over-do lapping the valves. It is unlikely, but possible, to remove too much, and doing so will ruin the valve seat.
You want to have a consistent gray line on the valve and the seat. It does not have to be in the middle of the valve, just an even ring all around.
Caution: Make sure all remnants of the valve grinding compound is cleaned. The compound is like fine sand and will cause engine damage if not completely removed.
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.
Read: Motorcycle Valve Job