Valve Lapping Tool

Small engines with overhead valves should have their valves lapped periodically. I thought I once saw a number on how many hours between lappings, but cannot find it. A tool for turning the valve is needed. I did not want to pay almost $15 (including shipping) for a piece of dowel and two suction cups. So, I made my own cranking tool with spring tension from steel scraps for lapping valves. The second photo shows how you know if your engine has overhead valves. Look for the cover imprinted with OHV.

Tools

1. Drill press and bits
2. Hacksaw or metal cutting bandsaw
3. Grinding wheel
4. Thread tap
5. Welder (not needed if made from wood rather than steel)
6. Angle head grinder with cutting wheel
7. Center punch

Materials

1. 1/4 inch steel scrap
2. 1/4 rod
3. 8-32 machine screw
4. Spring from an old self-opening umbrella

Select a drill bit the same size as the valve stem or slightly larger. The diameter of my valve stems is 1/4 inch. I drilled two holes about 1 1/4 inches apart into some scrap 1/4 inch steel. If tools are limited, wood could be used. It could even be thicker than 1/4 inch.

I used a metal cutting bandsaw to cut a body for my crank tool. Drilling first ensured that neither of the holes were too close to the edge. I ground away any sharp or rough edges and rounded one end. I left the other squared off for easier placement of a threaded hole for a locking screw.

I used a center punch to locate precisely a hole I would tap later for an 8-32 screw. For those who use the Metric system, an 8-32 screw would be close to an M4 screw. 32 means fairly fine threads. I used a vise for the drill press to hold the work piece. All of this could be done with a handheld battery drill. Some years ago I bought an 8-32 tap packaged together with the proper size numbered drill bit for the tap.

Carefully tap threads in the hole for the locking screw. Select a screw that is long enough, but not too long.

I used a stick welder to secure a piece of 1/4 inch rod in the other hole to be used as a crank handle.

Some years ago my wife gave me one of those compact umbrellas that opens by itself when a button is pushed. Key parts are plastic, and one day a long spring from the umbrella uncoiled inside my car. That spring is exactly the right size and strength for this project. I placed the valve in its guide and let it settle against its seat. I measured the length of stem poking through the engine head and subtracted for the thickness of my crank mechanism. I cut a spring a tiny bit longer than the space for it. The spring did not cut with a hacksaw. I had to use an angle head grinder with a cutting wheel. I ground the ends smooth so they will not dig into any metal on the engine head when the crank is turned. Place the spring on the valve stem. Hold the valve onto its seat. Slide the crank onto the valve stem. Gently tighten the locking screw. I quickly realized my spring was too long and adding too much tension to the valve in its seat. I shortened the spring just a little until it added what I felt was the right amount of tension. Should you need to add tension, place a washer or two onto the valve stem.

In using my lapping tool I will smear some valve grinding compound around the mating face on the valve and slide the stem into the valve guide. Then I will place the spring and lock the crank handle on the stem. I will use the crank to turn the valve one direction and then the other.* I will alternate that pattern until I am content the optimum amount of lapping has happened. From a video I watched, a rough raspy sound at the beginning quiets considerably when the surfaces are restored and fit well. Then clean away the remains of the abrasive compound. The intake valve is larger than the exhaust valve, so that helps to distinguish them.

Here is some information on accessing the valves and reassembly, as well as setting the correct valve lash.

*The valve lapping tool using suction cups on its ends does change the rotational direction. Over 50 years ago I was in a Sears store. They had some new old stock items that appeared to be on a closeout. One was a valve lapping tool with a crank and a gearbox. Turning the crank caused the tool to rotate in one direction for a few turns and then reverse by itself for almost the same number of turns before it reversed again.

Valve grinding compound can be purchased at your auto parts store. The first photo shows the Permatex product. The second photo shows some compound smeared on the valve face. I found I could push against the spring tensioner, hold it for a second or two, and some grinding compound would reposition itself between the mating surfaces to refreshen without removing the valve from the engine head. (See the third photo.)

Install the valve in its guide. Be careful not to let grinding compound into the valve guide. Place the spring and the crank handle on the stem and lock with the retaining screw. A little motor oil on the valve stem is helpful. (See the photo.) Crank a few turns in one direction, then a few turns in the opposite direction. You should be able to see mating surfaces ground to fit one another on both the seat and the valve.

When finished be certain to clean away all remnants of the grinding compound. Oil or solvent on a rag helps with compound removal. An initial oil change after about three hours of use is good to wash away anything left from the lapping process.