Integral Valve Seat Repair
Article by Mark Trotta
Vintage small engines will have either one of two types of valve seats--insert, which are replaceable, or integral, which are not replaceable. Many old flathead engines have the integral style. So what can we do when they need repair?
There's basically two ways to repair integral valve seats. If they are badly burned or pitted, they may need to be re-ground with a grinding stone or a valve seat cutting tool. But if they're just slightly corroded, integral-style seats can often be renewed by 'dressing' them with Emery cloth.
Valve Seat Renewal
Upon disassembly of this 1968 Tecumseh H25 flathead, I found corroded valve seats.
Taking a sheet of Emory cloth, I cut out several 2" x 2" squares and poked a hole in the center.
Starting with a piece of coarse Emory cloth, I stuck the square onto the valve, reached into the valve chamber from the other side, and turned the other end of the valve back and forth with my fingers, all the while keeping a slight pressure on the valve head.
I rotated the squares back and forth on the seat, switching from coarse to medium to fine grit as needed.
The intake and exhaust ports were also cleaned up using strips of Emory cloth wrapped around my finger.
Valve Seat Refinishing
When non-replaceable valve seats are too badly pitted for renewal, they can often be re-ground using a grinding stone or valve seat cutting tool. These are specialty tools which are generally too expensive to buy for a one-time use.
Neway Valve Seat Tool
I have several Neway valve seat cutting tools. With Neway valve cutters, you only need to buy the cutter sizes needed for the job at hand. Additional cutter sizes can be purchased if needed.
On Tecumseh and many other small engines, valve seats are ground to an angle of 46 degrees. Check a shop manual for proper specifications.
The purpose of lapping the valves is to see if the valve and seat make an air-tight fit. You'll need some 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.
CAUTION: Make sure you remove all the valve-lapping compound from all surfaces, it is highly abrasive and could harm your engine.
Some seats may need two or three sessions of lapping, but you do not want to over-lap the valves. Removing too much material may ruin a valve seat.
If you are unable to achieve a consistent grey ring around the valve head, the seats are probably too worn and will require refinishing.
Flathead Valve Adjustment
Though small engine flathead valves are adjustable, there is no actual "adjuster". The only way to decrease lash on flatheads is by slightly grinding the stem. The only way to increase lash on flatheads is by replacing the valve.
If flathead valve adjustment is needed, it requires the valve be removed from the block. Fortunately, this rarely needs to be done.