My recommendation is to never consider lapping valves sufficient. Guides must be checked first, and replaced if needed, then reamed to correct clearance. Valve seats need thorough inspection . Only a qualified machine shop can replace damaged or sunken seats. Installed valve height is critical to ensure a long term repair. Stretched valves should be replaced. Seats should be cut to ensure valves seat properly, then valves can be lapped. Each valve is the inspected to see where it contacts the seat, if too close to the edge of the face, the valve will overheat. If satisfactory, coat the valve seats with Prussian blue or similar, and cut the (usually) 60 degree and 30 degree angles to achieve a "three angle valve job." There are specs for the final seat width, with the exhaust seat typically wider than the intake seats in order to help dissipate heat. Skip any of these steps, you are risking doing the job over sooner than you wish.Air cooled engines require far more care than water cooled engines.
