"The Lawn Mower Repair Man's Front Page"
By Dave Aker
A method I have used succesfully to set the magneto air gap (distance between stationary laminations of the coil and the rotating magnets) on small engines with coil under the flywheel is as follows.
Remove flywheel and clean ends of coil laminations. These ends must be clean to ensure plastic tape adheres properly. Using plastic electricians tape, place one layer of tape across each leg of coil. Replace flywheel, rotate flywheel 2-3 revolutions and remove. Check tape for marks from magnet contact.
Keeping in mind the thickness of common plastic electrician's tape is about .008-.009 in., a fairly accurate setting of air gap can be performed using this method. Use more than one layer of tape if necessary. Note; Do not stretch tape when removing from roll as this will reduce thickness. Also if may be helpful to wrap one piece of tape around all lamination ends including point housing to help hold tape in place. If you have extra sticky tape, just cut a short piece to fit end of each coil lamination.
Out of curiosity, I even use this method to check gap on engines that do not need air gap adjustment simply to see what the gap is.