Lotsa people have asked me in the past on how to measure head clearance, and with all this talk on head clearance here are two methods that I use. Feel free to correct me if I'm wrong.

1) Probably the simplest method to measure existing head clearance with all the shims in place is to do the solder test. Remove the plug from the motor. Take a length of solder of about three inches in length and bend one end into an L shape. A small .25" of bend should be plenty. Rotate the flywheel unti the piston just closes the exhaust port. Push the L part of the bend into the plug hole and turn the solder so that the bent part contacts the piston liner wall. Rotate the fly wheel completely over tdc and back down the liner. Pull the solder out and measure the squished part for true head clearance. This is an okay test to give you an idea on head clearance. Make sure you don't break any solder off in the combustion chamber!!

2) The second method I use requires removing the combustion chamber head.
Here I take two short threaded screws with washers and screw them in the head screw holes until the washers contact the pistion liner. This is to hold the liner in place when you turn the motor over. Gently!! Turn the flywheel over until piston reaches tdc. On some new motor this can be tough because the piston really wants to push the liner out of the crankcase. Measure the distance from the top of the piston to the top of the liner and write that down. Then measure the distance between the top of the head button to the start of the combustion chamber dome and write that down. Subtract those two numbers and you have your head clearance. If you took the shims off the head button while measuring you must subtract that number from the difference to get head clearance.