1: Take out the spark plug
2: Take a piece of solder, about 6 inches long and bend the last inch 90 deg.
3: With the piston at just after BDC, insert the short 90 deg end of the solder into the sparkplug hole and orient so it is touching the cylinder wall, and in line with and on the same axis as the wrist pin.
4: Roll the engine over in the normal direction of rotation, turn the flywheel with your hand. Be careful as it may turn a bit stubborn because you are using the solder to show you the clearance between the piston crown and squish band in the combustion chamber by flattening it between the two. Cycle it two times or more to ensure complete compression if the solder.
5: Pull the solder out and measure the thickness of the solder at the very end that got flattened out. This is your squish clearance measurement.
Note: If you want to be even more accurate, remove the cylinder, exposing the piston.
1: Cut 4 small pieces of solder, about 1/2 to 3/4 inch long.
2: Using a dab of grease, stick all four pieces to the top of the piston with one end as near the edge of the piston crown as possible while allowing for installation of the cylinder. If the piston crown as viewed from above from the operators perspective were a clock face, position them at 12:00, 3:00, 6:00, and 9:00
3: Make sure crankshaft is positioned after BDC but well before TDC in normal direction of engine rotation.
4: Install and bolt down cylinder
5: Perform step #4 above
6: Remove cylinder and measure all four pieces of solder.
Tip, use small gauge solder for this. It will compress much easier and give you more accurate results.
