Bumping clearance is checked by putting a lead ball or plastic gauges over the piston and then turning the compressor one revolution by hand.
By doing this the lead ball will compress and the thickness obtained is the clearance volume.
This thickness is measured with vernier caliper or micrometer and is then compared with the manufacturer’s value. Adjustments are made in case there is an offset in the value.
The clearance between piston and cylinder head while piston at TDC is called bumping clearance. This can be measured in different ways. One common method is, remove the valves from the top of the piston. Put a lead ball of sufficient diameter into the cylinder. Slowly turn the flywheel one revolution by hand. Take out the lead piece and measure its thickness, which gives the bumping clearance.