Bumping clearance as the name signifies is a clearance given so that the piston of the reciprocating compressor would not bump into its cylinder head. Routine jobs like lifting the cylinder head to change the low pressure or first stage valves can change the bumping clearance if the correct thickness gaskets are not used or if the head is over tightened thus squeezing out the gaskets.
Many engineers miss this vital adjustment during overhaul of the compressors and efficiency and free air delivery of the compressor suffers.