Bumping clearance is the clearance between the cylinder head and the top of the piston at TDC. This clearance is provided to avoid the piston from banging the cylinder head in unloaded condition.
Bumping Clearance is measured by using a lead ball. or Feeler gauge.
Bumping clearance changes either due to worn-out bearings or improper cylinder head gasket thickness.
Bumping clearance can be adjusted either by changing the thickness of the cylinder head gasket. Or by adding shims to the bearings.