Piston Ring End Gap
Piston-to-wall clearance is measured to ensure the rings form a tight seal. The top compression ring contains compression and passes heat to the cylinder wall.
Piston rings have a split to install over the piston. The size the piston ring gap is crucial. It must be large enough to allow heat expansion and small enough to contain compression. Too much piston ring gap results in compression leaks and blowby, and too little gap results in the ends touching, resulting in scuffing, binding, and possible catastrophic engine failure.
Most manufacturers recommend measuring the piston ring end gap to ensure a tight seal. First, squarely place the top compression ring in the cylinder bore (no piston). Next, measure the ring gap at the top and bottom of the bore with the appropriate feeler gauge blade and compare them to the manufacturer's specifications. Some manufacturers recommend filing to expand the ring gap if it is below specifications.