Piston to Cylinder Wall Clearance
Piston to wall clearance is measured to ensure the rings form a tight seal. The top compression ring is responsible for containing compression and passing heat to the cylinder wall.
Piston rings have a split to install over the piston. The size of this 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 that the piston ring end gap is measured to ensure a tight seal. First, place the top compression ring squarely 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's below specifications.