Checking the piston ring end gap with a feeler gauge.Piston rings are added to pistons in order to ensure that the piston can, at all times, and at all temperatures, form a sliding, gas tight seal between itself and the cylinder bore.

In order to ensure the correct piston rings are being fitted, during assembly you should check the piston ring end gap. There are two measurements involved here: one when the ring is free and not fitted, and the other when the ring is inserted in the bore.

 Piston ring free gapPiston ring gap, ring in bore

See also the entry for piston ring groove gap.

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