Piston ring end gap, the distance between the ends of a ring when it's in the cylinder bore, needs to be very precise for proper engine operation. It involves inserting the ring into the bore, measuring the gap, removing the ring, carefully grinding a tiny bit off the end, then inserting the ring again to measure the gap. If you accidentally remove much material, you're basically screwed and need to get a new ring and start over.
There are three rings, the top, bottom and oil rings. The ring gaps are ideally 60 degrees apart so that the gaps do not line up with each other.
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