The seal between the piston and the cylinder bore is maintained by cast iron piston rings which are located in grooves machined near the top of the piston. The rings are gapped to allow for thermal expansion,although at first sight the piston rings of certain models of older classic cars for sale may look like modern rings, the latest piston rings are thinner, made of better material and are of different shape as found in the innovative W12 engine of the new Bentley continental for sale.
More springiness can be imparted by using narrower and thinner rings, by making the inner and outer surfaces eccentric, so that the ring is thicker opposite the gap, and by hammering treatment of the inside of the ring opposite the gap area. This also results in the advantage of a more even pressure on the cylinder wall all round.
Whereas the older rings were cast in sand moulds, newer production is from tubes cast centrifugally in metal moulds. Also, the outer bearing surface of the ring is often plated with chromium and is slightly tapered to assist runningin and to give a good seal.
Porsche are one manufacturer on the leading edge of piston ring technology, and their latest design can be found n the engine of their latest Porsche 911 turbo for sale here.
Often, the lowest ring has radial corners or an L-section to maintain a controlled amount of oil on the cylinder bore. Over the years there have been a great many designs to control the oil. If there is too much oil, there is excessive carbon formation in the engine; if there is too little oil, there is a risk of seizure.
Cord rings, for the control of oil and maintenance of a good seal in tapered and worn bores, were made of thin dished steel rings and were fitted four to a groove in a W fashion. Wellworthy made a Duaflex ring of two rounded-edge rings, held apart in the groove by a wavy spring and prevented from rotating by a backing spring locked into one of the oil return holes drilled in the groove base.
Often the lowest ring had a circumferential groove from which radial holes passed oil, scraped from the cylinder wall, to holes drilled in the piston from the ring groove.