Keystone is one of the compression ring types that is durable under heavy loads and is highly effective in preventing ring stick (a phenomenon in which the ring sticks to the ring groove).
As with barrel-faced, some keystone rings have barrel-faced contact surfaces that make contact with the cylinder wall.
In addition, there are two types of keystone: single-sided keystone where only one side (upper surface) is tapered, and double-sided keystone where both sides are tapered.
Mainly used in top rings.

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