When it comes to the function of the piston rings the top ring and oil rings functions are fairly straight forward. The top rings purpose, commonly called the compression ring, is to create a complete seal to hold compression when the piston travels upward during the compression stroke. The lower ring, the oil control ring, controls and evenly spreads oil across the cylinder wall before the second ring wipes the remaining oil off of the cylinder wall. Napier style piston rings feature a tapered, undercut step on the outer diameter on the second ring, creating a hook-shaped profile. This decreases the surface area that contacts the cylinder, reducing friction, while also encouraging oil that is wiped off away from the cylinder wall, thus improving efficiency. Named after the British engineering firm Napier & Son, this ring setup is more efficient than step and taper ring styles. We recommend using a Napier Second Ring when applicable and you can find one on our 124″ Big Bore Kit.