When a cylinder is new or overhauled the surface of it's walls are honed with abrasive stones to produce a rough surface that will help wear the piston rings in. This roughing up of the surface is known as "cross-hatching". A cylinder wall that has been properly "cross hatched" has a series of minute peaks and valleys cut into its surface. The face or portion of the piston ring that interfaces with the cross hatched cylinder wall is tapered to allow only a small portion of the ring to contact the honed cylinder wall. When the engine is operated, the tapered portion of the face of the piston ring rubs against the coarse surface of the cylinder wall causing wear on both objects.
Each tiny groove acts as the oil reservoir holding oil up to the top level of the groove where it then spreads over the peak surface. The piston ring must travel up and down over this grooved surface, and must "hydroplane" on the oil film retained by the grooves. Otherwise, the ring would make metal-to-metal contact with the cylinder wall and the cylinder would quickly wear out. |
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