What is a piston ridge?

Answer 1

Most people confuse ring ridge with piston ridge. The piston is wrapped in several rings which seal the combustion gases above the piston. These are made of a very hard material (usually cast steel). As the piston goes up and down they ride against the wall of the combustion chamber and wear it away. The part of the cylinder wall above where they reach that does not wear, resulting in a raised ridge. This MUST be removed when new rings are installed, as the old ring has also worn down, The new one will smash into the ridge and, being a hard brittle material, may break up, leaning jagged pieces of cast steel trapped between the piston and wall to tear grooves in both and ruin the engine.

The rings fit tightly into the piston grooves, but there is some slack. The edges of the groove must match the angle between the wear surface of the rings and the top/bottom, so the rings are supported across their full width and circumference. It looks like 90 degrees, but usually it is a little less, so the pressure of the moving piston forces the rings outward into the chamber wall, to improve the seal. Every time the piston reverses direction the rings hit the edges of the groove. Over time this pushes up ridges in the softer metal of the piston above and below the groove. These are piston ridges. They are cause no problem as the piston is a softer metal than either the rings or chamber wall. If they are high enough, they will show as bright bands above or below the ring grooves in the pistons. If they piston rides are more than a few thousandths wide, the grove may be over sized enough that the piston should be replaced, as the walls of the groove no longer support the ring properly. I have never seen piston ridges on a motor with under a hundred and fifty thousand miles, except for K-Cars and some of the little Chevy's from the seventies. The cause there was not mileage, but less than ideal metal in the pistons.