Four‑stroke outboard motor pistons fire only once every two revolutions, reducing heat load. These outboard motor pistons have more rings than two‑stroke versions to control oil. The ring pack on a four‑stroke outboard motor piston includes an oil scraper ring.

Because the oil is separate from fuel, four‑stroke outboard motor pistons run cleaner. The piston crown of these outboard motor pistons may have valve relief cutouts. Over time, carbon buildup on outboard motor pistons can cause pre‑ignition.

When rebuilding an older engine, measure the outboard motor pistons for taper and ovality. A worn outboard motor piston will make a knocking sound as it rocks in the cylinder. The wrist pin bore in an outboard motor piston must be perfectly aligned.

Some outboard motor pistons have a bronze bushing in the wrist pin hole for less wear. Using the wrong oil grade can glaze the skirts of the outboard motor pistons. For a reliable rebuild, always hone the cylinders before installing new outboard motor pistons.

After break‑in, the outboard motor pistons will have a even wear pattern across the skirt. Many four‑stroke outboard motor pistons are designed to run for over 2000 hours. Properly maintained outboard motor pistons keep the engine quiet and fuel efficient.