The piston skirt is the part of a piston that extends the lowest. It is tasked with keeping the piston from rocking excessively in the cylinder. It is typically machined with small grooves to aid in holding and transporting oil to the cylinder walls to provide proper lubrication. In some high-performance applications, the piston skirt may be coated with a type of chemical which aids in lubrication and prevents scuffs from occurring on the cylinder wall.
In a combustion engine, the pistons are sealed inside of the cylinder walls by the piston rings. The rings, as they are called, are making contact with the cylinder wall while the piston rides up and down, centered by the rings in the cylinder wall. At the top and bottom of each stroke, as the piston is changing direction, the piston rocks. It is then that the piston skirt makes contact with the cylinder wall, setting the piston straight once again to continue its journey.
Depending on the stroke of the crankshaft and the length of the connecting rod, the piston skirt could be in danger of making contact with the crankshaft at the very bottom of its travel or stroke. This is especially common in high-performance engines known as stroker motors. In a stroker motor, the engine's stroke has been altered by swapping to a longer throw on the crankshaft and also changing the length of the connecting rods. The piston pin location is changed, creating a new location for the piston to connect to the connecting rod.
In the typical stroker application, the engine block needs to have notches ground in the bottom of the cylinder walls to prevent the crankshaft and connecting rods from making contact with the block. Often, the bottom of the piston skirt must also be clearanced to avoid making contact with the connecting rod as it spins around the crankshaft throw. When clearancing a piston skirt, it is wise for individuals to make the same cut on both sides of the skirt. This maintains balance within the reciprocating components. The most minute removal of material can cause the components to be dangerously out of balance, leading to catastrophic engine failure.
One of the telltale signs an engine has been operated while low on oil is a scuffed piston skirt. As the piston travels up and down the cylinder wall, the lack of oil can result in the piston's skirt scrubbing or scuffing the cylinder wall, leaving the telling scuffing sign. When this occurs, it is often time to replace the pistons as well as to bore the cylinders to remove the scuff marks.