Piston Skirt coatings are an excellent way to prevent wear and increase lubricity. However, measuring a coated piston can vary significantly from an uncoated piston. We walk you through the process of measuring a coated piston.
In the JE Pistons and SRP piston lines there are three available skirt coatings: Perfect Skirt, our premium JE skirt coating; Tough Skirt an extremely durable spray-on coating, and SRP Pro skirt coating, which helps during break-in and low-oil conditions. In the below article, we will walk you through how to measure each style of piston skirt coating.
The purpose of measuring any piston is to establish the piston-to-wall clearance, or the distance between the piston skirts and the cylinder bore surface. This distance is incredibly important as it allows for thermal expansion of the piston during engine operation. Forged pistons, by nature of their aluminum base material expand slightly more than cast pistons, necessitating a small amount of extra piston to wall clearance.
Your pistons include JE Perfect Skirt, a revolutionary, patented, new piston coating that significantly reduces engine noise, friction, and skirt wear. Measuring a piston with Perfect Skirt coating is very similar to measuring a conventional JE piston but requires a few special considerations. To ensure your pistons are properly installed, follow the below measurement guide.
WARNING: Do not machine the cylinder bore larger or alter the provided piston-to-bore clearance spec to account for the coating. This will nullify all of the benefits of Perfect Skirt and could accelerate wear.
Measuring Perfect Skirt Coated Pistons
As with any engine application, having a precise bore diameter measurement is critical. It doesn’t matter whether this measurement is done at the machine shop (for each bore) or at home, with a precision,dial-bore gauge (not a snap gauge).
In order to properly measure your pistons, an outside-diameter, blade micrometer is required. Do not use a sliding caliper, or feeler gauge as these methods do not provide acceptable accuracy.
Pistons are not round and in fact have a cam shape. For this reason, it is critical to measure them at the gauge point to avoid a false reading. The gauge point for your Perfect Skirt coated pistons, as measured from the bottom of the piston, is specifically listed in the included instructions and will fall within the coating window. This window is an intentional design feature that allows the piston to be measured beneath the coating.
When measuring your Perfect Skirt coated pistons, take care to place the micrometer blades in the skirt window and not on top of the skirt coating. Measuring on top of the coating will yield false readings.
Piston-to-Bore (Piston-to-Wall) Clearance Formula
Once you have successfully measured your cylinder bore and piston diameter, use the following formula to determine your piston-to-bore clearance. Reference this number to the specs provided with your piston set.
With careful attention to detail and proper tools, your Perfect Skirt–coated pistons are destined for a long, long street life, quiet operation, and excellent performance. For more info on Perfect Skirt, such as how it works, or what piston applications will include it, click HERE.