Coated Pistons: Measuring Advice From JE Pistons

Coating technology for engine internals is becoming more commonplace today as a proof-positive way to increase wear characteristics and overall life of performance and racing engine components. JE Pistons has provided some informative advice for measuring a coated piston for overall piston diameter. These measurements contribute to the formula between the piston skirts and the cylinder bore, better described as piston-to-wall clearance.

Measuring piston-to-wall clearance is essential when building your new engine. In extremely critical motorsport engine build applications, the thermal expansion of the piston requires exact tolerances. Most commonly used in racing are forged pistons which expand more than cast pistons, necessitating this attention to detail.

Essentially, there are correct tools and measuring procedures and, unfortunately, common incorrect ways to measure coated piston diameters. For example, the engineers at JE highly recommend never using a dial caliper when measuring piston diameter.

Using more simplistic devices such as a caliper can result in incorrect readings up to .003-inch. Different piston manufacturers may vary with piston-to-wall clearance, depending on their piston materials used. Whatever your specific tolerances may be, they are typically measured down to .0001-inch of accuracy.

Exact Cylinder Bore and Piston Diameter Measurements

Measuring your engine block cylinder bores requires a precision dial-bore gauge. Pretty much, that is the end of the discussion there. High-end dial bore gauges are the tool of choice. Similar to using incorrect dial calipers when measuring pistons, a snap gauge is another tool that will not provide the needed accuracy.

Measuring piston diameter with the popular JE Perfect Skirt–coated pistons requires measurements using a blade micrometer. Shown beside a traditional cylindrical-ended unit, the blade micrometer will accurately measure the piston’s skirt within the provided piston coating window. Measurements made when contacting the coating surface will cause inaccurate readings.

All pistons are not perfectly round; in fact, they are described as the diameter being a “cam” shape. By following the specific “gauge point” locations that JE Pistons carefully points out in its illustrated instructions, you will achieve the proper measurements to match your cylinder bore and derive appropriate piston-to-wall clearances.

A dial caliper does not have the accuracy to reliably measure a piston's precision skirt, and should not be used for any piston measurement. The JE specification sheet included with all piston sets illustrates all proper measuring points for your coated pistons.

With careful attention to detail and proper tool usage, your Perfect Skirt–coated piston will help eliminate piston slap as well as prevent premature skirt wear. It only takes a few simple points of detail in engine assembly to incorporate coated pistons into your engine building regimen.

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About the author

Todd Silvey

Todd has been a hardcore drag racing journalist since 1987. He is constantly on both sides of the guardwall from racing photography and editorship to drag racing cars of every shape and class.
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