Video: How to Install Perfect Skirt–Coated Pistons

October 24, 2018 / by Evan Perkins

JE's revolutionary, Perfect Skirt piston coating prevents piston slap and and skirt wear, but requires a few special considerations during installation. Follow along in this step-by-step video as we discuss proper measuring techniques. 

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.

JE's revolutionary, new Perfect Skirt coating helps eliminate piston slap as well as prevent premature skirt wear. Installing a set of Perfect Skirt–coated pistons is simple and requires only a few slight changes from the norm.

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

Bore Diameter

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).

Measuring a Perfect Skirt–coated piston requires the use of a blade micrometer (left) instead of the traditional cylindrical-ended style (right). The blade end is necessary to ensure the mic' measures the base piston diameter and not the coating.


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.

Gauge Point

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.

Shown is the spec sheet included with every set of JE Pistons. Noted are the gauge point location and the specified piston-to-wall clearance (below the coating).

Looking for pistons? Click HERE to see JE's full piston line. 

Measuring Window

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.

All Perfect Skirt–coated pistons feature a window designed to allow the base material to be measured. The thickness of the coating is intentionally larger, and precisely controlled. The coating takes up the slack in the bore while the piston is cold and compresses as the piston reaches operating temperature.

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. 

Cylinder Bore Diameter – Piston Gauge Point Diameter = Piston-To-Bore Clearance

Sample problem:

4.001 bore - 3.9960 piston = .0050 clearance. 

It is important NOT to use a caliper to measure your piston diameter.The length of the caliper's jaws will actually catch the edge of the coating and yield an inaccurate reading.

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.


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Written by Evan Perkins

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