How to Size O Rings


All standard O-rings come in are sized by the "AS568-" system, after the dash is a series of three number that determines the physical size of the O-ring. There are three parts of the O-ring that are measured to determine its size: the outside diameter, inside diameter and cross section or width. Finding the proper replacement O-ring is important to ensure a proper fit and seal of the component the O-ring is attaching to. Measuring the O-ring only takes a few minutes and requires only one tool.

Things You'll Need

  • Micrometer
  • Lay the O-ring on a clean, flat and dry surface.

  • Close the micrometer jaws and press the "reset" button to calibrate it prior to measuring.

  • Place the rear jaws of the micrometer in the center of the O-ring. Slowly open the micrometer until the inner jaws lightly touch the inner part of the O-ring. Do not stretch the O-ring, or the measurement will be inaccurate. Read the measurement, known as the inside diameter, given by the micrometer and take note of it.

  • Open the micrometer jaws and place them over the outside of the O-ring. Slowly close the jaws until the lightly touch the outer part of the O-ring. Do not compress the O-ring with the jaws, as this will cause an inaccurate measurement. Take note of the measurement, known as the outside diameter, given by the micrometer.

  • Open the micrometer jaws and place the bottom jaw through the O-ring, so that the O-ring is hanging on the lower jaw. Close the micrometer jaws until they are lightly holding the O-ring, do not press too hard and squash the O-ring, as this will provide an incorrect reading. Take note of the measurement, known as the width or cross section, given by the micrometer.

  • Read the O-ring sizing chart (see Resources) and compare the inside diameter, outside diameter and cross section or width to the chart to get the proper "AS568-" number of the O-ring.

Tips & Warnings

  • The O-ring's inner diameter and cross section have a range of tolerance, so do not be alarmed if it does not match up exactly with the measurements on the chart. The tolerances are listed on the chart.

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