How to Measure a Cylinder Bore

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A dial bore gauge used to measure bores has three symmetrical anvils connected to the gauge body. The anvils touch the side walls of the bore to give an accurate -- usually to within 0.001 mm -- measurement. Measuring the cylinder bore at multiple heights within the cylinder shows you whether the cylinder is out of round or tapered.

Things You'll Need

  • Dial bore gauge
  • Machinist's ruler
  • Set the dial bore gauge to zero by attaching the measurement head that corresponds with the estimated diameter of the cylinder and adjusting the calibration knob next to the dial until it reads zero. Use a ruler to take a quick measurement across the cylinder bore, if you do not know the estimated diameter of the cylinder.

  • Insert the measurement head into the top of the cylinder, rocking the bore gauge back and forth slightly until the head is level across the cylinder. Remove and replace the measurement head with the next smallest head if the one you are using will not fit into the mouth of the bore. Repeat the process until you are able to take your measurement. Take the measurement then remove the gauge.

  • Turn the measurement head to a 90-degree angle from your original measurement, then repeat the process of taking a measurement across the top of the cylinder bore. Write down this measurement alongside the first measurement, marking both of them as taken at Position No. 1.

  • Reinsert the measurement head into the cylinder, this time taking a measurement across the cylinder at a depth of approximately 1 inch. Rock the gauge slightly to level the measurement head, then write down the smallest measurement given on the gauge.

  • Take another measurement at the same depth, again turning the measurement head to a position at a 90-degree angle from the first measurement. Record this measurement along with the original measurement for this depth, marking them both as being taken at Position No. 2.

  • Repeat the process of taking two perpendicular measurements two more times, each time moving the gauge approximately 1 inch deeper for each set. Record each pair as being taken at positions No. 3 and No. 4, respectively.

  • Remove the dial bore gauge, then examine your recorded readings. Each set of measurements will show you the variations in the cylinder width at a given depth, allowing you to see variations in the cylinder's shape. Check the specs for the engine to find what the allowable tolerances are in the cylinder bore.

Tips & Warnings

  • For increased accuracy, a third measurement may be taken at each height. If you wish to take three measurements at each height, adjust the positioning of the measurement head by 60 degrees for each measurement instead of 90.
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