The DIY Repairer’s Guide to Measuring Hydraulic Cylinder Tube Diameter

Hydraulic excavator boom cylindersThe main parameter for a hydraulic cylinder tube is its internal diameter. But more specifically, its internal diameter and concentricity along its entire length. They way to check this is with an internal micrometer in at least two planes at 90 degrees to each other.

You’re looking for two things: to get a nominal “size” on the tube and to make sure this size is consistent along its length. Obviously, if the I.D. increases markedly at some point along its length, then the tube has ‘ballooned’ and must be scrapped.

A standard internal micrometer works well for hydraulic cylinder tubes which are no longer than your arm! Anything longer than this requires special equipment. For long, large diameter cylinders, repair shops use a special bore gauge. At the measuring end it has two wheels on one side and a spring-loaded pin on the other. Change in the position of this pin is displayed on a dial gauge. Extension pieces are then added as required for the length of the tube – making the device into an extended T shape. The readout is attached to the end of the required extension pieces.

Once the nominal bore diameter has been set according to the tube being measured, the operator ‘drives’ the bore gauge along the length of the tube while watching the readout for run-out in the nominal diameter. As mentioned above, this process is repeated in at least two planes to check for ballooning or out-of-round.

This equipment is expensive and so you are not likely to rush out and buy one of these unless you repair a lot of large diameter cylinders. But there is a very simple technique that can virtually negate the need to measure the I.D. of any hydraulic cylinder, large or small, to check for ballooning of the tube. And this is: paying careful attention to the condition of the piston seal. If the piston seal is intact and not showing and signs of extrusion, then it’s most unlikely that the tube is ballooned or out-of-round.

But for a hydraulic cylinder repairer, there are other reasons for needing to measure the I.D. of a tube along its entire length. If the bore is pitted or scored, honing will be required to remove it. If the tube I.D. is standard size (not already oversize) the procedure will usually involve honing the tube to 0.030″ oversize. And this has to be checked by measurement.

Once at 0.030″ oversize, the bore is inspected again and if all scoring and pitting has been removed, 0.030″ oversize piston seal and wear (bearing) bands are installed. If scoring and pitting is still obvious at 0.030″ oversize, the bore is honed to 0.060″ oversize and inspected again. Assuming all scoring and pitting has been removed, 0.060″ oversize piston seal and wear (bearing) bands are installed. If unacceptable pitting or scoring remains at 0.060″ oversize, the tube is scrap.

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5 thoughts on “The DIY Repairer’s Guide to Measuring Hydraulic Cylinder Tube Diameter

  1. I WISH YOU WERE MY TEACHER BACK IN 1973
    at that time i was looking out for my future ,i could not tolerate Hydaulic classes.
    your Hydraulic hobby facinate me, your wife and yourself are both correct when the question about giving up your hydralic intrest.
    i still do not like hydralic and probably i make mistake in spelling it. surely i love your dedication to your faveriote subject

  2. I was told that you only Hone out to .010 of the original size I presume that the seal kits are only available to this size?

    • Ian, as a general rule you can hone the tube to .010″ oversize and still use standard size seals. Beyond that you hone to suit oversize seals (if available): .030″ oversize or .060″ oversize.

  3. Do you know of if there is a more modern way of measuring ram bores I am still using comparators which you set with micrometers.I measure tubes up to 15 inches diameter by 12ft long




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