Dial Indicator Contact Points - In General


  • Contact points for test indicators are shown on page 21.


Q & A


Q: Do we need to calibrate the dial indicator after changing contact points?

  • Unlike test indicators, you do not need to calibrate the dial indicator. The size, length or shape of the contact point has no effect on the indicator's accuracy. You will, of course, have to reset to zero but you would do this as a matter of course anyway.

What kind of point to use


Dial indicator and digital indicator contact points are fully interchangeable between manufacturers. There is only one deciding factor, and that is the thread diameter. For your average dial indicator you'll have a choice of inch 4-48 threads, or metric M2.5 threads. If the contact point can be screwed into the end of the spindle, then you can use it without any effects whatsoever on the indicator's accuracy.

[You may want to check our page devoted to indicator calibration.]

Some dial indicators which are not built to AGD standards will need specialty threads. This applies to the tiny dial indicators made by B&S, Starrett or Mahr-Federal; possibly others.

Flat points are used when measuring round surfaces such as rods, cylinders and spheres. Rounded points are used when measuring flat surfaces. If contours need to be indicated, then a contact point with a ball tip should be used. Thin chisel points are used for grooves and slots. A special rolling wheel is used when measuring stock. You can even buy blanks which you can machine to your own specifications.

Ruby, Teflon and Nylon


These contact points are used for special applications, typically in optics.

The ruby point does not expand or contract with heat and cold. It is also less likely to scratch delicate surfaces. It's also the material that will hold up to silicon carbide surfaces and it will not conduct electricity making it suitable for CNC operations.

The Teflon point is softest and made of a solid Teflon ball. It is least likely to scratch—and ideal for mirror finishes—but is not as long lasting as the others.

The specific Nylon we use for the contact points is Nylon 6/6 and it is not glass filled. The glass filled (fiber glass basically) will give Nylon a better holding shape, but it is abrasive and defeats the purpose of using the Nylon ball.

Thread size


The way some people bandy the terms 4-48 and M2.5 about, you'd think this was an innate sense we should have been born with.

4-48 is the thread size in inches, and M2.5 is in metric. They're different, and one won't fit into the other. Just because your indicator is graduated in inches or in millimeters doesn't automatically tell you which point to use. There are metric reading indicators which have inch threads, and vice versa.

To make sure, take out your micrometer or calipers and measure the diameter of the threaded portion of the contact point which fits your indicators. Allowing for some variations in manufacturing, the 4-48 thread will have a diameter of about .108" to .110" while the metric M2.5 thread will be about .1" (that's 2.5 mm).

Threads which are just a bit undersize will have one major advantage: if they ever break off inside the spindle, it will be easier to get the broken part out. You'll have no control over this; it's just a side note.

What are they made of?


Unless otherwise noted, these contact points are hardened steel, usually black anodized but some variation in color can occur from one batch to another.



BesTest and TesaTast Indicator Repair Manual … page 177
Starrett 711 Last Word Indicator Repair Manual … page 199
Interapid Indicator Repair Manual … page 208
Compac Test Indicator Repair Manual … page 194
Mitutoyo Test Indicator Repair Manual … page 102

New! Companion Reference Guide for Test Indicators … page 178


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This page's most recent revision: 1 APRIL 2018
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