Maximizing Hole Measuring with Bore Gages

Published on: 
Turbomachinery Magazine, May/June 2023, Volume 64, Issue 3

With a variety of bore gages to choose from, understanding the types of gages available is key to selecting the best fit for each application.

Accurate hole measuring in turbomachinery components is crucial for safety and performance, ensuring adequate pressure and flow throughout the system. An inaccurate measurement could lead to scrapping the component(s) and material, which is costly.

Turbomachinery applications that have holes include discharge nozzles, eye of an impeller, shaft, shaft sleeve, intake valve, and suction flange, which have hole diameters that range from tapered and straight to through hole and blind. To guarantee accuracy, each of these examples necessitate using the correct bore gage.

When selecting a bore gage, the following factors play a role:

  • Application or range of measuring
  • Accuracy
  • User’s comfort level
  • Budget for tools

It is useful to review the types of gages available to select one that best suits the application. Bore gage types include telescoping, dial, electronic, and small-hole tool gaging: two- or three-point anvil contact, with a choice between a vernier or electronic readout type, including wireless data transmission for SPC on many electronic types.


The choice between two- or three-point contact is important. A two-point contact rod-type inside micrometer is usually lighter, easier to handle, and more versatile over long ranges—from approximately 6 – 107 inches (150 – 2,700 mm). Any two-point contact gage, regardless of range, can probe a hole better than a three-point contact gage.

Most three-point contact tools have setting rings to ensure accuracy. If you require close tolerance work with a two-point contact inside micrometer, set the tool to a ring gage or an outside micrometer. A three-point contact gage can be seated in position more quickly than a two-point gage.

Usually, the three-point gages can also be read to a finer accuracy. The three-point gage will tell the maximum true diameter that can enter the hole faster than a two-point contact gage.

Three-point gages are accurate to 0.0001 inch (0.002 mm), but overall, accuracy depends on best practices and technique. To ensure accuracy, these practices should be followed:

  • Make sure that there are no specks of dirt between the clamping surfaces of the tool rods or
  • gage heads.
  • Tighten all rods uniformly—not too tightly, not too loosely, but a
  • firm assembly.
  • Assembling long sections should be done vertically or horizontally with support.
  • Because temperature can affect long rods used in these gages, they should be assembled in the same environment in which they will be used.


Telescopic bore gages are used for determining the true size of holes, slots, and recesses with a limited measuring range, usually up to 6 inches (150 mm).The ends of both contacts are hardened and ground to a radius to allow proper clearance on the smallest hole that the gage will enter. These tools must be slightly rocked in the hole being measured to ensure that the tool is on the proper diameter before it is locked and with-drawn. The final hole size is obtained by measuring over the gage contacts with a micrometer.


Dial bore gages are available in two- or three-point contact with a choice of dial reading, imperial, metric, or fractional graduations. Measurements are taken by comparison, so some type of setting master should be used as a reference standard. Set the gage as close to the hole being measured as possible to maximize the hole measurement.


Vernier bore gages require the combined ability to read and add fine graduations to obtain a measurement, whereas electronic digital bore gages feature an LCD numerical display for fast-go/no-go visibility measurement results.

As previously mentioned, electronic gages enable measurement data collection and documentation for SPC purposes, which is an advantage over dial or vernier gages. In addition, some electronic gages are IP67-rated for protection against harsh ambient shop environments with coolant, dust/dirt, etc. There are also unique pistol grip gages with trigger activation that allow for fast access to the hole being measured. Both vernier and electronic gages are available in two- or three-point contact styles.


Small-hole gages are well-balanced tools that are ideal for accurately measuring small holes, slots, grooves, and recesses in all kinds of turbine parts. Popular features include a hardend-ball measuring surface with two-point contact. Having a radius on each gage that is less than the minimum diameter that will be measured provides the two-point contact necessary for maximum accuracy. Smooth, sensitive adjustment for a better feel also provides more accurate measurements. Most of these gages have a safety stop that prevents the gage from going beyond its measuring range to avoid any breakage.


When machining bores, challenges with fit and out-of-roundness may result in lobing. Out-of-roundness can be caused by centerless grinding (5-lobing), a worn or out-of-balance spindle (irregular-lobing), work holding with a three-jaw chuck (3-lobing), and other issues. In an internal roundness scenario, lobing often occurs when applied forces on a limited wall thickness surpasses the material’s yield limits. The result impacts the roundness of the internal diameter.

Conversely, external roundness can be impacted when machining a material where its strength is inconsistent through the workpiece, for example, coatings. Out-of-roundness can be identified by irregular travel of an indicator or parts not fitting properly and is either asymmetrical (irregular lobing) or symmetrical (regular lobing). Empirically discovering that the fit is incorrect can be too late and very costly to rework.

Selecting the most appropriate bore gage for the turbine components being made will significantly minimize these challenges. Sometimes, roundness geometries are important enough to utilize metrology equipment that is specially designed to measure it. This is a costly but a very effective way to profile the specific roundness of a part. If parameters are established for the finished part, specific operations can then be monitored in-line with a bore gage. This can reduce the expertise required, empower operators, and improve the end results of the finished parts.

Roundness or ovality (a basic subset of roundness) is typically not specified. But understanding the character of an internal diameter can build confidence when qualifying your work. An effective internal diameter requirement with an overall tolerance is more typical. Bore gages can improve measurement confidence compared to using a slide caliper or other two-point instruments, including greater resolution, accessibility to depth, and ease-of-use.


Sometimes custom gaging solutions are available to solve specific turbomachinery application challenges. For example, a dovetail slot gage can accurately measure internal and external holes in gas turbines and aerospace engines. Fan blade gage versions provide internal measurements of the dovetail seat face width and bottom lobe width. A turbine blade version offers internal single-side measurement for setup, and symmetry and internal double-side measurement for overall width. An external dovetail gage offers measurement of the dovetail slot with ball contacts as well as on machine measurement.

For challenging perpendicular bore holes, a valve cavity-seat pocket gage is helpful. Using a one-piece right-angle adaptor, measurements can be made at 90 degrees to the entry of the core. Optimized anvil form and guide pieces assist the entry and correctly position the measuring head for higher accuracy and ease-of-use.

Overall, gages determine the quality and accuracy of the machined hole or bore and optimize hole measuring.

Tim Cucchi is the Continuous Improvement Manager for Assembly at The L.S. Starrett Co. Founded in 1880 and headquartered in Athol, MA, The L.S. Starrett Co. ( is a global manufacturer of precision-measuring tools and gages, optical comparators and vision systems, force, and hardness testing solutions, and more.