# Bore gauges

Selecting, mastering and interpreting bore gauges for internal diameter checks without confusing resolution, local size and bore form.

Reviewed: 2026-11-01

Tags: inspection, dimensional-metrology, bore-gauges

Record depth, direction and reference with an internal-diameter reading. A diameter does not describe the complete bore form.

The underlying limitation is geometric. The EMRP IND62 TIM guide, hosted by PTB, explains that opposed two-point diameter measurements cannot recover odd-lobed profiles such as triangular or five-lobed shapes. Its worked application is external roll measurement, not a bore-gauge calibration procedure, but it demonstrates why a diameter variation profile is not a complete roundness profile. Three-point handheld readings likewise must not be substituted for a specified full-profile evaluation without validating the method's response. [Hemming and Widmaier, *Good Practice guide to measure roundness on roller machines and to estimate their uncertainty*, section 3.1, undated PDF](https://www.ptb.de/emrp/fileadmin/documents/tim/GPG_roundness_measurement.pdf)

## Instrument families

| Instrument | Reading and practical distinction |
| --- | --- |
| Two-point comparative bore gauge | An indicator measures displacement relative to a mastered setting. Interchangeable anvils establish the nominal range; the indicator's travel is not the complete diameter range. |
| Inside micrometer | A screw or electronic readout reports size directly after its reference is established. Two-point versions still require careful alignment. |
| Three-point internal instrument | Three expanding contacts provide self-centering within the intended geometry. Scale, digital and pistol-grip versions exist; self-centering does not replace setting or verification. |

Mahr describes its three-point instruments as automatically self-centering and offers the three readout/actuation variants above. This concerns locating the measuring head, rather than a guarantee that every bore shape is fully characterized. [Mahr, 3-point inside measuring devices, undated product page](https://metrology.mahr.com/en-int/products/group/handmesstechnik-messschrauben-3-punkt-innenmessgeraete/)

## Mastering, alignment and contact

Use a suitable calibrated setting ring or another approved setting arrangement. Record the reference's assigned size and applicable certificate correction, rather than assuming the nominal engraving is exact. Distinguish **setting the reference** from **calibrating performance over a range**: a successful zero check alone does not establish all indication errors. NPL treats calibration, traceability and uncertainty as separate parts of a defensible dimensional measurement. [NPL, *Fundamental Good Practice in Dimensional Metrology*, GPG80, July 2005; corrected reprint October 2012](https://eprintspublications.npl.co.uk/3360/1/mgpg80.pdf)

Alignment has two different traps: a tilted measurement can span an oblique section, while an off-centre measurement can span a chord. Mitutoyo identifies positive axial and negative radial misalignment errors for inside micrometers. Follow the specified rocking procedure to locate the valid turning point; do not choose whichever reading is closest to nominal. Its two-point setting guidance specifies a **maximum indication in a ring**, but a **minimum indication between outside-micrometer faces**. Those instructions are setup-specific, so a generic “rock to minimum” rule is unsafe. [Mitutoyo, Inside Micrometers, undated technical guidance](https://www2.mitutoyo.co.jp/eng/about-metrology/knowledge/inside-measurements/)

Contact conditions matter after alignment. Mitutoyo warns that a Holtest can read differently across the anvils and at their tips; establish its start point under the same contact condition used on the part. A bottom-adjacent measurement therefore needs a compatible head and matching setting procedure. [Mitutoyo, contact-position guidance](https://www2.mitutoyo.co.jp/eng/about-metrology/knowledge/inside-measurements/)

As a bounded model example, the **Bowers XT500** manual requires clean mating faces when changing anvils and recommends the same orientation during mastering and measurement. Its large-bore preset procedure uses the setting-ring value and the anvil certificate information. Follow that instrument's ratchet and preset sequence rather than transferring force or turn-count instructions to another gauge. The source identifies itself as **INST-81-1, Issue 1**; it shows no calendar publication date. [Bowers XT500 three-point manual, English instructions](https://s3.eu-west-1.amazonaws.com/bowersgroup/assets/supportportal/Bowers-INST-81-1-Iss-1-XT500-3-Point-Bore-Gauge-Manual.pdf)

## Diameter is not complete form

Measure at defined depths and directions when investigating taper or directional size differences. Keep the individual readings: averaging can hide a local extreme relevant to assembly. This sampling is a process check whose coverage must be specified, not an automatic cylindricity assessment.

For a form-controlled feature, specify the appropriate roundness/cylindricity measurement, sampling and evaluation separately. Record whether a reported value is a local contact size, a range of sampled sizes or a formally evaluated geometric characteristic.

## Resolution, accuracy and uncertainty

A concrete catalogue example separates the terms. Mitutoyo's **Series 511, catalogue page C-21**, lists **511-722** for **35–60 mm**, with a **2109SB-10 indicator graduated at 0.001 mm**. The same page lists series accuracy as **2 µm** and indication stability as **1 µm**. Those are manufacturer specifications for the listed equipment; a 1 µm graduation is not a claim of 1 µm measurement uncertainty. The standalone catalogue page is undated, so confirm the specification of the actual supplied configuration. [Mitutoyo, Series 511 Standard Type, C-21](https://www.mitutoyo.com/Images/CatalogUS-1003/resources/_pdfs_/US-1003_Catalog_135.pdf)

For the actual task, evaluate uncertainty from the reference, indication performance, repeated insertion, operator alignment, contact effects and environment. NPL distinguishes resolution, accuracy, repeatability and uncertainty and explains combining uncertainty contributions. Do not simply add catalogue accuracy to a display increment and call the result a complete uncertainty budget. [NPL GPG80, measurement terminology and uncertainty chapters](https://eprintspublications.npl.co.uk/3360/1/mgpg80.pdf)

Temperature must be controlled or accounted for. NIST identifies **20 °C** as the reference temperature for dimensional measurements; this does not mean every shop measurement must physically occur at exactly 20 °C. Record conditions and address thermal expansion when relevant. There is no universal waiting time that establishes equilibrium for every part size and material. [NIST Engineering Metrology Toolbox FAQ](https://emtoolbox.nist.gov/faq/faq.asp)

## Worked interpretation and practical selection

**Illustrative calculation, not a product performance claim:** a ring's assigned diameter is 50.002 mm. The gauge is zeroed in that ring; its verified sign convention makes a larger bore positive. A part reads +0.006 mm, giving:

`local diameter = 50.002 + 0.006 = 50.008 mm`

Treating the ring as exactly 50.000 mm would instead report 50.006 mm, introducing a 0.002 mm offset. Acceptance still requires the drawing limits, measurement uncertainty and agreed decision rule. Neither the arithmetic nor a good zero return establishes unsampled bore form.

Before selecting equipment, specify diameter range, required reach, blind-bottom clearance, contact location, interruptions and wall stiffness. Verify that the selected contacts can reach the intended surface without bridging an edge or distorting the part. Catalogue extension availability alone does not validate the completed setup: the Series 511 reference, for example, offers optional deep-hole extensions, whose suitability must be checked for the application. [Mitutoyo Series 511 catalogue](https://www.mitutoyo.com/Images/CatalogUS-1003/resources/_pdfs_/US-1003_Catalog_135.pdf)

A practical inspection record includes gauge/head identification, reference and correction, setting and closing reference checks, depth/direction, individual readings, environmental conditions and acceptance rule. Investigate poor repeatability or zero drift before releasing results; retain enough detail to distinguish a changed part from a changed measurement setup.

## Related reference

[Inspection and quality documents](../../sourcing/documents/index.md)