Measuring a bore with a dial bore gauge

What it is

A dial bore gauge is a two-point contact instrument for bores: a dial indicator on top, an interchangeable measuring head below, and a self-centering bridge that keeps the measuring line through the bore centre. It reads differences, not absolute sizes, so it always needs a reference of known size.

In China the product family is covered by GB/T 8122-2024 (current); the standard has carried the name 内径指示表 since the 2004 edition — the 1987 edition was titled 内径百分表, the name shops still use. It is not an inside micrometer: the contact is two-point with a centering bridge, and the size comes from the setting plus the reading.

Setting the gauge to size

The gauge is zeroed on a reference of the nominal size. Three references are in common use (textbook: 金莹, 公差配合与技术测量): a master ring gauge — accurate and the easiest, which is why shops that check the same bore in batches keep one; a gauge-block stack clamped between the measuring faces — accurate but fiddly; and an outside micrometer set to the size — convenient, but the setting inherits the micrometer’s own error.

When mounting the dial, pre-compress it before locking it — enough that the reading can swing either side of zero. Hold the gauge by its insulated handle during both zeroing and measuring, not by the metal.

Measuring: rock, and read the minimum

Tilt the head to slip it into the bore, then straighten it. Rock the gauge gently in the axial plane — the plane through the bore axis. Away from the true diameter the two contacts span a chord, which reads larger; at the turning point of the swing the span is the diameter itself, and that is where the pointer stops and comes back. Read the minimum at that point.

The sign matters: reading = actual bore − setting size. A reading on the plus side means the bore is larger than the reference. A reading taken before the turning point, or with the gauge still tilted along the bore, is a chord or a diagonal, not the diameter.

A worked example

The setting ring is Ø28.00 mm and the bushing bore is specified Ø28 +0.05/0. Set to zero on the ring, rock in the bore, and read +0.030 mm at the turning point: bore size = 28.00 + 0.030 = 28.030 mm — inside 28.00 to 28.05, so the bore passes.

Practice points

Measure in at least two directions 90° apart and at more than one depth; the spread between readings shows out-of-round and taper (see measuring a bore). Clean the bore first — a chip under a contact point reads as a local diameter change.

For loose tolerances a caliper is faster; choose by the tolerance. The dial bore gauge earns its place when the tolerance is tighter than a caliper can judge repeatably, or the bore is deep and a caliper cannot be rocked properly.

Standards

The product standard is GB/T 8122-2024 (current), which replaced GB/T 8122-2004. Verification of the dial indicator on top follows JJG 34-2022 (current), which replaced JJG 34-2008 and JJG 379-2009.

Planned for the lab

Dial bore gauges are planned for KELIBRON (intake group G2). The Ø28 bushing bore already in the lab will be the first workpiece; its Ø28 setting ring (or a gauge-block stack) arrives with the same intake. Until then, practise the rock-for-the-extreme reading on the caliper bore scene below — the geometry error it teaches is the same one the bore gauge avoids.