Hole Micrometer

A hole micrometer measures internal bore diameter directly — no rocking, sweeping or feel required. The three-point design, which Mitutoyo brands as the Holtest, seats three anvils at 120° around the bore so the head self-centres and reads true diameter on the first pull. That’s what separates it from a two-point internal mic or a telescoping gauge, where the reading depends entirely on operator technique. Because it self-centres, two operators get the same number, and you can take readings at several depths to check a bore for taper or bellmouth.

Three-point anvils only travel a few millimetres, so Holtest sets come in narrow steps — 6–8mm, 12–16mm, 25–30mm and up. Each size is zeroed against a matching setting ring, so factor a ring into your decision if you’re buying used. Very small bores under about 6mm use a two-point Holtest instead, where three contacts won’t fit.

We stock new, new-old-stock and inspected used 3-point hole micrometers — mechanical thimble, digital and dial models — from Mitutoyo and others, alongside RSK, SK and Ocean setting rings. Every used tool is checked against gauge before it’s listed.