A bore micrometer measures the internal diameter of a cylinder or bore — the job an outside micrometer and a caliper cannot do accurately. During a classic-engine rebuild it tells you whether a bore is worn past its service limit or gone oval before you commit to a rebore. This guide covers the technique and the honest accuracy you can expect in a home workshop.
What a bore micrometer is
A bore micrometer is an internal-diameter tool. The common three-point type has anvils spaced at 120 degrees so it self-centres in the hole and reads the true diameter directly — no rocking required. That makes it fast and repeatable for size. It is a different tool from a two-point internal micrometer; the differences between the families are covered in our internal micrometer guide.
Two ways to measure a cylinder bore
There are two honest routes, and they are good at different things:
- Three-point bore micrometer: self-centres and reads true diameter quickly, but because it averages across three contacts it is weaker at capturing ovality.
- Two-point method: a dial bore gauge, or telescopic (snap) gauges transferred to an outside micrometer. You gently rock a two-point tool to find the minimum reading, and by measuring at different orientations it reveals ovality that a three-point tool tends to average out.
For classic-engine work where you pulled the crank, found scoring and the machine shop said ‘measure the ovality first’, the two-point route usually tells you more.
Measuring wear, ovality and taper
Map the bore rather than taking one reading. Measure the diameter at two positions 90 degrees apart at the same depth — the difference is the ovality. Repeat near the top, the middle and the bottom of the bore to reveal taper from ring travel. Compare the worst reading against the manufacturer's service limit; classic petrol engines usually quote wear and clearance limits to 0.01–0.02 mm. Because classic manuals are often imperial, decide up front whether you are working in thou or millimetres and stick to it.
Where the PROSTER outside set fits
The external side of a rebuild — piston skirts, gudgeon pins and crank journals — is measured with an outside micrometer, and telescopic gauges are read across an outside micrometer's anvils too. The PROSTER 6-piece outside set does exactly that job at £126.13. Be clear on scope: PROSTER sells the outside set, not the bore tool itself, so pair it with a dedicated internal or dial bore gauge for the cylinder ID. See the outside micrometer guide.
PROSTER Outside Micrometer — 6-Piece Metric Set
Six metric frames covering 0–150 mm in 25 mm steps, carbide measuring faces and a ratchet stop, with free UK delivery, 30-day returns and a 12-month guarantee.
Honest limits
A three-point tool averages ovality, so where a bore is clearly oval a two-point gauge is the better call. Borderline or sub-0.005 mm decisions — and the final call on whether to rebore — are best confirmed by a machine shop. Let the block and the tool reach the same temperature before the measurements that matter.
Frequently asked questions
Can I measure a cylinder bore with an outside micrometer?
Not directly, but you can with telescopic (snap) gauges: lock the gauge in the bore, then measure across it with the outside micrometer. For a direct reading use a bore micrometer or a dial bore gauge.
How do I check a bore for ovality?
Measure the diameter at two positions 90 degrees apart at the same depth; the difference is the ovality. A two-point dial bore gauge or telescopic-gauge method captures this better than a self-centring three-point tool, which averages it out.
Do I need a three-point bore micrometer or a dial bore gauge?
A three-point bore micrometer self-centres and reads true diameter quickly; a two-point dial bore gauge is better when you specifically need to map ovality and taper.
Does PROSTER sell a bore micrometer?
No, PROSTER's set is a 6-piece outside micrometer set; it reads telescopic gauges and measures pistons and journals externally, while the bore itself needs a dedicated internal tool.