Bore Gauge Buying Guide: Dial Bore Gauges, Telescopic & Inside Mics

A dial bore gauge is the quickest way to measure a bore's diameter, taper and out-of-round to a hundredth of a millimetre or better, but it only compares: you set it to a known size first, then read how far the bore differs. If you just need the odd hole size checked, a telescopic gauge and an outside micrometer will do the job for less. For larger bores, an inside micrometer reads size directly. This guide covers each type, what range and resolution mean, and how to pick the right bore gauge in NZ for your work.

Types of bore gauge

Dial bore gauges

A dial bore gauge has a fixed anvil on one side of the head and a moving contact on the other. The movement is transferred up the handle to a dial indicator. A centring guide keeps the gauge on the true diameter, and you rock it gently through the bore: the reading where the needle turns back is the true diameter. Interchangeable anvils let one gauge cover a band of sizes, such as 18-35 mm.

The dial bore gauge is a comparator. It tells you how far the bore is from the size you set it to, not the size itself. That makes it fast for checking many bores against one nominal, for example engine cylinders, bearing housings or production bores.

Telescopic gauges

A telescopic gauge is a transfer tool. A spring-loaded plunger expands inside the bore or groove, you lock it with the knurled end of the handle, withdraw it, then measure across the contacts with an outside micrometer or caliper. It has no scale of its own, so your result depends on your feel and on the micrometer you read it with. They are cheap and handy for one-off checks.

Small-hole gauges

Small-hole gauges (also called ball gauges) work the same way as telescopic gauges, but for holes too small for a telescopic plunger. The split ball or contacts are expanded in the hole, then measured with a micrometer. If your holes are below the smallest telescopic gauge size you have, this is the tool to look for.

Inside micrometers and 3-point bore micrometers

A rod-type or tubular inside micrometer is a micrometer head with extension rods. It measures across two points and reads the size directly off the thimble, which suits large bores and internal spans where a dial bore gauge would need a big setting standard. A 3-point bore micrometer has three contacts that self-centre in the bore, which makes it easier to get a repeatable reading.

What to look for in a bore gauge

Range

Range is the band of bore diameters a gauge can be set up for. On a dial bore gauge it comes from the interchangeable anvils and washers supplied in the kit. Pick the range that puts your common bore sizes in the middle of the band, not at the edges. On an inside micrometer, range depends on the extension rods supplied. On a telescopic gauge, each gauge covers one band, so sets are sold to cover a wider span.

Resolution (graduation)

On a dial bore gauge, the resolution is set by the dial indicator fitted to it. A 0.01 mm indicator is the general workshop choice. A 0.001 mm indicator suits fine fits such as bearing seats and reamed or honed bores, but it needs a careful setting standard and a steady hand to be worth having. Do not confuse resolution with accuracy: a fine dial on a poor setting standard still gives a poor answer.

Depth

Check how deep the gauge can reach. Mitutoyo gives a measuring depth for each of its bore gauges (for example 100 mm on the 18-35 mm model), and our listing notes that optional extension rods can be attached for measuring deep holes.

Contact points and setting accessories

Carbide contacts resist wear far better than steel. Kits that include washers as well as anvils let you set the gauge in finer steps between anvil sizes.

Setting a bore gauge to a micrometer or ring

Because a dial bore gauge compares, you must set it to the nominal size before every job:

1. Fit the anvil (and washer if needed) that suits your nominal bore size.
2. Set an outside micrometer to the nominal size and lock it. A setting ring of the nominal size is better if you have one, because the gauge then sees the same shape it will measure.
3. Place the gauge between the micrometer faces (or in the ring) and rock it until you find the turning point of the needle.
4. Rotate the dial bezel so zero sits at that turning point.
5. Measure the bore the same way. Readings either side of zero show how far over or under the nominal size it is.

Measure at several depths and at 90 degrees to each other to find taper and out-of-round. Let the gauge and part reach the same temperature, and hold the gauge by its grip.

Bore gauges and inside measuring tools in stock

Mitutoyo lists an accuracy of 2 µm for each of the four 511-series bore gauges above, and a measuring depth of 100 mm (18-35 mm models) or 150 mm (35-60 mm and 50-150 mm models). The kits include the dial indicator, anvils and interchangeable washers.

To set a bore gauge or read a telescopic gauge you need an outside micrometer covering the same size. The Mitutoyo Outside Micrometer is listed in metric ranges from 0-25 mm to 225-250 mm at 0.01 mm graduation, plus a 25-50 mm x 0.001 mm model and imperial sizes. Mitutoyo specifies its 103-137 (0-25 mm) with carbide-tipped measuring faces and a ratchet stop.

How to choose by job

• Engine cylinders and larger housings: the 50-150 mm dial bore gauge, set to a micrometer of the same size, shows taper and out-of-round quickly.
• Bearing seats, bushes and small precision bores: the 18-35 mm dial bore gauge with the 0.001 mm indicator (511-721-20), set to a ring or a 0.001 mm micrometer.
• Occasional hole and groove checks: the telescopic gauge set covers 8-150 mm with six gauges, and you read it with a micrometer you may already own. Buy single gauges if you only work in one size band.
• Large bores and internal spans: an inside micrometer reads directly, with ranges up to 50-1000 mm on the tubular extension rod type.

Care and use

• Clean the bore and the contacts before measuring. Swarf or a film of oil between the contacts and the bore throws the reading off.
• Re-check the zero against your setting standard during long runs, and always after a knock.
• Store the gauge in its box with the indicator cover fitted, and keep the anvils and washers together with the gauge.
• With telescopic gauges, lock lightly and withdraw squarely. Rocking it out at an angle changes the size you captured.

FAQ

What is the difference between a dial bore gauge and a telescopic gauge?

A dial bore gauge has its own indicator and shows the bore's deviation from a set size while it is in the bore, so it reveals taper and out-of-round. A telescopic gauge only captures the size, which you then measure with a micrometer. The dial bore gauge is faster and more repeatable. The telescopic gauge is cheaper.

Do I need a micrometer to use a dial bore gauge?

Yes, or a setting ring. The gauge has to be zeroed to a known size before it can tell you anything about the bore.

Which bore gauge range should I buy?

Choose the range where your most common bore sizes fall comfortably inside the band. If you work across many sizes, a 50-150 mm gauge covers the widest band of the Mitutoyo models above.

Is 0.01 mm resolution enough?

For most engine and general machining work, yes. Go to 0.001 mm only if your tolerances need it and you have a setting standard good enough to support it.

Browse dial bore gauges, telescopic gauges and inside micrometers in Bore Gauges & Inside Micrometers, or see Mitutoyo, Micrometers and Measuring Tools.