On a production bore held to tenths, an air gage (air gauge) is usually more accurate in practice than a dial bore gauge, because the result doesn't depend on the operator. A dial bore gauge has to be rocked to find the true diameter, and different people find it slightly differently. An air plug centers itself and reads in seconds. The dial bore gauge wins on flexibility: one gauge covers many sizes.
Side by side
| Air gage (air plug) | Dial bore gauge | |
|---|---|---|
| How it reads | Opposed air jets, non-contact, self-centering | Contact points; rock to find the minimum reading |
| Operator influence | Very low: insert and read | High: feel, rocking and alignment affect the reading |
| Readout resolution | Digital readouts to .00005″ or finer | Typically .0001″ per division (varies by indicator) |
| Speed | A second or two per reading | Slower; each reading needs rocking |
| Part surface | Not touched; no marks on soft or polished bores | Contact points can mark soft or finished surfaces |
| Size range | One nominal size per plug, a few thousandths of range | Wide range with interchangeable anvils and extensions |
| Form checks | Out-of-round, taper and multiple diameters with the right jets | Diameter only, one position at a time |
| Data | Digital readout to SPC, Excel or a PLC | Manual recording, unless fitted with a digital indicator |
Why accuracy is really about repeatability
Both gauges are comparators: each is set to a master and reads the difference. What separates them on the shop floor is how much the reading changes from one try, or one operator, to the next. That spread eats into your tolerance. A common rule is that the gauge should use no more than about 10% of the tolerance, which a gage R&R study will show. On tight bores, air gaging passes that test far more easily, because the jets center the reading and there is no feel involved.
When a dial bore gauge still makes sense
- One-off or low-volume parts with many different bore sizes
- Wide tolerances, where the dial bore gauge's operator spread is small compared with the tolerance
- Very deep or very large bores beyond practical air tooling
When to switch to air
- Production runs of the same bore size
- Tolerances of a few tenths, where operator spread becomes a real share of the tolerance
- Soft, thin-walled or polished bores that contact points could mark or deflect
- Blind bores measured close to the bottom (see the blind air plug)
- When you need data, not handwritten readings
Questions
How accurate is an air gauge?
Air gages repeat in the sub-micron range, and digital readouts resolve .00005″ or finer. Real accuracy depends on the masters, clean regulated air and stable temperature.
Is an air gauge more accurate than a bore gauge?
In production, usually yes, because an air plug centers itself and removes operator feel from the reading. A dial bore gauge is more flexible across sizes.
What is the function of an air gauge?
It measures a bore or shaft diameter without contact, by reading the back-pressure of air escaping between its jets and the part. See how air gaging works.
Can one air plug measure different bore sizes?
No. Each air plug is built for one nominal size with a working range of a few thousandths of an inch. A dial bore gauge is better when sizes change constantly.
New to air gaging? Start with how air gaging works, then browse air plug gages.