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Air Gage Basics

Guide · Updated October 27, 2026

Air gaging measures size by how much air escapes between a jet and the part. It is non-contact, fast and reads to tenths or better, which makes it common for production bore checks.

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Step by step

Supply clean air: Use filtered, regulated, dry air.
Master it: Set the gage on min and max setting masters, adjusting zero and magnification.
Insert the plug: Put the air plug into the bore. It's self-centering.
Read: The column or digital display shows the deviation from nominal.
Re-master: Re-master on the maker's schedule or when the temperature changes.

Pros and cons

Pros: fast, no contact, good for bore taper and out-of-round checks.
Cons: each tooling piece fits a narrow size range, and it needs clean air and masters.
Masters are certified rings and plugs, traceable through gage blocks.

Worked example

You are checking a 0.7500 +0.0004/−0.0000 in bore with an air plug and a column or dial readout. Set the gage with two masters: a 0.7500 in LO master ring and a 0.7504 in HI master ring. Adjust zero so the LO master reads 0 and the magnification so the HI master reads +0.0004 in (on a column with 0.00005 in graduations that is 8 divisions). A part that reads +0.00015 in is 0.75015 in, inside tolerance. Check linearity: a third master at 0.7502 in should read +0.0002 in (4 divisions). If it reads +0.00025 in, the scale is 25% off at mid-range, so reset the magnification before running parts.

Common problems

Readings that drift during a shift usually mean supply pressure changes or a dirty filter; recheck the masters every hour or two. A sudden jump means a nicked jet or a chip in the plug. Low readings on rough bores happen because air leaks through the peaks, so air gages suit finishes around 32 µin Ra or finer.

Worked example: setting with two masters

An air plug for a 0.5000 in bore with ±0.0005 in tolerance is set with a minimum master at 0.4995 in and a maximum master at 0.5005 in. Insert the min master and set the zero; insert the max master and adjust magnification so the reading spans the 0.0010 in band. Bores reading between the two marks are good.

Air gages are fast and non-contact, but they average over the jet area. Supply air must be clean and dry; water or oil in the line changes the reading.

Quick shop check

Worked example: a bore spec of 0.5000–0.5006 in is mastered with a min ring of 0.5000 and a max ring of 0.5006 in. Set the display to read 0.5000 on the min ring, then adjust magnification so the max ring reads 0.5006 (a 0.0006 in span). A part that reads 0.5004 is 0.0004 above the low limit and 0.0002 under the high limit, so it passes with 1/3 of the band left; a reading of 0.5007 rejects. If the two masters no longer read their sizes after re-mastering, clean the jets before trusting any reading. Common problems: a gage that drifts during the shift usually indicates temperature change or a dirty jet. Recheck with masters every few hours.

FAQ

How does an air gage work?

Back-pressure or flow between a jet and the part changes with the gap, and the display converts that to size.

How accurate is air gaging?

Commonly 0.00005 to 0.0001 in resolution over a narrow range.

How is it mastered?

With min and max setting masters, usually ring or plug gages.

Does surface finish matter?

Yes. Rough surfaces can read differently than contact gages.

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General shop information only — follow your gage manufacturer's instructions and your quality system.