An air probe (also called an air plug or air spindle) measures an inside diameter: a bore or hole. An air ring measures an outside diameter: a shaft, pin or spool. Both work the same way, with recessed jets that read size without touching the part.
Side by side
| Air probe | Air ring | |
|---|---|---|
| Measures | Inside diameter (bores, holes) | Outside diameter (shafts, pins) |
| Our sizes | .059″ to 8.260″ | .061″ to 7.500″ |
| Styles | Thru-hole, blind, super-blind | Center-jet, shoulder-jet, extra-close |
| Set with | Ring masters | Setting discs |
| Sized from (Dual Master) | Minimum bore − clearance | Maximum shaft + clearance |
How each works
Air flows from the readout through the gage body and out of two or more opposed jets. The gap between each jet and the part controls how much air escapes, and the readout converts that into size. Because the jets are opposed, a probe or ring held slightly off-center still reads correctly: one gap opens as the other closes by the same amount.
Choosing a probe style
- Thru-hole: jets set back from the nose; longest wear life. Use whenever the bore goes through.
- Blind: jets close to the nose, to measure near the bottom of a blind bore.
- Super-blind: jets as close to the nose as possible, for the deepest reach, with faster nose wear.
Choosing a ring style
- Center jet: jets at mid-width; best wear life.
- Shoulder jet: jets near the leading face, to measure close to a shoulder.
- Extra-close: jets as close to the face as possible.
Rings come with two or three jets; three jets 120° apart catch three-lobe out-of-round that two jets can't see.
What about shafts you can't pass through a ring?
Use an air snap gage. Its open C-frame slips over a shaft from the side, so the part can stay on the grinder or between centers.
Browse air plug gages and air ring gages, or price either one online.