In short: an air plug takes its reading at the jet, so it can't measure closer to the bottom of a blind hole than the distance from the nose to the jet. A super-blind air plug reads as close as .050″ (1.27 mm) from the bottom on small sizes and about .075″ on most others, and needs a bore at least .08″ to .18″ deep depending on size. A through bore needs a land at least 2 jet diameters wide, typically .036″ to .100″. Tables for every size are below.
Two numbers decide whether an air plug fits your part
Customers ask us two versions of the same question: how close to the bottom of a blind hole can you measure? and how much land do you need? Both come down to where the jet sits and how big it is.
- Dim A is the distance from the end (nose) of the plug to the centerline of the jet. The reading is taken there, so in a blind bore Dim A is how far above the bottom you are measuring.
- Jet diameter sets how much bore wall the jet needs under it. The whole jet must be covered by the surface you're measuring, plus some margin, or the back-pressure leaks away and the reading is unstable.
Minimum bore depth = Dim A + 1½ jet diameters. Drawing not to scale; the land above the jet is exaggerated for clarity.
The rules we use
| Plug style | What limits it | Rule |
|---|---|---|
| Thru-hole (APT) | Width of the land (the straight, at-size bore wall) | Minimum land = 2 × jet diameter |
| Blind (APB) | Depth of the bore | Minimum bore depth = Dim A + 1½ × jet diameter |
| Super-blind (APS) | Depth of the bore | Minimum bore depth = Dim A + 1½ × jet diameter |
Why 1½ jet diameters? Half a diameter puts the whole jet inside the bore, and the extra full diameter of wall above it keeps the air from escaping out the top, so the reading is stable right down at the bottom. For a through bore, a land of two jet diameters gives the same margin on both sides of the jet.
Measure from where the bore is at size. None of these numbers include a chamfer, radius or fillet at the bore entrance. Add that length to the depth or land.
Dual Master air plugs
| Bore size | Jet | Thru-hole: minimum land | Blind: reads at / minimum depth | Super-blind: reads at / minimum depth |
|---|---|---|---|---|
| .059–.073″ (1.50–1.85 mm) | .018″ | .036″ (0.91 mm) | .080″ / .107″ (2.72 mm) | .050″ / .077″ (1.96 mm) |
| .073–.120″ (1.85–3.05 mm) | .023″ | .046″ (1.17 mm) | .080″ / .115″ (2.91 mm) | .050″ / .085″ (2.15 mm) |
| .120–.183″ (3.05–4.65 mm) | .042″ | .084″ (2.13 mm) | .085″ / .148″ (3.76 mm) | .065″ / .128″ (3.25 mm) |
| .183–.323″ (4.65–8.20 mm) | .047″ | .094″ (2.39 mm) | .095″ / .166″ (4.20 mm) | .075″ / .146″ (3.70 mm) |
| .323–2.510″ (8.20–63.75 mm) | .050″ | .100″ (2.54 mm) | .095″ / .170″ (4.32 mm) | .075″ / .150″ (3.81 mm) |
| 2.510–5.865″ (63.75–148.97 mm) | .050″ | .100″ (2.54 mm) | .105″ / .180″ (4.57 mm) | .085″ / .160″ (4.06 mm) |
| 5.865–8.260″ (148.97–209.80 mm) | .050″ | .100″ (2.54 mm) | .125″ / .200″ (5.08 mm) | .105″ / .180″ (4.57 mm) |
"Reads at" is Dim A: how far above a flat bottom the measurement is taken.
Single Master air plugs
Single Master tooling uses one fixed jet size for each amplification group, so the land is the same at every size.
| Bore size | Low amplification (D-2500 to D-8000) jet .048″, minimum land .096″ |
High amplification (D-10000 to D-32000) jet .023″, minimum land .046″ |
|---|---|---|
| .059–.120″ | — | Blind .115″ · Super-blind .085″ |
| .120–.183″ | Blind .157″ · Super-blind .137″ | Blind .120″ · Super-blind .100″ |
| .183–2.510″ | Blind .167″ · Super-blind .147″ | Blind .130″ · Super-blind .110″ |
| 2.510–5.865″ | Blind .177″ · Super-blind .157″ | Blind .140″ · Super-blind .120″ |
| 5.865–8.260″ | Blind .197″ · Super-blind .177″ | Blind .160″ · Super-blind .140″ |
Values are minimum bore depth. The reading is taken at the same Dim A as the Dual Master table. Not sure which group you have? Identify your amplification and tool code.
Air rings: how close to a shoulder
The same idea works in reverse on a shaft. Slide an air ring on until its face stops against a shoulder, and the reading is taken Dim A from that shoulder. The shaft needs at least Dim A + 1½ jet diameters of straight, at-size length from the shoulder. On a plain shaft with no shoulder, the minimum land is again 2 × jet diameter.
| Shaft size (Dual Master) | Jet | Shoulder-jet ring | Extra-close ring | Center-jet ring |
|---|---|---|---|---|
| .061–.073″ | .018″ | .102″ (.070″ and up) | — | .277–.308″ |
| .073–.120″ | .023″ | .110″ | — | .316″ |
| .120–.183″ | .042″ | .138″ | — | .344″ |
| .183–.323″ | .047″ | .166″ | .151″ | .352–.446″ |
| .323–.760″ | .050″ | .170″ | .155″ | .450″ |
| .760–3.010″ | .050″ | .200″ | .160″ | .575–.635″ |
| 3.010–7.500″ | .050″ | .210″ | .165″ | .700″ |
Values are the minimum straight shaft length from the shoulder. Low-amplification Single Master rings (.183″ and up) are within .003″ of these figures; high-amplification rings use a smaller jet and need about .035–.040″ less. A fillet or undercut at the shoulder holds the ring off the shoulder, so add its length.
When a standard plug won't reach
- Drill-point or radiused bottoms. The tables assume a flat bottom. A cone or corner radius stops the nose early, so add the height of the cone or radius, or ask us about a special nose shaped to the bottom.
- Narrow lands between grooves or cross holes. A slot jet covers a long, narrow land that a round jet can't. We quote these from your print.
- Sizes near a band edge. The jet size is chosen from the plug body diameter, which sits slightly under the bore size, so a bore right at a band edge can fall in the neighboring row. The online configurator shows the exact minimum for your size.
What to send us
A print or sketch showing the bore diameter and tolerance, the total depth, the bottom shape (flat, drill point or radius), any entrance chamfer, and where grooves or cross holes fall. With that, we can tell you which plug reaches and where the jets will land.
Questions
How close to the bottom of a blind hole can an air gage measure?
As close as Dim A, the distance from the nose to the jet centerline: .050″ to .105″ for super-blind plugs and .080″ to .125″ for blind plugs, depending on size, on a flat-bottom bore.
How deep does a blind hole need to be for an air gage?
At least Dim A plus 1½ jet diameters, about .08″ to .20″ depending on plug style and size. Add any entrance chamfer or radius.
How much land does an air gage jet need?
At least two jet diameters of straight, at-size surface: .036″ to .100″ for Dual Master plugs depending on size, .096″ for low-amplification and .046″ for high-amplification Single Master plugs.
What is the difference between a blind and a super-blind air plug?
A super-blind plug has its jets closer to the nose, so it reads nearer the bottom and fits shallower bores. The trade-off is a shorter nose that wears sooner. Use a blind plug when it reaches, and a thru-hole plug whenever the bore goes through.