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Plug Gage Basics

Guide · Updated October 28, 2026

A plug gage is a precision cylinder used to check a hole. A go/no-go plug has one end at the smallest allowed hole size, which must enter, and one at the largest, which must not.

Back up a gage with blocks

Verify a hole or set a bore gage with an exact stack built in the free Gage Block Calculator.

Step by step

Find the hole limits: Read the minimum and maximum hole size from the drawing, for example 0.5000/0.5005 in.
Select the gage: GO = minimum hole size; NO-GO = maximum hole size. Check the markings and class on both members.
Clean everything: Wipe the gage and the hole; chips and coolant give false results.
Check GO: Insert the GO member with light hand force through the full depth that matters. It must enter.
Check NO-GO: Try the NO-GO end square to the hole. It must not enter beyond a small lead-in.
Record and store: Accept only if GO goes and NO-GO does not; wipe, oil lightly and store the gage.

Worked example

A hole is toleranced 0.5000 +0.0005/−0.0000 in. GO plug = 0.5000 in, NO-GO = 0.5005 in.

With class X gages (about ±0.00004 in at this size), the GO is made slightly over its nominal and the NO-GO slightly under (the "tolerance in" convention), so gage error can only reject a borderline part, never pass a bad one. Effective accepted range is about 0.50004 to 0.50046 in.

Classes and materials

Common gagemaker classes from tightest to loosest are XXX, XX, X, Y, Z and ZZ. Tool steel is standard; chrome-plated or carbide gages last longer on high-volume work. Pin gages are the low-cost cousin, sold in sets by 0.001 in.

Tips

Never force or twist a plug gage; a seized gage damages both the gage and the part.
Let parts cool first: a warm part can accept a NO-GO that will reject later.
For the full go/no-go concept: go/no-go gaging basics.
For quick checks across many sizes: pin gage set basics.
Plugs show size, not location: how to indicate a bore true covers position. For shafts, see ring gage basics.

FAQ

What is a go/no-go plug gage?

A gage with a GO end at the minimum hole size and a NO-GO end at the maximum. A good hole accepts GO and rejects NO-GO.

What size is the NO-GO end?

The maximum allowed hole size, adjusted slightly by the gagemaker tolerance toward the inside of the part tolerance.

What is the difference between a plug gage and a pin gage?

Plug gages are usually handled, marked GO/NO-GO and made to a stated class for a specific tolerance; pin gages are sold in sets by size for general checks.

Can a plug gage tell me the actual hole size?

No. It only tells you whether the hole is within limits. Use a bore gage or small hole gage for a number.

Is there a free gage block calculator?

Yes. The Gage Block Calculator at gageblockcalc.com builds any stack, free with no login.

Related guides and calculators

Build your stack in seconds

The free Gage Block Calculator finds the fewest-block combination for inch and metric sets, with wear blocks. No login.

Questions? Ask a machinist.

General shop information only — follow your tooling, machine and gage manufacturers' instructions and your quality system.