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Thread Gage Basics
Guide · Updated October 28, 2026
Thread gages accept or reject threads against their class limits. A thread plug checks a tapped hole; a thread ring checks a bolt or threaded shaft. The GO member must thread in fully by hand, and the NO-GO must not go more than a few turns.
Size the tap drill too
See the tap drill chart, and build exact check stacks with the free Gage Block Calculator.
Step by step
- Read the thread callout: For example 1/4-20 UNC-2B: size, pitch, series and class. B is internal, A is external.
- Choose plug or ring: Use a thread plug for internal (B) threads and a thread ring for external (A) threads, in the same class.
- Clean and deburr: Blow out chips and remove burrs; a chip in the root makes a good thread fail GO.
- Run the GO member: Thread the GO in by hand, with no wrench, through the full thread length. It must go.
- Try the NO-GO: Thread the NO-GO by hand. It may enter, but should stop within about three turns.
- Accept and store: Accept if GO goes and NO-GO stops; wipe, oil and store. Rings are adjustable and must be set against a master.
Worked example
A tapped 1/4-20 UNC-2B hole, 0.50 in deep. The GO plug threads in by hand to full depth. The NO-GO plug starts and stops after about 1½ turns. The thread is good.
If the GO stops halfway, the tap may be worn, the hole may have chips in the bottom, or the tap drill was too small and the tap loaded up. If the NO-GO runs in freely, the thread is oversize, often from a worn or oversize tap or a tapping head pulling the tap.
Tips
- Never force a thread gage with a wrench; it damages the gage and hides a bad thread.
- Check the pitch first with a thread pitch gage.
- Measure external pitch diameter with a thread micrometer or three wires.
- Plain holes and shafts use plug gages and ring gages.
- Tap speeds affect thread quality: tapping speeds and feeds.
FAQ
How do you use a go/no-go thread gage?
Thread the GO member in by hand through the full length; then try the NO-GO, which should stop within about three turns.
What does 2B mean on a thread?
Class 2 fit for an internal thread. 2A is the matching external class.
Why is the GO thread gage not going in?
Chips, burrs, a worn or wrong tap, or a tap drill that was too small. Clean the hole first.
Can I adjust a thread ring gage?
Most thread rings are adjustable and must be set to a master setting plug by a qualified lab or gage room.
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
- Thread pitch gage basics
- How to use a thread micrometer
- How to measure threads with three wires
- Tap drill chart explained
- Tapping speeds and feeds
- Go/No-Go gaging basics
- How to Cut Threads on a Lathe
- How to Cut Internal Threads on a Lathe
- How to Tap on a Mill
- How to Cut Acme Threads on a Lathe
- How to Chase Threads
- How to Cut Left-Hand Threads on a Lathe
- Helicoil Thread Insert Basics for Machinists
- How to Use a Center Gage (Fishtail)
- NPT Pipe Threading Basics for Machinists
- How to Use a Pipe Tap (NPT)
- How to Use a Threading Die and Die Stock
- How to Use a Thread Mill
- All free machinist calculators
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General shop information only — follow your tooling, machine and gage manufacturers' instructions and your quality system.