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How to Check Plug Gage Wear

Guide · Updated October 28, 2026

Every time a GO plug enters a hole it loses a little size. Eventually a worn GO plug passes holes that are too small, and bad parts ship. A simple comparator check against gage blocks catches wear before it matters.

Step by step

1. Soak the gage: Let the plug and gage blocks sit on the plate together so they reach the same temperature.
2. Build the nominal: Wring a gage block stack equal to the plug nominal size.
3. Zero the comparator: Set a comparator stand or bench mic to zero on the stack.
4. Measure the GO end: Check near the entry end, the middle and the back, in two directions 90° apart.
5. Check the NOGO end: Measure it too; it should show little wear, but damage or nicks still matter.
6. Compare to the limit: Retire or downgrade the gage when the GO end reaches the wear limit.
7. Record it: Log the readings and date so you can see the wear rate and set the check interval.

Worked example

A 0.5000 in GO plug for a 0.5000/0.5010 hole. Zero a comparator on a 0.5000 in gage block stack. The GO member reads −0.00002 at the back, −0.00008 at the middle and −0.00015 at the entry end, so it has worn 0.00015 in where it enters the part. If the shop wear limit is 0.0001 in below nominal, this plug is retired. Readings that are larger at the entry end and smaller at the back are the classic wear pattern.

Where plugs wear

The first few millimeters of the GO member take most of the wear, because they enter every hole. Bell-mouthed or tapered readings point to wear; a flat spot or burr points to damage from dropping or forcing the gage.

Setting the interval

Base it on use: a production cell gage checked hundreds of times a day may need a monthly check, while a toolroom gage may go a year. Shorten the interval if your log shows steady wear.

Tips

• Basics: plug gage basics and go/no-go gaging.
• Comparator: comparator stand setup.
• Pins: pin gage sets.

FAQ

Which end of a plug gage wears?

The GO end, mostly near the entry.

How do I measure plug gage wear?

Compare it to a gage block stack with a comparator or bench mic.

What is a typical wear limit?

Set by your quality system; often a small fraction of the part tolerance.

Can a worn GO plug be reused?

Sometimes as a smaller size or a reference, but not for its original size.

How often should plug gages be checked?

Based on use; busy production gages more often than toolroom gages.

Related guides and calculators

• Plug Gage Basics
• Go/No-Go Gaging Basics
• Comparator Stand Setup
• Pin Gage Set Basics
• Ring Gage Basics
• Gage Block Calibration Interval
• All free machinist calculators

General shop information only — follow your machine and tooling manufacturer's instructions and your shop's safety rules.