Buying guide · Pin gages

Pin Gage Sets: Class ZZ vs Z vs X, Plus vs Minus, and Buying Used

A pin gage set is one of the cheapest ways to check holes fast, and one of the easiest gages to buy used. But the class, the plus or minus marking and the condition of the pins decide whether the set will actually tell you the truth about a hole. This guide covers what Class ZZ, Z and X mean, which way plus and minus go, what comes in the common sets, and how to check a used set with a micrometer before you pay.

Ask about a pin set: 508-343-1219 Open the full gage checklist

Classes ZZ, Z, X
Plus vs minus
Picking a class
Common sets
Checking a used set
When to walk
FAQ

1. What Class ZZ, Z and X mean

The class is the gagemaker tolerance: how close the pin itself is made to the size stamped on it. It says nothing about the hole you're checking. The classes for plain cylindrical gages are defined in ASME B89.1.5.

Class ZZ

.0002 in (0.005 mm). The class most full pin sets are sold in. General shop-floor hole checks.

Class Z

.0001 in (0.0025 mm). Half the tolerance of ZZ. Common for dedicated GO and NO-GO pins on tighter holes.

Class X

.00004 in (0.001 mm). Precision inspection and gaging holes toleranced in tenths. Classes Y and XX also exist but are less common in sets.

2. Plus pins vs minus pins

Every pin in a class carries its tolerance in one direction. A plus pin is made at or above its marked size; a minus pin is made at or below it. The rule that keeps the gage from passing a bad hole:

GO pin, at the minimum hole size: plus. A plus pin can never be smaller than the minimum, so it can't slip into an undersize hole.
NO-GO pin, at the maximum hole size: minus. A minus pin can never be larger than the maximum, so a hole that's even slightly oversize will take it and get rejected.
On a used set, find the plus or minus marking on the case, the cert or the pins themselves (sets over about .060 in are usually marked on the pin). If you can't find it, you don't know what you're buying.

Catalogs don't all explain plus and minus the same way. Go by the direction the tolerance is applied, not by the word on the box: the gage's own tolerance should always sit inside the part's tolerance.

3. Picking the class for the job

A common metrology rule of thumb, published by gage makers, is the 10 percent rule: the gage tolerance should use no more than about 10 percent of the part tolerance. Worked out:

A hole at ±.001 in has a .002 in band. 10 percent is .0002 in, so Class ZZ fits.
A hole at ±.0005 in has a .001 in band. 10 percent is .0001 in, so step up to Class Z.
Holes toleranced in tenths need Class X or a different method (bore gage, air gage, CMM).
Buy the loosest class that does the job. A tighter class costs more and doesn't make a worn pin good.

4. What's in the common sets

Pin sets are usually sold by range in .001 in steps, so you can count what should be there. For example, a .061-.250 set holds 190 pins (.061 through .250). Other common ranges include .011-.060, .251-.500, .501-.625, .626-.750 and .751-1.000. Check the seller's case layout against the range on the lid.

Material: plain hardened tool steel is standard. Black-oxide pins (Vermont Gage calls theirs Black Guard) resist rust and show wear as the black turns silver. Carbide pins wear much longer and cost more.
Metric sets run in 0.01 mm steps. Don't mix them into an inch set.
A set is worth much more complete. Missing sizes are usually the ones that got used the most.

5. Checking a used pin set before you pay

Bring a micrometer you trust (a 0-1 in mic with a recent check covers most sets), a clean rag and a few minutes.

Count the empty slots against the case layout. Write down the missing sizes.
Read the lid or cert for range, class, and plus or minus. Match it to the markings on a few pins.
Look for rust, pitting and dark fingerprint stains. Rust on a pin's measuring surface changes its size; light surface staining that wipes off doesn't.
Look at the ends. Pins wear first at the lead end, where they enter the hole. A bright band or a visible taper there means wear.
Mic a handful of random pins, plus the most common sizes (the ones a shop reaches for daily). Measure near the end and near the middle; a difference tells you it's worn.
Check for nicks and burrs by rolling a pin slowly between your fingers or along a flat surface.
On a black-oxide set, look for pins that have gone silver. Those are the worn ones.
Ask for the cert. A cert shows the pins were in size when they were checked, not after years in a drawer. Have the pins you'll use for acceptance re-checked.

Your shop micrometer won't resolve .0002 in reliably on every pin, so the mic check is a screen for wear and damage, not a calibration. If the set is going to be used to accept parts, budget for a lab check of the sizes you need.

6. When to walk away

Rust or pitting across many pins.
No way to tell the class or plus/minus direction.
Lots of empty slots in the sizes your work actually uses.
Pins mixed from different sets, classes or makers in one case.
Several random pins mic under size at the lead end.

Rule of thumb: a used pin set is worth what its good, in-size pins are worth to your work. Count those, not the case.

Pin gage set questions

What is a Class ZZ pin gage?

Class ZZ is the widest gagemaker tolerance class for plain pin gages: .0002 in (0.005 mm) on the pin's size. It is the class most full pin sets are sold in and covers general shop-floor hole checks.

What is the difference between Class ZZ, Z and X?

The class is how close the pin itself is made to its marked size. Class ZZ is .0002 in, Class Z is .0001 in and Class X is .00004 in. A tighter class costs more and is only needed when the part tolerance is tight enough that a ZZ pin would use up too much of it.

Should I buy a plus or minus pin gage set?

For hole checks, the GO pin at the minimum hole size should be plus and the NO-GO pin at the maximum hole size should be minus, so the gage's own tolerance can never accept a bad hole. Many shops keep a minus set for general use; check how each seller marks plus and minus, because not every catalog explains it the same way.

How many pins are in a .061-.250 pin gage set?

In .001 in steps, a .061-.250 set has 190 pins (.061 through .250). Count the empty slots on a used set before you agree on a price.

Is a used pin gage set worth buying?

It can be, if most pins are present, there is no rust or pitting, and a spot check with a micrometer shows the pins you check still read on size. Treat any cert as a starting point and have the pins you'll use for acceptance checked.

What does Black Guard mean on a pin set?

Black Guard is Vermont Gage's black-oxide treatment for pins. It helps resist rust, and as a pin wears the black turns silver, which shows you which pins have seen the most use.