Buying guide · Gage blocks
Gage Block Sets: Grades, the 81-Piece Set, and Checking a Used Set
A gage block set is the length reference the rest of the inspection bench gets checked against: mics, height gages, indicators, snap gages. That makes a used set either a bargain or a quiet source of bad readings. This guide covers what the grade marks mean, what a standard 81-piece set should hold, and how to check a used set on the bench before you pay.
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1. What the grade marks mean
The grade tells you how close each block was made to its marked size (and how flat and parallel its faces are). The current US standard is ASME B89.1.9, which lists a calibration Grade K and Grades 00, 0, AS-1 and AS-2. On a used set you will also see older marks:
- ASME B89.1.9 (current): 00 is the tightest, then 0, AS-1 and AS-2. As a published example for blocks up to 1 in, the limits are about ±3, ±6, ±12 and ±24 millionths of an inch (gagesite.com B89.1.9 tolerance table). Longer blocks get wider limits.
- Federal GGG-G-15C (older): Grades 0.5, 1, 2 and 3. Pratt & Whitney Measurement Systems describes 00 as roughly equal to old Grade 1, 0 to Grade 2, and AS-1 to Grade 3, with AS-2 looser than Grade 3 (P&W grade notes).
- AA, A, B (oldest): still stamped on plenty of sets in old toolrooms. They're no longer current, so treat them as "how it left the factory," not as today's size.
2. Which grade a shop actually needs
- Floor and inspection-bench use (setting mics, checking height gages and indicators, stacking for a sine bar): a shop-grade set, AS-1 or AS-2 (or an old Grade 3 or B set in good shape), is what most job shops use.
- Masters for checking your other blocks and gages are usually a tighter grade (0 or 00) and get handled as little as possible.
- Check what your customer's quality requirements or your own gage-control procedure call for before you pay extra for a tighter grade. A worn Grade 0 set is not better than a clean AS-1 set.
- Rule of thumb from gage makers (the same 10 percent idea used for pin gages): the reference should use only a small part of the tolerance you're checking.
3. What's in the common 81-piece set
The most common inch set is the 81-piece set. You can count it against this layout:
- 9 blocks, .1001 to .1009 in (steps of .0001)
- 49 blocks, .101 to .149 in (steps of .001)
- 19 blocks, .050 to .950 in (steps of .050)
- 4 blocks, 1, 2, 3 and 4 in
- Some sets add two wear blocks (often carbide) that go on the ends of a stack so the working blocks don't take the wear. Check the lid or the case layout.
Smaller sets (36-piece, 9-piece "space" sets, long-block sets) and metric sets exist too. Whatever the set, the empty slots are usually the sizes that got used the most, and a missing 1 in or .100-series block hurts more than a missing odd size.
4. Checking a used set before you pay
Bring a lint-free wipe, a little solvent, a loupe or phone macro lens, and a micrometer you trust. Wear finger cots or gloves if you can; fingerprints rust steel blocks.
- Count it. Lay the case open and match every slot to the layout on the lid. Write down missing and swapped blocks (a block in the wrong slot often came from another set).
- Match the paperwork. Maker, grade and serial on the case should match the cert, if there is one. Blocks are usually marked with size and often serial; check a few.
- Look at the gaging faces under light. Rust spots, pitting, gray staining and fingerprint-shaped marks are the big ones on steel blocks. Light oil film that wipes off is normal; marks that don't wipe off are in the surface.
- Check edges and corners. Dings on an edge can raise a burr onto the face. On ceramic sets look for chips.
- Wring a few pairs. Clean two of the most-used blocks (the .1001-.1009 and 1 in blocks get handled most), then slide them together with light pressure. Clean, flat faces should wring and hold. A pair that won't wring after cleaning usually has a burr, a worn face or rust.
- Feel for burrs. Draw a clean face lightly across a clean fingernail or a flat stone surface; a burr catches. A gage block stone can take a small burr down, but don't buy a set planning to stone every block.
- Mic a few blocks. A shop micrometer can't see millionths, so this only catches gross wear, wrong sizes or mixed sets, not grade. It's still worth doing.
- Check the case. A good case with a dry lining and rust-preventive paper or oil protects the set. A wet, moldy or crushed case usually means the blocks inside had a hard life.
None of these checks tells you a block's size to its grade. That takes a calibration against masters on a comparator. The bench check tells you whether the set is worth paying to have checked.
5. Certs, calibration and what the seller can honestly say
- A cert shows the blocks measured in size on the day they were checked. An old cert on a set that has been in a drawer since is history, not a current size.
- If the set will be used to accept parts or to check other gages under a quality system, plan to have it calibrated before you rely on it, and price that into your offer.
- A set sold "as-is, not calibrated" is not a bad set. It just means you're paying for the steel and the case, and the check is on you.
- Sellers: say what you know (maker, grade on the case, count, missing sizes, cert date if any) and nothing more. See the seller checklist.
6. When to walk away
- Rust or pitting on the gaging faces of many blocks, especially the small .100-series blocks.
- Many blocks that won't wring after cleaning.
- Mixed blocks from different makers or grades in one case, with no way to tell which is which.
- Several of the most-used sizes missing.
- A price based on a high grade stamp with no way to show the blocks are still close to it.
Rule of thumb: a used block set is worth what its clean, wringing blocks are worth to your work, less the cost of a calibration if you need one. Count those, not the lid.
Gage block set questions
What are the gage block grades?
The current US standard, ASME B89.1.9, uses Grades 00, 0, AS-1 and AS-2, plus a calibration Grade K. Grade 00 is the tightest and AS-2 the loosest. Older sets are often marked with Federal GGG-G-15C grades (0.5, 1, 2, 3) or the even older AA, A and B, which are no longer current.
Which gage block grade does a job shop need?
For setting and checking micrometers, height gages and indicators on the floor, a shop-grade set (AS-1 or AS-2, or an old Federal Grade 3 set) is usually enough. Grades 0 and 00 are normally kept as masters for checking other blocks and gages. Buy the grade your work and your customer's quality system actually call for.
What is in an 81-piece gage block set?
The common inch 81-piece set is nine blocks from .1001 to .1009 in, forty-nine from .101 to .149 in, nineteen from .050 to .950 in in .050 steps, and four from 1 to 4 in. Some sets add a pair of wear blocks. Count the case against that layout before you agree on a price.
Is a used gage block set worth buying?
It can be, if the set is complete or nearly so, the gaging faces are free of rust and burrs, the blocks you'll use most still wring, and the case and grade marking match. Treat the old cert as history; if the set will be used to accept parts, have it calibrated before you rely on it.
Why won't my gage blocks wring?
Usually dirt, old oil, or a burr or nick on a gaging face. Clean both faces with a solvent and a lint-free wipe and try again. A block that still won't wring after cleaning has a raised burr or worn face; a gage block stone can take a burr down, but a face that's rusted or pitted is done.
Steel, carbide or ceramic gage blocks: which is better used?
Steel blocks are the most common and the cheapest, but they rust from fingerprints and humidity, so check used steel sets hardest. Tungsten carbide blocks wear much longer. Ceramic blocks don't rust and wear well but are more brittle, so check edges and corners for chips.