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How to Check a 1-2-3 Block

Guide · Updated October 27, 2026

1-2-3 blocks are setup workhorses, and they get dropped, burred and worn. A quick check on the surface plate shows whether a block is still good for precision setups.

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Step by step

Clean and stone: Wipe the block and stone off burrs lightly.
Check size: Zero an indicator on 1, 2 and 3 in gage block stacks, then compare each dimension of the block.
Check parallelism: Sweep the top with an indicator while the block sits on the plate, for each pair of faces.
Check squareness: Use a master square or an indicator on a height gage up a side face.
Match pairs: Compare a pair of blocks: they should read the same on each face.

Typical specs

Good blocks are about ±0.0002 in on size and square within 0.0002 in.
Mark any block that's out of spec and keep it for rough work.

Worked example: checking squareness and size

Set the block on a surface plate and measure each of its three sizes with a micrometer at four corners. A good set should read 1.0000, 2.0000 and 3.0000 in within about 0.0002 in, and the two blocks of a pair should match each other more closely than that. Then stand the block against a precision square or a cylindrical square and sweep the upright face with a test indicator. Over 3 in of height, 0.0002 in of indicator change means about 0.000067 in per inch of squareness error.

To check parallelism, run a test indicator on a height gage across the top face with the block on the plate: total indicator runout over the face is the parallelism error. Flip the block 90° and repeat for each pair of faces. Burrs around the tapped and through holes are the most common cause of bad readings, so stone every face lightly with a flat hard Arkansas stone before measuring, and wipe the plate and block clean.

Worked example: using a 1-2-3 block as a height stack

Need a 4.000 in setup height? Stand one block on its 3 in side and set a second block on it on its 1 in side: 3.000 + 1.000 = 4.000 in. Two blocks that each read 0.0001 in over size give a stack of 4.0002 in, so errors add. For 5.000 in, use the 3 in and 2 in sizes. Wring or seat the blocks firmly, wipe each face, and check the finished stack with a height gage or against gage blocks before you trust it for a setup.

FAQ

How accurate are 1-2-3 blocks?

Good ones are about ±0.0002 in on size and square within roughly 0.0002 in.

How do I check 1-2-3 block size?

Zero an indicator on gage block stacks of 1, 2 and 3 in and compare. Build any reference size free with the Gage Block Calculator at gageblockcalc.com.

Are matched pairs important?

Yes, for parallel setups the pair must read the same.

How do I fix a burred block?

Stone it lightly. Burrs throw setups off.

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General shop information only — follow your gage manufacturer's instructions and your quality system.