← AskMachinist · Guides · Calculator · 🔎 Search

How to Use 1-2-3 Blocks for Setups

Guide · Updated October 28, 2026

1-2-3 blocks are hardened, ground blocks 1 × 2 × 3 in, usually sold in matched pairs. They give three exact heights, square corners and threaded or clearance holes, so they double as parallels, angle plates and height references.

Check the blocks

Check the blocks against gage blocks before trusting a setup. The free Gage Block Calculator builds the stack.

Step by step

Check the pair: Match a pair before use; see how to check a 1-2-3 block.
Clean: Wipe the blocks and the table or vise.
Choose the height: Lay the block on its 3 × 2 side for 1 in, on 3 × 1 for 2 in, or stand on end for 3 in.
Use as parallels: Put a pair under the part in the vise; tap down until they will not slide.
Square to the table: Stand a block on the table and clamp a part to its face for a quick 90° setup.
Use the holes: Bolt blocks together or to the part through the clearance holes; threaded holes take screws.
Store oiled: Lightly oil and keep the pair together.

Worked example: building a height

You need the part top 4.250 in above the table. A block on end gives 3.000 in; add a gage block stack of 1.250 in (1.000 + 0.250) for 4.250 in. Or lay one block for 1.000 in and stack another on end for 1.000 + 3.000 = 4.000 in, then add a 0.250 in block. Fewer interfaces give less error: each wrung or seated face adds a little.

How accurate are they?

Better 1-2-3 blocks are ground to about ±0.0002 in on size with square and parallel faces in the same range; cheap ones can be several tenths out. Treat them as setup tools, not gage blocks, unless you have measured them.

Rigid setups

Bolt two blocks together through the holes to make an L or a wider base. Do not hammer clamping screws into tapped holes; strip the threads and the block is scrap.

Indicating from a block

Clamp a block to the table with its 3 in face along X and indicate it straight: it becomes a quick fence for repeat parts.

Common problems

Part rocks: burr on a block or the part. Height off by 0.001 in: chip under the block. Block magnetized after grinding: demagnetize so chips do not cling.

Tips

Worked example: building heights from 1-2-3 blocks

A matched pair of 1-2-3 blocks gives 1.000, 2.000 and 3.000 in directly, and stacked combinations give 4.000 in (1 + 3), 5.000 in (2 + 3) and 6.000 in (3 + 3). For a 5.250 in setup, stand one block on its 2 in face over a 3 in block and add a 0.250 in gage block.

Check a pair by standing both on a surface plate and running an indicator across them; good blocks match within about 0.0002 in. Use the tapped holes to bolt blocks together for an L or a step instead of clamping across ground faces.

FAQ

What sizes are 1-2-3 blocks?

1 × 2 × 3 in.

Can I use them as parallels?

Yes, as a matched pair.

How accurate?

Good ones about ±0.0002 in; check yours.

What are the holes for?

Bolting blocks together or to the part.

Can they replace gage blocks?

Not for inspection; they are setup tools.

Related guides and calculators

Build your stack in seconds

The free Gage Block Calculator finds the fewest-block combination for inch and metric sets, with wear blocks. No login.

Questions? Ask a machinist.

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