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How to Measure a Groove or Recess with Gage Blocks
Guide · Updated October 27, 2026
Groove and slot widths are awkward to mic. A gage block stack makes an exact plug gage: if the go size slips in and the no-go size won't, the slot is in tolerance.
Need an exact reference size?
Build any gage block stack with the free Gage Block Calculator. Free, no login.
Step by step
- Build go and no-go stacks: Use the minimum and maximum slot widths. The free Gage Block Calculator finds the stacks.
- Clean the slot: Clean the slot of chips and burrs.
- Try the go stack: It should slide in with light contact.
- Try the no-go stack: It should not enter.
- Check depth: Set an indicator on a block stack at the depth, then compare the groove floor.
Tips
- Use wear blocks on the outside of the stack for repeated checks.
- Don't force the stack. Forcing it scratches the blocks.
- For bigger grooves, use stacks in a holder with jaws.
Related: wear blocks, go/no-go gaging.
Worked example: building a 0.3875 in groove check stack
A groove is drawn at 0.3875 in wide. Using an 81-piece inch set, take the last digit first: 0.1005 in, leaving 0.2870; then 0.137, leaving 0.150; then 0.150. Stack 0.1005 + 0.137 + 0.150 = 0.3875 in. If the stack enters the groove with a slight drag and a 0.3880 in stack (swap 0.1005 for 0.101) does not, the groove is between 0.3875 and 0.3880 in.
Keep the stack square to the groove walls; a tilt reads the groove smaller than it is. Wipe the groove clean because a single chip can hold the stack out.
Second worked example: a 0.3753 in groove stack
To check a groove nominally 0.3753 in wide with an 81-piece inch set, take the tenths block first: 0.1003 in leaves 0.2750 in. Then 0.125 in leaves 0.150 in, which is a single block. Stack: 0.1003 + 0.125 + 0.150 = 0.3753 in, three blocks.
Build go and no-go stacks a tenth or two either side, for example 0.3751 in (0.1001 + 0.125 + 0.150) and 0.3755 in (0.1005 + 0.125 + 0.150). The go stack should slip in with light drag and the no-go should not enter. Let blocks and part reach the same temperature before judging tenths.
Quick shop check
Common problems: wringing blocks with dirty faces. Wipe each face with a clean cloth before wringing.
FAQ
Can gage blocks measure a slot?
Yes, as go/no-go plugs at the minimum and maximum widths.
What about groove depth?
Zero an indicator on a gage block stack at the nominal depth and compare.
Will slot checks wear my blocks?
Somewhat. Use carbide or steel wear blocks on the outside of the stack.
How do I find the stack?
Enter the size in the free calculator at gageblockcalc.com.
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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 gage manufacturer's instructions and your quality system.