Plug Gage

A plug gage is a fixed-limit inspection tool that verifies the inside diameter of a hole or fit of an internal thread by producing a GO/NO-GO decision rather than a numeric measurement. The GO end represents minimum acceptable size and should enter smoothly; the NO-GO end represents maximum acceptable size and should not enter.

On the shop floor, plug gages are the go-to check for drilled, reamed, bored, or tapped holes in CNC cells and inspection stations. After a critical bore is completed, the operator uses the GO member to confirm the hole is no smaller than minimum acceptable size and the NO-GO member to confirm it is no larger than maximum. The result is an immediate accept/reject decision that supports high-throughput production and removes interpretation errors from micrometers or CMM reports. The 10% rule allocates roughly ten percent of product tolerance to the gage itself, so the inspection tool does not consume too much allowed part variation. GO tolerance is applied plus on the minimum-hole side; NO-GO tolerance is applied minus on the maximum-hole side. This makes plug gages especially valuable for tight-tolerance holes, where fixed-body gages give fast, repeatable checks without rocking behavior.

• ⚠️Wrong tolerance direction on GO/NO-GO members: GO applied minus can accept undersize holes; NO-GO applied plus can accept oversized bores. Gages made in-house without limit-gage logic send bad parts downstream.
• ⚠️Forcing the gage into the hole: GO plugs must enter with light hand pressure. Shoving masks burrs, taper, or undersize conditions, damages the gage, and false-accepts bad holes into downstream operations.
• ⚠️Wrong gage style for thread verification: A plain plug gage checks bore diameter only, not thread pitch diameter, form, or flank condition. Using one on tapped holes lets damaged threads, incomplete taps, or wrong pitch reach final assembly.