A bore concentricity gage having a collet at one end adapted to be expanded into close fitting engagement with one of two axially spaced bores on a workpiece and a hollow concentricity plug at its opposite end adapted to be received in close fitting engagement with the other bore. The plug has an accurately concentric bore therein adapted to be engaged by a rotatable "go" bushing journalled on a mandrel having an expander for the collet thereon.
