An apparatus for checking 
diameter and roundness of crankpins ( 42 ) in the course of the 
machining in a 
grinding machine, includes a Vee-shaped 
reference device ( 70 ), a gauging device with a movable feeler ( 67 ) associated with the Vee-shaped device ( 70 ) and a movable support ( 5, 40, 41, 60 ) device for the Vee-shaped 
reference device. The movable support device is coupled ( 5 ) to the 
grinding-wheel slide ( 1 ) and includes, in working condition, two 
parallelogram structures ( 40, 41 ) in series enabling substantially translation displacements of the Vee-shaped device ( 70 ). One of the two 
parallelogram structures ( 40, 41 ) includes an axially movable stem ( 32 ), with end surfaces ( 38, 39 ) that, in working condition, contact the external surface of bearings ( 15, 19 ), thereby setting the distance separating two of the four vertexes ( 11, 7, 14, 18 ) of the 
parallelogram ( 40 ), whereas when the apparatus is in the 
rest position, at least one of said end surfaces ( 38, 39 ) is separated from the surface of the associated bearing. During the checking, the direction of the feeler ( 67 ) displacements remains substantially unaltered.