Abstract: Grinder apparatus, especially for simultaneously grinding two axially-aligned, but radially and longitudinally spaced annular valve seat surfaces, that includes a rotatable shaft, a plate, rotating on the same axis of rotation as the rotatable shaft, disposed about one end of the rotatable shaft and extending laterally outwardly from the rotatable shaft, two grinders rotatably mounted on the plate on axes parallel with, but spaced at different distances from the axes of rotation of the rotatable shaft and the rotatable plate, the end surfaces of the grinders being disposed at different distances from the outer face of the plate, a pair of motors for separately rotating the rotatable shaft and the plate and a drive for transmitting rotational movement of the rotatable shaft to the grinders mounted on the plate, such that one of the grinders engages one of the annular valve seat surfaces during the grinding operation and the other grinder engages the other of the annular valve seat surfaces during the grinding