Abstract: A gauge for use in connection with the timing of a diesel engine and adapted to measure the spacing between an injector body surface and a surface of the plunger of the injector, the surfaces being parallel, the gauge consisting of a gauge body restable on the plunger surface and a gauge stem restable on the body surface and extending slidably through the gauge body parallel to the direction of plunger movement, the stem having a planar surface normal to its axis which will be moved into coplanar relation to a corresponding planar surface of the gauge body when the spacing being measured is correctly adjusted, the coplanar relation of these surfaces, or any lack thereof, being readily detectable by touch when passing a fingertip thereover.