For providing cam timing, a sprocket wheel housed in a timing cover is attached to a cam shaft by means of an intermediate adjusting arm placed in a recess of the sprocket wheel. The adjusting arm is rigidly fixed to the cam shaft and moveably secured to the sprocket wheel. Adjustment of the relative angular positions of the sprocket wheel and the cam shaft is achieved by repositioning a screw in the adjusting arm which is inclined to the face of the sprocket wheel so as to facilitate its rotation without removal of the timing cover.