A tire building machine for marking the centerline of a continuous piece of plystock is described as having, (I) a carriage which is movable along a trackway which is normal to the direction of travel of the piece of plystock being marked, (II) a pair of sensors carried by the carriage for monitoring the opposing marginal edges of the piece of plystock, (III) a device centrally disposed on the carriage between the sensors for marking the centerline of the plystock, (IV) a mechanism for adjusting the spacing between the sensors when the width of the plystock varies, and (V) a mechanism for moving the carriage and attached sensors, in unison, along the trackway, when the piece of plystock wanders from the pathway which the sensors are initially set to monitor. Also disclosed, is a mechanism for covering each of the marginal edges of the piece of plystock with a pair of unvulcanized rubbery gum strips, and a mechanism for monitoring movement of the strips into contact with the piece of material and adjusting such movement when required.