114 may be mounted either on arm members 58 or links 60 when it may necessary to measure the radius of a corner 116 finally at least one arm 58 may be replaced by an inspection device not shown which is mounted on the shaft 68 attention is now directed to fig1 in which an alternate form of the positioner 56 a is shown positioner 56 a includes a central hub assembly 85 comprising a central shaft 68 having a support 64 attached to one end thereof a cylindrical support 84 may slide on the opposite end of the shaft 68 the shaft 68 may be connected to a pneumatic cylinder assembly 82 which linearly displaces the shaft 68 causing the latter to slide through support 84 linear displacement of shaft 68 likewise displaces support 64 connected between support 64 and support 84 and 86 may be four pairs 87 of arms 88 extending diagonally from shaft 68 and pivotally connected at their medial sections at a pivot point 100 the outer ends of arms 88 are connected by links 90 having rollers 62 on its opposite ends the links 90 each include a slot 94 receiving a guide pin 96 on the outer end of one of the arms 88 in effect the slot 94 and pin 96 form a lost motion mechanism so that the wheel 62 on each link 90 remains in contact with a cavity wall 22 24 26 as transporter 32 traverses the cavity 28 regardless of changes in the distance between the walls in use the pneumatic cylinder 82 applies a retraction force to shaft 68 thereby biasing support 64 to move toward the right as viewed in fig1 as support 64 retracts toward support 84 the outer ends of.