Acme Threaded Collar, Trap Thread, M20x4, Stainless Steel, OD 40mm Trapezoidal threading allows for higher axial holding power than traditional threaded collars. Tightly controlled face to bore perpendicularity is maintained (TIR of Threads are double tapped to ensure a burr-free finish which extends shaft life and allows for easier installation. Ruland clamp style shaft collars do not mar the shaft, are infinitely adjustable, and have superior holding power when compared to set screw collars.