Park Tool HMR-4 Shop Hammer The Park Tool HMR-4 Shop Hammer was specifically designed with bike work in mind. I t features a fiberglass shaft, soft touch grip (for comfort), and a dual head-steel on one side and a replaceable plastic head on the other side. Features 13. 5" long (345mm). Steel striking face: 1-3/8" (35mm) diameter Plastic face: 1-11/16" (43mm) diameter 24 ounce head (680g) Manufacturer's Part No: HMR-4 UPC: 763477004390 | Park Tool | Hmr-4 Shop Hammer Blue