Onguard MastifX4P Quattro Bolt Locking Mechanism locks shackle on four sides Z Cylinder is bump proof and resists picking, pulling or drilling Rounded design and centered keyway eliminates pry points Co molded crossbar cover and double rubber coated shackle protects finishes All metal end caps provide additional protection against hits and drops Automatic keyway covers helps seal lock internals against the elements 10mm (0. 39) titanium reinforced, hardened steel