Series II Commercial Grade Urethane Dumbbells by American Barbell offer an exclusive locking design used to permanently attach the solid urethane heads to each solid steel dumbbell handle. A traditional bolt-together dumbbell, such as a Pro-Style dumbbell, uses an internally threaded dumbbell handle shaft that accepts a 5/8" fine thread Allen bolt to lock the individual plates and handles together as one. While the Pro-Style dumbbell is a very strong dumbbell design used in commercial gyms, it is hamstrung by this Allen bolt that can often loosen when the dumbbells are dropped. To solve this problem American Barbell reversed the design of the Pro-Style dumbbell therefore threading the dumbbell handle shaft itself with a course thread and then giving it a massive steel locking nut that will not back off even under the most severe impact. The solid heads and entire dumbbell itself stays locked together and secure removing any liability issues. American Barbell Series II Urethane Dumbbells were originally designed and distributed by GP Industries as the GP Series II Uni-Lock Dumbbell (DBS2), and continue to be produced today under the American Barbell brand name with the same industry-respected engineering and commercial quality. The Series II Dumbbell has a black smooth surface urethane head with permanently molded and easy to read number increments in gray. This dumbbell has a solid steel core, high-strength steel hard chrome plated handle, and a premium-grade German urethane head coating that is 90 times stronger than rubber. American Barbell premium urethane is super tough and is bonded to the steel core unlike the majority of dumbbells available today. Urethane will never dry rot and is odorless. This dumbbell is made to withstand severe training environments. 5-year commercial warranty against factory defects.