Heads are cast completely around and through two high carbon Roll Pins permanently embedded in the steel handles. No matter how much they're abused, they won't break. Specially designed hexagonal shape prevents rolling and assures safer workouts. Raised silver numbers for easy readability and quick selection. All edges and corners are ground smooth for safe handling. Cast from ASTM grade 20, the highest quality gray iron casting available. 15 to 100 lb. dumbbells feature extra-thick, stress proof, knurled steel alloy handles.