Speedmaster alloy main girdles are machined from 6061 T6 billet aluminum that creates an additional main web structure to strengthen your block's main webbing. The forces of higher rpm and power-adders can overwhelm main bearing caps and fasteners, causing the caps to walk, fastener threads to fatigue, and your main web to crack! Speedmaster main cap girdles dampen the harmonics that caused the main caps to move in the first place and effectively tie all five main caps together for enough brawn to produce big horsepower capabilities with a stock block!. Speedmaster has designed these main cap girdles to reinforce the alignment of the main caps in high performance applications and counter the limited webbing and weaknesses of those stock blocks. The main girdles are anodized for good looks and durability.