When U-bolts are tightened to secure the leaf pack in the vehicle from factory, the metal is stretched as a normal part of this process. It is important that U-bolts be replaced when a suspension system is changed, as a stretched U-bolt has lost its integral strength. The U-bolt will appear tight when re-used, but in fact it isn’t, this puts undue pressure on the centre bolts in the springs, causing them to move or snap which in turn allows the diff to move and the leaves to spread. Tough Dog U-bolts are manufactured using high tensile steel & are designed with the correct length of thread for larger, heavy duty leaf spring packs.