Over time the valverain on the 5.4 & 4.6 3v Ford engines defelope a knocking noise. This noise is commly caused by the cam phaser moving back & forth. Eventually this will set a CEL(check engine light) or even set the engine in LIMP mode. The most common repair is replaceing the cam phaser, however that wont fix the design flaw with them. Limiting the movement of the cam phasers will elimiate the lash between the phaser and prevent future damage or failure. 

These cam phaser limiters are designed to allow the use of aftermarket springs and camshafts while still retaining the stock phaser. The limiters are two piece anodized billet aluminum and designed to fit within the stock phaser. These limiters are designed to limit the total travel of the cam phaser. They eliminate the durability issues associated with the phasers and allow the use of larger aftermarket camshafts and springs.