A retaining assembly for coupling a fluid end component to a fluid end casing. The retaining assembly includes a pair for clamp members, each of which has a pair of rails that define a wedge-shaped groove therebetween. The groove applying an increasing amount of pre-load force or tension to a flange of the fluid end components and a flange of the fluid end casing to drive them together. The amount of pre-load tension applied can vary and can be determined by the user and amount of tightening of the clamp members.