A prepaid communications system includes a wireless call handling device for receiving wireless calls from wireless customers, a packet data serving node for receiving wireless packet data calls from the wireless call handling device and transmitting the calls to a communications network and an administration system in communication with the packet data serving node. An adjustment member is provided for mounting each lifter to a shaft, wherein the adjustment member is eccentric relative to the shaft, such that rotation of the adjustment member modifies the position of the lifter and in turn modifies the valve clearance between the lifter and the valve. A resilient stopper at the opposite end is arranged to contact the engine casing if the displacement of the displacer exceeds a predetermined limit. A material property matrix k is then calculated based on the relationship {f}=k{x}. When a mating plug is inserted into the cavity, the leading edge of the mating plug contacts the more forwardly-positioned sliding surface before reaching the stop surface. A light source and a method of forming a light system are further included.