An improved valve assembly (100) includes a primary spring (142) which extends along a valve stem (144). A spring retainer (150) holds the primary spring (142) on the valve stem (144) and has a thread convolution (152) which engages a thread convolution (154) on the valve stem (144). The spring retainer (150) has a force transmitting surface (182) which engages a reference surface (184) on the valve stem (144) with a retainer preload force which is at least as great as a primary spring preload force. Shims (158–162) are provided between the primary spring (142) and the spring retainer (150) to enable a desired primary spring preload force to be obtained.