Abstract: A fixture for testing and servicing a MSIV of a nuclear power generation plant includes a cover plate for bolting to the valve body and a support member extending from the plate for supporting the valve stem. A pressure cylinder attaches to the support member for loading the stem to conduct a leak rate test. Means are provided for pressurizing the valve body to perform a leakage test. Stem bearings spaced apart along the stem provide accurate stem alignment, and prevent bending of the stem under test loading. Seals between the fixture, the body and the stem permit pressurization of the valve body above the poppet, permitting a leak test of the MSIV without reassembly of the actuator structure. In another embodiment, a collar having an axially directed thrust face mounts on the stem and bears against a thrust bearing to permit rotation of the stem during assembly.