A steam sterilizer having a door movable relative to an opening between a first open position and a second closed position. A plurality of spaced-apart roller assemblies are aligned along edges of the door to align the door relative to the opening as the door moves between the open and closed position. Each of the roller assemblies are comprised of a cylindrical roller having an outer annual recess extending along the periphery thereof. The recess is dimensioned to receive a lateral edge of the door. The roller is mounted on a shaft and is movable against a biasing force axially along the axis of the shaft.