Abstract: A housing, having at least a top wall and a spaced parallel bottom wall, has aligned openings in the top and bottom walls and a path extending between the aligned openings. The housing is mounted over a well bore so that the openings are aligned with the well bore. In the housing are a pair of blocks which are pivotable from an open position, in which the path is cleared, to a closed position, in which the blocks extend across the path. Wire line tools are mounted through the housing into the bore and thence into the well. When the tools are raised from the well, the blocks are in their closed position whereby to prevent the tools from falling back into the well should the wire break.