I have a Clark manhole cover in the patio with a recessed lid to take the patio slabs. The retractable handles are covered by a plastic strip held down by two screws. Problem was the screws rotated but did not unscrew. It felt like the threaded inserts were turning in the plastic moulding. Soaked them in plus-gas for a few days but finally drilled the heads off the screws. Was suprised to find the manhole still held fast. Does anyone know if the whole screw needs drilling out to release the lid?