Piston Desk Clock Great desk or mantle piece ornaments for £55.00 A lovely small clock made in a piston crown.The classic clock face is approximately 50 mm in diameter and the black piston is approximately 105 mm across. A Piston clock in its natural state A lovely clock for £55.00 Another lovely small clock made in a piston crown.The classic clock face is approximately 50 mm in diameter and the black piston is approximately 95 mm across.