this invention relates to oil well production , particularly to a method and apparatus for downhole stimulation of oil production from a well by generating cyclic shock waves in and around the wellbore . high pressure air passes through gas supply conduit , downwardly through pilot passage of pilot valve into internal piston valve chamber and high pressure chamber . when the pressure in chambers and exceed 80 % of the line pressure from supply conduit , pilot valve shifts vertically , exposing the upper end of piston valve to high pressure , and thereby urging the piston valve downwardly to open the high pressure chambers and into the low pressure chamber , thus creating an abrupt charge of gas into the well . the movement of line pressure through high pressure passage to a position behind the annular shoulder of the piston valve causes the piston valve to return to its start position , and the differential pressure across pilot valve causes it to move back to its start position .