the upper end of the hammer piston 20 and exerts a downward force on the hammer 20 sufficient to dislodge the collet arms 26 from the abutment shoulder 10e and permit the hammer 20 to drive downwardly to impact the firing pin 24 fig2 d and thus effect the detonation of the primer 14 it should be noted that the establishment of a fluid pressure differential below the packer between the casing annulus and the region below hammer 20 will also release the hammer due to the provision of a radial port 10f in hammer housing 10 in the event that for any reason the impact force produced by the release of hammer 20 is insufficient to produce detonation of primer 14 the construction of this invention permits the hammer to be retracted to its original elevated position and released again such retraction is produced by releasing the fluid pressure from the casing annulus and applying a fluid pressure to the bore of the tubular work string 3 such fluid pressure passes downwardly into the bore 5c of the crossover tool 5 it cannot pass through the bore 5c due to the provision of the temporary blanking plug 7 it therefore flows outwardly through the radial ports 5b and into the annulus between the crossover tool 5 and the casing 2 but at a point below the set packer 2 the fluid flow down the lower casing annulus flows through the radial port 10f provided in the wall of the hammer housing 10 and impinges upon the bottom.