tool of any other suitable shape having a lateral size greater than that of the aperture 82 of the tool retention member 80 the tool retention member 80 can be removed from the housing 11 as described below to allow a variety of work tools 20 with different configurations of the tip 46 to be attached to the tool assembly 10 as the piston 18 strikes the work tool 20 the force of the piston 18 is transmitted through the work tool 20 to the tip 46 in the general direction of arrow 22 moreover this force is applied to a hard object such as rock concrete or asphalt in order to break up the hard object the upper and lower housing members 12 14 are connected by a housing engaging structure 50 in the illustrated embodiment the housing engaging structure 50 includes for example four lugs 52 provided equidistantly about the circumference of a plug portion 54 of the upper housing member 12 these are shaped such that as the plug portion 54 is inserted into the lower housing member 14 the lugs 52 can pass between four projections not shown for example formed on the internal wall of the socket portion 56 the upper housing member 12 is then rotated by for example 45 degrees relative to the lower housing member 14 so that the lugs 52 engage beneath the projections 58 prestressing structure in the form of four jacking.