has radial inner teeth 24 to be engaged into matching seats 25 obtained in the base of piston head 22 the outer surface of ring 23 may be smooth like most known rings or it may have a groove 26 which in this example has a fret design as can be seen in the drawings but may also have an annular or a different profile radial apertures 29 in the wall 21 align with seats 9 when the piston is mounted on support 2 thus allowing for the insertion of keys 10 the latter lock wall 21 to support body 5 preventing it from turning or moving axially clamping the piston by means of keys is the preferred solution of the invention because the piston is locked securely to support 2 both rotationally and translationally however this is not the only feasible method for example a conceivable alternative may be a traditional threaded system allowing the piston to be screwed onto piston body 5 or else a bayonet type system both of which are known in the art for cooling piston 20 channels 30 are obtained in cylindrical wall 21 and extend parallel to one another along the wall generatrices between an annular distribution chamber 32 encircling the front end of support body 5 and an annular collection chamber 33 the collection chamber is arranged at the wall base.