wall of the cyliner may contract and be compressed in the recessed portions which absorb them further the piston presents at least one cavity opening on the periphery of the first bead of the piston and at least one mobile piece partially occupying this cavity and which moves therein due to the movement of the piston to prevent without other deterioration any considerable deposits of calamine application_number US 89120078 A description referring now to the drawings fig1 and 2 partly show a piston 1 according to the invention after a long running in period and at an average operating temperature this piston is at the end of compression 1 cm from its top dead centre this piston is represented by a part of its first or upper bead 3 and it is animated by a reciprocating movement inside a cylinder 2 shown partly beneath the first upper bead 3 is partly shown the first ring 4 of the piston this ring being housed in a transverse annular groove 5 according to the invention the piston 1 presents on its periphery alternating projecting portions 6 and recessed portions 7 portions 6 may be constituted by threads of trapezoidal cross section separated by triangular sectioned grooves 7 constituting the recessed portions calamine is usually deposited in these grooves indicated by dots in the figure the threads 6 and grooves 7 may be obtained by a.