outwardly from crankcase 15 in the embodiment of fig1 distal extremities 37 and 38 are adapted by means of machined threads to accept positioning means in the form of knurled and threaded knobs 60 and 61 respectively a side support plate 39 has flat front and rear surfaces 40 and 41 respectively and a circular opening 42 communicating between said front and rear surfaces rear surface 41 is adapted to abut against the gasket receiving surface 16 of the crankcase clamping means in the form of split flange 43 having tightening screw 59 is attached to front surface 40 the clamping means has a circular opening 44 coaxial and condiametrical with circular opening 42 both openings accommodating shaft 33 clamping means 43 secures front shaft 33 when tightened by means of allen screw 59 thereby preventing movement of the shaft and bearing block 67 transverse relief slot 64 partially penetrates the diameter of clamping means 43 and longitudinal relief slot 65 penetrates the wall of clamping means 43 in order to facilitate clamping centering plug 45 has upper and lower surfaces 46 and 47 respectively and an annular abutment shoulder 48 upwardly recessed from said lower surface 47 thereby defining upper and lower cylindric sectors 49 and 50 respectively upper sector 49 extends upwardly from shoulder 48 to upper surface 46 lower sector 50 extend downwardly from shoulder 48 to lower surface 47 the lower sector is adapted for insertion into cylinder 12 in close circumferential conformity with the walls of the cylinder abutment shoulder 48 is adapted to define a lower.