12 in which portion the cylinder bore hole 10 u2032 is increased in diameter towards the cylinder face wall 12 cylinder section 2 u2032 this portion 10 u2032 of the cylinder bore hole with increasing or increased diameter envelops at least a portion of the forward piston section 303 u2032 of the piston 303 when the piston as represented in fig1 in dashed lines is in its top dead center tdc in the variant of the third embodiment of the piston cylinder unit illustrated in fig1 it is not required that the piston is provided with the piston section 337 with reduced diameter in the portion of its piston face wall though this is not impossible either also in the variant according to fig1 the piston 303 can be provided with a ventilation groove 333 a pressure compensation circumferential groove 335 a piston side fluid bearing forward annular nozzle arrangement 330 u2033 or with combinations thereof as has already been described in conjunction with the first variant of the third embodiment the piston cylinder unit according to the invention and this also applies for all embodiments forms an element of a linear compressor in an advantageous embodiment wherein the compressed fluid is a gas for example air the fluid bearings are thus configured as gas pressure bearings.