a different length in order to obtain a different stroke length of the piston 40 as shown in fig4 a cylinder 30 u2032 is provided that has the same diameter as the cylinder 30 but is of a longer length the cylinder 30 u2032 can easily and quickly be substituted for the cylinder 30 by removing the manifolds 20 and 24 from the heads 12 and 14 and removing the cylinder 30 from the heads the cylinder 30 u2032 is then connected to the heads 12 and 14 in the manner discussed above and the manifolds 20 and 24 are connected back to the heads 12 and 14 in the latter contest the lengths of the manifolds 20 and 24 will have to be increased to accommodate the increased length cylinder 30 u2032 of course in the event the cylinder 30 becomes damaged or wears out it can easily be replaced with a new cylinder in the same manner as discussed above the embodiment of fig5 is similar to that of fig1 4 with the exception that the manifold 20 has been replaced with two inlet ducts 50 and 52 having heads 50 a and 52 a respectively bolted to the upper surfaces of the end heads 12 and 14 respectively as viewed in fig1 the ducts 50 and 52 register with two inlet openings formed in the upper surfaces of the heads 12 and 14 respectively to introduce gas into the heads in the same manner as described in connection with the embodiment of fig1 4 similarly the manifold 24 has been replaced with two outlet ducts 54 and 56 having heads 54 a and 56 a respectively bolted to the lower surfaces of the end heads 12 and 14 the ducts 54 and 56 register.