30a offset from the intake ports 37 by about 22 5 u00b0 the exhaust from exhaust ports 36 communicates with an exhaust manifold not shown before exiting outlet 102 of duct or the shroud 104 in the preferred embodiment of the invention the cylinder 14 including the hub 25 the pistons 16 and the torque tubes 20 24 are formed of 6061 t6 511 aluminum alloy each of the torque tubes 20 and 24 is connected by means of a so called 34 scotch yoke 34 mechanism to a crank throw 38 on a crankshaft 40 rotatably journalled in the crankcase 12 as best shown in fig3 and 4 the scotch yoke mechanism comprises a power arm 41 formed from two mating arm halves 41a and 41b preferably made from the same aluminum alloy as the torque tubes each of the arm halves has a semicircular recess near its upper end as oriented in fig3 and a semi elliptical recess near its lower end the semicircular recess has upper and lower slots 42 as best shown in fig4 each torque tube is provided with a groove that registers with the slots 42 the arm halves are assembled onto the torque tube with the arm slots 42 registering with the torque tube grooves and a key member 46 is inserted between each of the slots and its registering groove to form a 34 woodruff 34 key that firmly secures the arm onto the tube with the tube thereby extending through the circular aperture formed by the registering semicircular recesses of the arm halves the arm.