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Recent studies have confirmed that it is possible to achieve perfect balance of all forces and moments created by pistons, connecting rods and crankshaft of the Collins family of 12, 8, 6 and 4 cylinder engines. With the Scotch Yoke mechanism there are no secondary forces, or higher order forces, and thus the counter-balancing needed can be determined exactly. The pure sinusoidal, translational motion of the pistons and connecting rods and the pure rotational motion of the crankshaft and bearing blocks makes the calculation for balance “trivial”. In all cases, except the 12 cylinder model, a single balance shaft rotating counter to the crankshaft rotaion, at engine speed provides complete cancellation of the moments created by the moving parts of the engine. Alternatively, two small balance shafts can be used, located in less space sensitive spots than the single balance shaft. In the case of the twelve cylinder engine, no balance shafts are required.