Like many motorcycle engineers, Turner wanted to build an engine that combined compact size and good balance with ample power. By the end of 1928, he had penned a novel four-cylinder whose bores were arranged square, two-by-two. This formation was essentially two twin engines coupled in parallel, with the whole thing timed by geared flywheels sat in the center of the crankcase.
These flywheels ran in opposite directions, so the gyroscopic effect of their rotation was effectively canceled, aiding smoothness. Turner also arranged the engine’s timing so that its pistons hit top dead center on diagonally opposite sides. Because no pair of cylinders was never allowed to fire on the same side of the engine, forces were evenly distributed front-to-back and left-to-right…