The 1937 models of Harley-Davidson, in order from top to bottom-45 Twin, 74 Twin, 80 Twin and 61 o.h.v. Twin. Some of the outstanding features on these models are: circulating pressure oiling system; double loop trussed frame on 61, 74 and 80; built-in instrument panel; welded steel tanks; speedometers driven by spiral gears in the gear box on the 61, 74 and 80; four speeds forward on 61, 74 and 80 with constant mesh design transmission with self-locking shifting clutch; and external ignition timer. The motors are air-cooled, four-stroke cycle twin cylinders, with side by side valves, Ricardo-type removable heads on 45, 74 and 80 Twins. Overhead valves in removable cylinder heads on 61 cu. in. Aluminum alloy, cam-ground, double-slot pistons and mirror-honed cylinders on all models. All main bearings roller bearings. Fully enclosed and lubricated valve gear. Chrome-iron seat inserts on intake valves and silicon-chrome-molybdenum seat inserts on exhaust valves on 61 cu. in. Extra deep cooling fins on all models