I'm not real familiar with the early Willys stuff so I will go with Jeep expert Jim Allen's explanation.....
"Roos� reliability benchmark was 100 hours at full power. One of his first tasks was to strap the 48hp engine to a dyno. It lasted 22 hours at 3,400 rpm��nuff said. Roos and his staff set to modernizing the engine, incorporating insert bearings, a fully counterbalanced crankshaft, aluminum pistons, a fully pressurized lubrication system, and a revised valvetrain. In just a few months, the new engine was ready for the dyno. It exceeded 100 hours at over 60 hp at a whopping 4,400 rpm. Success! The new engine was dubbed Go-Devil and first appeared in some of the 1939 Willys cars rated at 61 hp at 3600 rpm. In 1940, when the stylish new Willys 440 (4-cylinder, 1940) models appeared, they were all powered by the Go-Devil, as were the �41 441 and �42 442."