The Hien's engine was a licence-built Daimler-Benz DB601, but it was too sophisticated for the state of the 1940s Japanese industry. Both the Ha-40 and its derivative the Ha-140 were unreliable. Professor Suzuki devotes several chapters of his book "The Romance of Engines" to the Ha-40; Kawasaki compiled a report of their investigation into the Ha-40's problems during the war and Prof. Suzuki was fortunate enough to get his hands on a copy. One of the causes was incorrect heat treatment of the crankshaft, resulting in a troostite rather than martensite structure. Another was that Kawasaki couldn't achieve the surface roughness required for the crankshaft's roller bearings. Kawasaki tried to get a licence for the Bosch direct fuel injection equipment but failed, so a Mitsubishi system was fitted instead which had its own set of problems. Aichi also obtained a licence for the DB601 but its version, known as the Atsuta, was just as bad as the Ha-40. In the end the Ha-(1)40 engine plant was destroyed by a B-29 attack leaving Kawasaki with a pile of Ki-61 airframes without engines, so the engineers were forced to fit a Mitsubishi radial engine. The conversion was extremely successful and the "new" aircraft became one of Japan's best fighters of WW2, the Ki-100.
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