Not to be [persnickity] but it wasn't an [in-house] engine.
It was a Franklin 0-335, modified in house, to be water cooled.
"An air-cooled flat-6 engine, the Franklin O-335 made by Air Cooled Motors (and originally intended for the Bell 47),[22] fit, and its 166 hp (124 kW; 168 PS) pleased...