The Concept: An OHV + Inclined Cylinder that Changed the World of General Purpose Engines
Two engines were developed under the ZE01 project heading - one with 3.5 horsepower and another with 5 horsepower - and each was to share parts with the other. Yamaguchi, now the project leader, wanted to reflect in the new engines everything he had learned from his own experience. He understood what the ME had failed to achieve, but he understood equally well the weakness of engines from other manufacturers. Further, he knew why the customers had not been satisfied. Therefore, he was confident the new OHV engines would solve the problems inherent in the previous models. He was willing to introduce various ideas, as long as they did not raise the cost, and in this task he applied himself through long hours at the drawing board.
Yamaguchi's initial challenge was engine size, since with a general purpose engine the fuel tank is normally found on top. However, with an SV engine the tank can be mounted directly to the body. Moreover, an OHV engine has its cylinder head and valves positioned between the body and tank, thus increasing the height of the engine assembly. Increased height means a higher center of gravity and more significant amounts of vibration. It also affects engine manageability when installed in a work machine. Detachment of the fuel tank requires an extra function in order to feed fuel to the engine. Thus, the tank should remain connected to the engine, ideally above the body. However, installation compatibility was the most important issue of all, and most companies were still loyal to SV engines. No matter how good the performance was, no OEM company would adopt an OHV engine if it meant a change in specifications. The Honda engine, which lacked installation compatibility, had already made that clear.
Yamaguchi solved this problem with the help of the ME engine. First, he drew an outline of an ME, then he tried to fit the OHV engine into it. After all, the ME's dimensions had been determined through intensive studies of installation compatibility. Thus, an OHV that could fit within the ME's dimensions would ensure installation compatibility. After drawing many designs with different cylinder positions, Yamaguchi finally came up with the idea of setting the cylinder at an incline.
The idea was not so convincing to other staff members, though. In fact, the ZE engine would not be the first Honda engine for which an inclined cylinder had been attempted. The G25 engine released in 1966 had adopted this design with a cylinder placed almost horizontally. However, that engine design was fraught with problems, including heat damage. The staff, who remembered the experience, was very skeptical. "We've already had enough anguish," they said. "Why the inclined cylinder now? What's wrong with the vertical layout?'
Yamaguchi was in fact involved with development of the G25 engine, so he knew well the struggles it had produced. "There was no meaning in developing OHV engines that couldn't promise installation compatibility," Yamaguchi recalled. "If the ideal design for a general purpose engine means an inclined cylinder, we must face the obstacles and overcome them. We had already had the facts of engineering. We couldn't afford to hesitate, now. We had to make the best use of that experience."
A series of long discussions took place, and through Yamaguchi's articulate passion, a consensus was finally found. Following the decision, the staff members visited farms in mountain areas in order to study how cultivators and other machines were used on the slopes and apply that data to laboratory tests. Consequently, the angle was set to 25 degrees to prevent the backflow of oil. With that, the inclined cylinder's specifications were at last determined.
A local market-compatibility test for the ZE Series model CX140, held in Thailand (Photograph on those days courtesy of Yoshinobu Yamaguchi)