P'art of the day: Honda NR750 Oval Piston

Todays p'art was developed for two wheels instead of four.

The oval piston was developed by Honda and first appeared on the NR500 in 1979. Ever the innovators, Honda were determined that 4-stroke engines could compete with the dominating 2-strokes. The Gran Prix bikes were limited to 4 cylinders so Honda used oval pistons so they could use more valves. Each cylinder had 8 valves and each piston had 2 conrods, it was effectively a V8 disguised as a V4.

The manufacturing processes necessary to maintain adequate cylinder sealing were extremely challenging to develop. Unlike normal pistons that can be turned on a lathe to achieve exact tolerances, the oval pistons had to be painstakingly machined. The piston rings were very challenging to produce for similar reasons.

After years of exhaustive development the engine finally developed the target figures. 130bhp at 19500RPM were unheard of for a naturally aspirated 4-stroke 500. Unfortunately due to weight issues the 4-stroke NR500 was not competitive, its only victory being the 500kms of Suzuka in 1981.

However all was not in vain. Honda amassed a vast cache of knowledge about unique manufacturing problems and 4-stroke performance from the project. These days 2-stroke engines are all but obsolete, due to emissions, economy and reliability issues, a testament to Honda's foresight. The Oval Piston engine was eventually used in the NR 750 of 1992. Their determination to pioneer new technologies meant that Honda remain at the forefront of naturally aspirated engine design, on two wheels and four, on road and track. 

If you want to be the best you have to take chances, Toshimitsu Yoshimura, an engineer who worked on the original project summed this up well by saying, "When I look back on it, I'm not sure if we were experimenting with cutting edge technologies or obsessed with foolish ideas".