Honda NR (RC40)
Honda made four pistons behave a little like eight, packing thirty-two valves into one of the most unusual road-bike engines ever sold.
Honda NR750 (RC40) (52647347730).jpg · Michael Gaylard from Horsham, UK · CC BY 2.0
More than a set of wheels.
Honda's oval-piston experiment began with the 1979 NR500 race project. Fourteen years of work eventually produced the road-going NR, launched in Japan in May 1992 for 5.2 million yen. The RC40 combined oval pistons, eight valves and two spark plugs per cylinder with electronic fuel injection, carbon-fibre bodywork and a polished aluminium frame.
Look a little closer.
Honda's Japanese launch sheet gives the piston three measurements: 101.2 mm across its long side, 50.6 mm across its short side and a 42.0 mm stroke.
Photos show an example of the model or generation. The pictured vehicle’s year, equipment, or modifications can differ from the specification described here.
Why Honda invented the oval piston
Grand Prix rules limited the NR500 to four combustion chambers, while rival two-strokes could make power at very high speed. Honda stretched each piston so it could fit twice as many valves and two connecting rods, chasing the breathing area of a V8 while remaining a V4 under the rules. The racing project struggled, but its back-torque limiter, materials work and engineering lessons reached later production machines.
Source: Honda Global · The Oval Piston Engine ↗A showcase, not just an engine
Honda surrounded the oval-piston V4 with CFRP fairings, titanium connecting rods, magnesium wheels, an inverted fork and a single-sided Pro-Arm swingarm. PGM-FI used eight throttle bores and eight injectors. The point was to demonstrate a complete next-generation sports motorcycle, not merely to sell an exotic cylinder shape.
Source: Honda · NR Japanese launch release, 6 April 1992 ↗Where these facts come from
- Honda · NR Japanese launch release, 6 April 1992
- Honda Global · The Oval Piston Engine
- Honda Global · 75 Years of Honda History: NR
Selected facts checked for this edition. Some source pages link to broader model families. Last reviewed October 1, 2026.