The Nr Legacy
THE NR LEGACY
OVAL PISTONS PAST, PRESENT AND FUTURE
KEVIN CAMERON
HONDA'S NR OVAL-PISTON CONCEPT HAS, DESPITE all the development lavished upon it, only one visible heir at the moment: the NR750 collectorbike, intended more to be seen than ridden. The NR is not, however, without an important legacy. NR research began in 1977, at a time when Honda was seeking a fresh engine paradigm to replace the milestone CB750 inline-Four and its twin-cam descendants, the 750F and 900F.
High performance is central to motorcycling, so rpm would continue to rise. Honda’s experience with ultra-revs in GP racing of the 1960s had shown the problems of inlineFours, which vibrate as two rocking 180-degree Twins, imposing heavy bending loads on the crankcase. Honda found that if the cylinder was made separate from the upper crankcase half, this vibratory bending caused scrubbing between cylinder base and crankcase that would destroy any gasket, leading to oil leakage. To stop this, Honda cast the complex, finned cylinder and upper case half of its racing Fours in one piece.
This wasn’t practical for production Fours. Later experience with the twin-cam endurance inline engines and the 750F, no doubt, showed similar case/cylinder flex troubles. Kawasaki, in its air-cooled Superbike engines of the 197882 period, had to use copper base gaskets to keep oil inside. Inline beam bending was a notorious problem.
How should Honda design a new generation of higher-
