Honda retired from top flight world championship GP racing at the end of the 1967 season and they didn't return until the early 1980s with Mick Grant on the oval-piston four stroke NR500 which was an unmitigated disaster. However the world no. 1 bike maker finally did what was unthinkable - build a two-stroke 500cc machine on which Freddie Spencer finally won Honda's first 500cc rider's crown. To Yamaha however went the honour of not only bagging the first ever 500cc rider's world championship title - the first Japanese bike maker to achieve this, but also being the first in the history of the sport to do it on a two-stroke machine! The seeds for Yamaha's move into the premier blue riband class of bike sport were sown when engineers decided to couple two of the firm's liquid-cooled, twin-cylinder TZ250 motors to come up with a 500cc four. This engine was a simple (if at all it could be referred as such then!) piston-ported design with a single petal reed valve. The first 500 engine was run in July 1972, and the machine, model number OW20, was tested in December by old Yamaha hand Motohashi. Designed by chief designers Naito and Matsui, the two-stroke four produced 80bhp at 10,000rpm and could reach 165mph.