As early as 1968, a V-four was considered as the way forward for the larger capacity Yamaha, the 500cc YZ648A intended for the GP classes and the 750cc YZ648 intended to compete in the USA. During 1970 the decision to make all engines as inline fours was taken, this meant a reduction in development and production costs as to compete in the US the 750 had to be produced in far larger numbers effectively making them production models rather than pure prototypes. First designed in 1971, the Yamaha OW20, broke the mould for the, until then, largely four stroke dominated premier 500 GP class. By clever design the inline engine was kept as narrow as a V-Four and remained very well balanced throughout its rev range, In the early part of 1972 this radical engine was fully developed and by September that same year a complete machine was up and running.
The very first race bike was a piston port design and initially deemed too peaky by the intended GP rider Jarno Saarinen. By the time of the first race of the 73 season the OW20 had sprouted reed valve induction, making the power delivery that little bit more controllable but at the expense of outright power. The new Yamaha looked stunning for its time, resplendent in the company white and red livery, armed with twin discs up front and a single disc rear.
The main opposition for that first season came from Giacomo Agostini, Phil Read and the factory MV, who having won the title for the previous fifteen years mostly unchallenged, would appear to have been caught napping by Yamaha’s secrecy. The Italian team taking their place on the grid armed with an outdated, over weight, three-cylinder, drum braked machine. The MV did produce more power, around 12bhp more than the Yamaha but the latter’s overall package was far superior. The MV team could only sit back and watch OW20 mounted Saarinen storm to an inaugural win, 16 seconds ahead of second place Phil Read. This success continued for Yamaha for the first few GP’s and it looked certain that Jarno would add a 500 title to his 250 crown of 1972, that was until the untimely death of Saarinen and Renso Pasolini at the Italian Monza circuit. Yamaha withdrew their team immediately and returned to Japan to retire for the rest of the season.
Yamaha returned the following season with a package that shocked the world just as much as when the OW20 first took to the track the season before. The current 500 world champ, and the darling of the legions of Italian fans, Giacomo Agostini had left MV through the winter of 73 to join the Yamaha factory, once again in complete secrecy supposedly for a figure of £55000.
The 1974 bike for the start of the season was the OW20, essentially the same as the previous year but by mid season Yamaha had given Agostini the OW23, a special lightweight version with magnesium cases and an overall reduction in size. It tipped the scales at 133kgs some 22kgs lighter than the OW20. It was not enough however and a combination of unreliability and sheer bad luck on Yamaha’s part awarded MV, with Ago’s bitter rival Phil Read on board, the title for 74.
For 1975 the basis of the lightweight OW23 remained but with a few changes and emerged as the OW26. The engine was completely redesigned to facilitate the fitting of a cassette gearbox, this feature was essential when considering the variety of circuits on the GP calendar and the narrow power band of the OW. The mechanics had to perform a complete engine strip prior to this development and for the 75 season onward all they had to do was remove the clutch and the whole gear cluster was easily at hand. The latest modifications were sufficient to hold off Read and a very highly developed MV four, at last the prestigious 500 title was finally Yamaha’s and firmly back in Ago’s grasp.
The official Yamaha team sat it out for the 1976 season returning to Japan to develop the next phase of the OW’s evolution although some of the old machines were allowed out into the hands of south American 350 world champ Johnny Cecotto. So bad was the support that he ran out of spares for the 500 by mid season and decided, reluctantly, to concentrate on the 350 and 750 classes.
Throughout the 500’s long, and reasonably successful, history the engine configuration changed many times, often without the knowledge of the worlds press. The bore and stroke fluctuated between a square 54mm x 54mm to the over square yet revvy 56mm x 50.6mm as did the inlet porting from straight piston control to reed valve and back again. The chassis also went through many changes and lost the twin shock arrangement in 1974 in favour of the “Monocross” system that the Yamaha PR men were keen to impress upon the buying public.
One thing that was not made public at the time was the early OW’s incredible thirst, so much so that quite a few GP’s were severely compromised by the need for fuel stops and at the 1974 Italian GP Ago actually ran out of fuel on the final lap while holding a massive lead. Later in 74 saw a new style aerodynamic seat that set the style for others to follow to this day, in actual fact it was an extra fuel tank enabling the OW to complete the long GP race distances of the time.
The OW’s also went through several permutations of barrel design initially there were two blocks of two cylinders just like the 250/350 and also the mighty 750. By 1977 and the version ridden by Steve Baker and Johnny Cecotto had sprouted four individual cylinders and heads in readiness for the next phase, the Power Valve. The 77 machine also saw a new bottom end design with an overall width reduction of 25mm and much smaller crankshafts to increase top revs. For this final pre Roberts configuration the bore and stroke reverted back to the 54mm x 54mm square version complete with its short tuned intake stubs, this was a desperate attempt to find some bottom end and mid range power.
The 1977 machine also saw the first of the cross over pipe arrangements creating more ground clearance by threading the #1 pipe through the frame to emerge just under the seat on the right hand side. The by-product of this was the heating of the air to the carbs and the rear shocker. This was one modification that could not be kept secret as the different pipe set up was clear for all to see. OW35 mounted Steve Baker finished second in the final championship standings to Sheene that year but, despite this undoubted success in his first GP season, did not return to the Yamaha fold for the 78 campaign.
Enter Kenny Roberts.
Yamaha 500 GP Bikes – 1973 to 1977 Gallery
[dmalbum path=”/wp-content/uploads/dm-albums/Yamaha GP Bikes 73-77/”/]