Yamaha 500 GP History – Part 3

Yamaha OW48R During the 70’s the Yamaha GP spec 500 was never made available in any form whatsoever to the public making it a very mysterious machine indeed, the only ones that existed would be the few closely guarded factory items, unlike the thousands of TZ250/350 twins and the hundreds of 750 fours produced. This “over the top” secrecy of the Yamaha team throughout the 70’s and early 80’s gave the world precious little factual information about the 500 racers and any official photographs of the time were taken in such a way as too hide essential features from the public gaze. Likewise at race meetings the OW’s would be quickly covered over following a practise session or actual race. Presumably the secrecy was not just to confound the other teams but also to please the publicity department. Much store has always been placed in the Yamaha RD (Race Developed?) ethos, so we, the public had to think that the 500 used reed valves and the same pistons etc as the RD250’s that we were all buzzing around on.

Many people thought the inline 500 engine was just a TZ750 bottom end with a pair of 250 barrels fitted, the engine layout is indeed identical but the crank cases of the 500 are very much smaller than the 750 and no components are interchangeable. The two machines were completely different as proven by the few privateer GP racers who performed the conversion of a TZ750 into a 500 by fitting TZ250 cylinders, the result being an uncompetitive and difficult to ride machine that was slower than a TZ350.

OW48RIn 1980 a TZ500 was produced for sale but it lacked the speed and the trick parts necessary to make it a competitive machine, it was actually a copy of the 1978 OW35, and a poor one at that, so it was already well out of date when launched. Labelled the TZ500G it arrived with mechanically actuated power valve assemblies (taken from the tachometer drive) rather than the electronic ones that Kenny Roberts had used to good effect when he won his three back to back titles between 1978 and 1980. The power valves were also a completely different shape as the factory items were of a guillotine design while the customer version feature the cylindrical valve in keeping with all of the Yamaha two stroke road machines from the YPVS onwards.

The TZ500G may have mimicked the basic engine configuration of the factory unit with its over square bore but, as a race machine, the production RG500 Suzuki was a far better bet, especially with Suzuki’s policy of the factories last years secrets being sold to the public. Yamaha were way behind in that respect and never made available the trick stuff that had been developed by Roberts and his crew. Even two years after Roberts last rode the OW48 the production version, the TZ500J, bore no resemblance to it technically. The bikes that the public could buy were not the same machine by any stretch of the imagination with at least 10bhp less power and woefully inadequate chassis. Just looking at the detailed engine shots of the TZ500J and the OW48R show little more than a passing resemblance.

Riding the different variants of the Yamaha 500, both factory and customer, yielded some surprising results. The pukka OW48R possesses, without doubt, the most power and, along with it, by far the narrowest power band. With the kick coming in strong at 10K and disappearing off into the next gear change at 12K sharp. That is it, nothing else. There is some power low down say from 6500 to 8ish but nothing that is going to win you a GP in a hurry so King Kenny must have had the little Yam screwed back to the stops to have achieved what he did.

Yamaha TZ500JThe production version of the day however is a very different animal with good usable power, albeit considerably less of it, from around the 7500 mark through to the redline of 11500. Both bikes handle very similarly, although the steel framed production version gives less feedback and the thinner forks (38mm TZ500 –40MM OW48) squirm around more under braking whilst cranked over. Both bikes feel small once moving and respond well to rider input, almost like a “big” 250. Agility is the name of the OW game and wherever Roberts would have lost out on top speed or acceleration he sure as hell would have made up with corner speed. The Roberts works machine has fully adjustable Kayaba forks with much thicker stanchion walls along with an anti dive unit (remember them?) controlled by fitting different jets into the oil damper. Actually this one works quite well and rarely does the front end pump up under braking unlike others from the period.

In complete contrast the TZ500G/H has basic oil damped forks, equipped with spring preload adjustment that could easily be mistaken for RD road bike units from the 70’s. Likewise with the rear suspension the OW48R possesses a trick remote oil reservoir unit and nitrogen to assist the spring rate whilst the TZ damper differs little from the early TZ250/350 type. As for wheels it has always appeared strange to me that even when every one from the late seventies onwards was using mag alloy wheels on everything, Yamaha were still selling the production race bikes with wire wheels (actually as late as 1985). So the TZ500 has spoke wheels while the OW48R has ultra lightweight cast magnesium Morris wheels, once again greatly aiding the turn in ability of the title winning machine. The TZ500 engine and chassis bear no resemblance to the factory item and it would be difficult, if not impossible, to interchange parts from one to another.

TZ500JThe 1981 season dawned and saw a completely new angle for Yamaha and the all-new OW54, an unashamed “if you can’t beat em join em” RG500 copy, complete with its square four configuration and disc valve induction. This new approach was not a success as quality control was completely out of the window, the bike did not handle well and when it did run the power was not controllable. While the factory boys of Roberts and Sheene had moved on to the square four design good old Joe public got the TZ500H, identical to the G in everyway, needless to say sales of the 1981 spec machine were very poor. Just looking at the grids both GP and nationally from the period and the amount of Suzuki’s compared to Yams is phenomenal, rarely was the ratio higher than 5-1 and to see a TZ Yamaha in the top ten would mean it was accompanied by some extraordinary riding talent. The only success at GP level for an over the counter TZ was by Dutch privateer Jack Middleburg who won a very wet Dutch TT on one although to achieve that he used a Nikko Bakker chassis and RG front forks!!

TZ500JThe sidecar boys can tell the real story of the production TZ engines as the unsuccessful solo power plant carried on for many years as a potent chariot pusher. Even the later ADM and Krauser labelled engines of the 95/96 seasons used the TZ500 crankcases as the basis, although the first thing to go was the troublesome Yamaha gearbox to be replaced by the several after market items that became available in the late eighties. With many engines using the Yamaha bottom end but with NSR500 Honda barrels fitted, output figures exceeding 190bhp were achieved by the sidecar racers. Perhaps the success of the aging Yamaha power plant in sidecar racing came about due to the height and bulkiness of the only real alternative, the RG500.

Yamaha Factory Machines

OW19 1973 First of the fours, twin shock rear suspension.
OW20 1974 Monoshock rear
OW 23 1974 Much smaller and lighter than previous
OW26 1975 Side loading gearbox championship winner
OW29 1976 No a full factory backed effort
OW35 1977 First of the up and under pipe routing
OW35 1978 Yellow and black later with power valve c/ship winner OW45
seen this year also
OW45 1979 Only minor changes to the previous year c/ship winner
OW48 1980 Alloy framed anti dive forks
OW48R 1980 Twin pipes routing out of seat unit ala RG500

Yamaha Production Machines

TZ500G 1980 a poor copy of the OW35 much heavier and using TZ250
TZ500H 1981 Few changes were made for this model
TZ500J 1982 The outer cylinder were reversed to aid ground clearance but
was still a shadow of the works OW48R it was based upon