1975 Yamaha YZ360B VS. 1974 Yamaha OW13

  In mid 1974, Yamaha released the worlds first single rear shock production motocross bike. The YZ250 and YZ360B. These bikes were the first long travel motocross bikes offered to the public. They had the appearance of being somewhat hand made, parts were drilled out, the bolts were dished for less weight and there were various magnesium parts on the bike. They instantly became the best production bikes of that year. Hakan Andersson had won the 250 World Championship in 1973 on the monoshock and the works riders were now riding the second generation works bikes, the OW series. Motocross bikes were now entering a hyper-evolution in suspension and chassis design. If you didn't have a monoshock or a bike with the rear shocks moved up, your race results suffered. The bike made more of a difference during this time, than at any other time other than maybe the late sixties when the two strokes took over the four strokes. It was really surprising just how much faster you could go through the rough sections on the monoshock. The faster you went the better it seemed to work. The question was, how much different was the production bike from the full blown works bike? I have ridden both and the works bike is light years better than the stock one. The main difference you feel is the weight. The works bike is over 30lbs. lighter than the YZB. The second is the power delivery. The works bike has a much broader powerband than the stock bike. The stock bike is unbelievably fast, but the powerband is narrow and there is a huge hit in the middle. The works bike is at least as fast only with a much broader powerband. The ergonomics of the works bike are also much better. The OW is much thiner and easier to move around on.

  The YZ360B featured here is a brand new bike (never had gas in the tank.) The OW13 is the bike that Canadian factory rider Tom Kratzer used in 1974.(see the feature of this bike in "The Bikes"). The photos and text in this feature will show just how much more advanced the works bikes were compared to the production bikes of the time.  

   Looking at the profile of these two bikes they appear similar. In actual fact there are almost no parts that will interchange with each other. While the YZB had just raised the bar on a production motocross bike, the OW by comparison, was far advanced to the YZ.

  From the left side you can see that the clutch actuators are different, with the works motor having the clutch arm going thru the top vs the production one going thru the side. This made for a much more positive feel. The ignition cover shape is different as a result. Notice the transmission shaft on the works motor, it is hollowed out and filled with a rubber plug. Every single nut and bolt, countershaft nut included on the OW motor is titanium. Carburetor on the YZ is a 34mm Mikuni and the OW uses a 36 or a 38. The crankshafts are different, with the OW having the crank pin in a different location, making it 370cc vs 351cc on the production model. Inside, the motors are of the same design but the works bikes parts (gear box, clutch etc.) are all hand made.

  Magnesium is used for the sand cast center and outer cases on the OW vs di-cast aluminum on the YZ center cases and di-cast magnesium on the outer cases. The cylinders are the same bore and both use a very thin iron liner. Porting is slightly different between the two bikes. The reed valves and manifolds are different as well. The OW had the option of one or two ring pistons and the YZ was limited to just one. Piston intake cutaways were different as well. Also the YZ had a choice of two pistons, a standard 96 and the optional 98 for when the bore got worn a little. The works bike had a choice of 10 different sizes! Notice the forged titanium kickstart lever on the OW vs the steel kickstarter on the YZ. The works motor is nearly ten pounds lighter than the YZ. The pipe on the OW is a combination of hydroform and cone sections and is made of very thin sheet metal. The stock pipe is much heavier and is made from heavy gauge stampings.

  With all the body work removed you can see the huge difference in the frames. The works frame is made from chrome moly and the YZ is made from a high quality mild steel. The geometry is much different between the two as well. Notice the backbone area on the OW, it is made from a series of formed sheet metal and tubes. The stock bike is made from all tubes. The shock body on the OW is hard anodized aluminum and the YZ is steel. The insides of both shocks are different as well. The OW was much more tunable than the stock shock. Rear wheel travel on the OW is just over 8 inches and the YZ is just over 6 inches. That is a big difference.

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