1974 Yamaha OW13

The 1974 OW13 shown above is one of 3 bikes known to exist and all three are in this collection. Engine displacement ranged from 360cc to 400 long and short stroke.

1974 Yamaha OW13

For 1974, the works Yamaha’s got a new name and were now labeled OW’s instead of YZ’s as the YZ name was now transferred to the production line. The 1974 OW 125 works bike was designated the OW11, the 250 now became the OW12 and the open bike became the OW13. This was the second generation of the monoshock works bike (dubbed the Mark II). The monoshocks had still not been offered to the public when the OW's were released. The first Yamaha production monoshock would be the 1975 YZB series that was released in mid 1974. Even though the production bike got rave reviews in the press, they were 35lbs. heavier, made less power and had less suspension travel than the 1974 OW works bikes.

The OW’s got an all-new frame design with new geometry and beefed up suspension that now featured over 8 inches of travel at each end. This was twice as much rear travel as the norm less than two years earlier! Motocross chassis and suspension design was now in hyper-evolution and things were changing very fast. The new frame was a work of art and very much more refined than the 1973 works YZ frames that were very strong but crude looking. After much testing in Japan the team developed a frame with improved geometry that provided better all around performance.

In designing the new frame, the Yamaha engineers made a structural error that would haunt them for nearly the whole season. The top rear motor mounts began to crack and break at an alarming rate and it seemed like nothing would stop this from happening. Gussets were added and other quick fixes but the frames continued to break. Frames were changed nearly every race and even then they would break. The problem got so bad that Yamaha actually pulled out of the 1974 Trans-AMA series because of this and it was Yamaha factory rider Pierre Karsmakers who took matters into his own hands. Frustrated and sitting at home, Pierre, out of his own pocket commissioned Profab out of Southern California to design and build a new stronger frame using the same geometry as the standard works frame. Profab owner Pete Wilkins accepted the challenge and made a beautiful frame utilizing monocoque style bracing that went the entire length of the backbone up to the steering head. With his new Pro-Fab framed works Yamaha Pierre re-entered the final 1974 Trans-AMA round at Saddleback Park and won the race. Yamaha immediately ordered 25 of the Profab frames.

The front suspension was beefed up and the tube diameter was increased to 36mm from 34mm. The tubes themselves were very thick to compensate for bending when landing from jumps. Prior to the upgrade, it was not uncommon to rotate the fork tubes 180 degrees in-between motos due to the forks bending. The works rear shock also found new troubles and the increased travel now caused the shock to get hot to the point that the aluminum shock body would overheat and turn oval. Higher quality aluminum and steel bodies remedied this problem.

The OW13 had several engine displacements from a 360 all the way to a full 400. This particular OW13 is a 370 and the extra 10cc’s was obtained from a different crank pin location. Riders were now demanding broader power-bands, as the tracks were getting rougher and the factory engineers were trying to accommodate this with larger displacements to increase torque. Riders now had several choices and different displacements were sometime used for different tracks. The engine cases were sand-cast in magnesium and were very similar in design to the 1973 works cases. The main benefit in this design was the more positive clutch pull. The 1974 OW’s were much more refined than the 1973 monoshocks and when the shocks worked and the frames didn’t break performed very well. The design was very good and provided a solid chassis in the new world of long travel suspension.

The OW13 featured in this chapter was ridden by Tom Kratzer; Tom and his fellow teammate Bill Mclean were both Yamaha factory riders from Canada who rode for the Canadian Yamaha importer. These two riders were two of Canada’s finest in the 70’s and were always a threat where ever they went. Both riders rode OW’s in the Canadian national championships and finished 2nd in their respective classes in 1974. On top of getting second in the Canadian nationals, Tom also entered this bike in the 1974 USGP at Carlsbad where he earned 15th overall against the worlds best. Both Tom and Bill entered a few select west coast Trans-AM’s in 1974 on the OW13’s as well.

In 2008 Canadian motocross legends Tom Kratzer and Bill Mclean both succumbed to cancer. Both riders were fans of the sport and often looked on their careers at Yamaha as some of the most memorable times of their lives. Tom and Bill will always be a part of the elite few that actually got paid to ride works bikes.

Both Tom and Bill were aware that their works Yamaha's were in this collection (Tom's 370 and Bill's 125 - the OW11). They both provided valuable information about their bikes and looked forward to seeing them with all the other works bikes in our museum.

Tom Kratzer, Pierre Karsmakers and Billy McLean pause for a photo during the 1974 Trans AMA series.


Hakan Andersson’s comments:

After I won the world championship in 1973 on the first monoshock we were very eager to get started on the 1974 works bike. We spent a lot of time in Japan developing the new bike using all of the data that we acquired from the 73’ season. There was some shock trouble initially due to the aluminum shock body. It wasn’t staying round when it got hot and the shock would stop working properly. Steel bodies and better heat-treated aluminum solved this problem. When the bike was finished it was better in every way. I was riding very well, I felt we had a good bike and I was ready to defend my world championship.

At the first grand prix in Spain I qualified fastest and was feeling very good on the new bike. In the first moto, the Russian Pavel Rulev was in the lead on the factory KTM. I was in second and moved along side to pass him. We were going up a big hill and there was a jump at the top. While we were side by side, Rulev went sideways when he hit a bump crashed into me. His bike landed on my wrist and broke it. I didn’t crash but I had to stop for a short while losing a few places. I was able to get up and going and managed to work my way up and finished third. I was in a lot of pain and my wrist was swollen very bad but still decided to race in the second moto where I finished 5th and got second overall that day. After the race we went to the doctor and we learned my wrist was broken. It was a bad way to start out the GP season and I missed the second round in Italy. I decided that I would try and race the third round in Tjeckoslovakien. While in the first few laps of practice I was going around the track very slowly to learn the track and after a very long downhill with a small jump there was a left corner before it went up again. I was going around the corner slowly and my bike accidentally went into neutral and I fell over. As I was on the ground, Miroslav Halm came flying over the jump at full speed and his bike landed right on my back. I didn’t see him at all. I broke a blood vein in my back and I didn’t start the race. When I went back to Sweden, the doctor tried to take out the blood and then I got a bad infection in my back. I missed the next five races and the 1974 season was gone. I felt I had a good chance to be world champion again and was very confident on the new OW Yamaha that we worked so hard on.

I did race in the Motocross des Nations at the end of the season in Husqvarna Sweden where we won but my frame broke in both motos at the end, all four motor mounts were broken and I was lucky to finish. Then we went to the states for the 1974 Trans-AM and it was decided I race four races and then go to Japan to begin developing the 1975 factory bikes.

Hakan Andersson on an OW12 during a 250 GP in 1974. Note the cut-away cylinder fins, Hakan's mechanic performed special work on the transfer ports that required cutting away a portion of the fins in that area. This gave Hakan's bike more mid range power. We have this very cylinder in our collection. See here.


Pierre Karsmakers comments:

We spent a lot of time in Japan working on the new OW works bikes for 1974. We focused on the chassis and also we worked on getting the front forks stiffer, we had problems in this area with the 1973-monoshock bikes. When the bikes were done they performed better than the monoshock bikes I rode in the 1973 Trans-AM series but they had shock and frame problems. When the shocks worked the bike was fantastic but at first the shock bodies were going oval after they got hot. The frames were breaking the whole year and we had many DNF’s because of it. The rear motor mounts were breaking every race and it got to the point where we were changing frames every race.

In the middle of the Trans-AM series Yamaha decided to pull out. I went to the Yamaha management in the US and told them I knew of a fabricator in California that could make a new frame that would last. The factory didn’t want stray away from Japan fixing the problem officially but told me if I wanted to take matters into my own hands that it would be OK. I took a standard works frame to Pete Wilkens of Profab and told him the problem and that I needed one with the same geometry but one that would last. The last trans-AM was at Saddleback and I wanted to compete in that race because I knew I could beat those guys.

With just three days until the Saddleback race, Pete had the new frame finished and it was a work of art. Profab was one of the best fabrication shops around and later we used them to make special parts when I was at Honda. We put the bike together and I went out to Saddleback for the final Trans AMA. The bike stayed together and I won the race. It worked perfectly, just like it was designed to in the first place. Yamaha was so happy that I won the race and the bike stayed together, that they decided to pay the cost of the frame. They ordered more frames and they sent some to Japan for evaluation.

Pierre Karsmakers passes #12 Jim Pomeroy enroute to the overall win at the 1974 Trans AMA finale at Saddleback. Pierre's OW13 featured the super trick Profab frame and a 400 long stroke engine. Pierre's bike is also in this collection.


 Tom Kratzer's works OW13 #25 (the very bike featured here) sits quietly in the pits next to Bill McLean's #35 bike at the USGP at Carlsbad 1974. Tom got 15th overall.

1974 Yamaha OW13 Photos

Detail Photos


    • The motor is 370cc's. The difference from the 360, is that the crank pin is in a different location, giving a longer stroke. This was to give it more torque. The crank cases and sidecovers are sand-cast magnesium. All the case screws including the centercase screws are titanium. The gearbox is a five speed unit with hand cut gears and hollow shafts. As with most works Yamahas of this era, the kick start lever is forged titanium. All transmission and crankshaft nuts are machined from titanium. Motor mount bolts and shift shaft are titanium. The clutch actuating arm goes through the top of the motor instead of through the side like the production model. This system gave a more positive clutch pull. The exhaust is comprised of hydro-formed cone sections gas welded together. The muffler is very small and does little to quiet the bike.


    • The top triple clamp is cast magnesium, while the lower is machined from billet aluminum. The fork tubes are knurled at the lower clamp for more grip and they are machined down just above the lower clamp for reduced weight. All the pinch bolts are hand machined titanium. There is a machined aluminum fitting fastened to the lower triple clamp that the fork gator is clamped to. This prevents any binding that might occur. The frame was inadequately gusseted at the steering stem. This led to many frame failures. Late in the season, US factory rider Pierre Karsmakers commissioned Pro-Fab to redesign and make a new frame. The frame was so successful that Pierre won the final round of the Trans-am at Saddleback with one. Yamaha immediately ordered 25 of them and even designed the 1975 worksbike after them. Two of his bikes with that frame are in this collection. The ignition coil is mounted right behind the steering stem and the black box is mounted between the front down tubes.


    • Fitted on this bike is the optional front mud fender. It has an over-fender pop-riveted on the front and a rubber flap riveted on the rear. Since the stock fender was so narrow, many riders opted for this set-up in all conditions.


    • The frame is tubular chrome moly with the backbone structure made from a series of fabricated plates welded together. The frame was designed from a clean slate and far different than the 1973 model. The shock has a hard anodized aluminum shock body and was internally adjustable with various valving jets. The early shock bodys elongated and were sometimes replaced with steel. The rear reservoir held nitrogen and acted as a spring. It could be tuned by adjusting the nitrogen pressure. The air box is made from very thin layers of fiberglass and the aluminum brackets are held by pop-rivets. These had to be watched carefully because once the rivets came loose, they would allow dust to pass. The air filter covers are hand formed in aluminum, connected by a titanium thru bolt.


    • After some swingarm failures on the 1973 bike, Yamaha used stainless steel skin sandwiched to a very thin piece of chrome moly for the diagonal section on the 74 model. It is strange, because a magnet will stick to anywhere on the swingarm except the diagonal part. It is very strong but also surprisingly heavy. The rear hub looks similar to the stock one, but the webbed reinforcing is bigger and stronger. It is sand-cast magnesium. Wheel spacers are machined titanium. Notice where the spokes enter the flange, it is unpainted. The reason for this is to keep the spokes tight and to detect cracks. The rear backing plate is sand-cast magnesium and has special sand-cast magnesium brake shoes. Titanium is used for the pivot spline, chain adjuster bolts and even the little brake adjustment nut. Those tires are brand new 1974 Trelleborgs.


  • Front forks are hand made by Kayaba. This was the first year Yamaha really tried to beef up the front end due to the long travel design. The tubes are 36mm and very thick and heavy. The lower sliders are machined out of billet aluminum. Notice the lower photo, you can see the stamping at the axle lug. This designates the year the fork was made and Kayaba's model code. The front brake cable fittings are titanium. Even the pin that connects the cable to the brake arm is titanium. This bike is trick!


Riders had a choice of 360, 370, 400 short stroke and 400 long stroke engines. Different crankshafts and pistons went with each configuration as well as intake manifolds. Above is a photo of Ake Jonsson's works cylinders for the OW13. We have every possible configuration in our collection from that era. Yamaha spent a fortune searching for the right combination for the ultimate open class 2-stroke engine. The other manufacturers all did similar R&D. In the future we will do a feature on early Yamaha open class engine development. Fascinating stuff!


Historical Photos

A brand new never started 1975 YZ360B sits next to our 1974 OW13. As good as the production bike was, it wasn't even close to the works bike at every level.