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   This motor was the talk of the motocross world in 1976 and rumors ran wild. The combination of water jackets and cooling fins had most confused. Some magazines actually reported that the top end was air-cooled and it was the lower end that was water-cooled. The fact is that just the top end is water-cooled and the fins are there as some sort of a safety net. The center cases are die-cast and very similar to stock except the cylinder stud spacing is wider to accommodate for the much larger transfer ports for more mid-range power. The OW27 wasn't the fastest in top speed but it made very good power and maintained it for the 40 minute plus two lap motos. This was a huge advantage in 1976 where many 125's made incredible power but only for a short time. It wasn't uncommon for a 125 to lose over 20% of it's power during the long 40 minute motos. The gear box has the same ratio's as the YZ 125X, only they are hand cut and drilled for lightness. Yamaha did experiment with a water-cooled works 125 in late 1975 but that was near the end of the Japanese Nationals and it never saw action outside of Japan.

    

    

 A close up of the sand cast cylinder and head. The bleed hole at the bottom of the cylinder served two purposes. First to drain the cylinder for servicing and second, it was mandatory to loosen the bolt after running the bike at warm up to remove any air that might be in the system. If you didn't do this, an almost certain a vapor lock would occur that would guarantee a seizure. You can see the original safety wire still on the bleed bolt that was wired up by Bill Buchka back in 1976. Below is a close up of the sand cast magnesium clutch cover that contains the water pump impeller.

    

    

As with most works bikes, the center cases are matched by hand at the factory and numbered. The kick start lever is made from billet machined steel with the foot rest turned on a lathe and welded into the lever. The kick start shaft is smaller in diameter than stock and uses a different spline pattern.

    

    

The motor is attached to the frame by machined aluminum motor mounts and to do so at the bottom rear, a special spacer was made. Notice in the photo below, there is a steel sleeve pressed into the spacer. This insured that the soft aluminum wouldn't cause the bolt to come loose and save weight at the same time. This is just one small reason why works bikes cost so much money.

    

    

    

The carburetor is a 34mm mikuni that is hand bored to the exact spec and in the bottom photo you can see OW27 stamped right under the choke lever. The reed cage is larger than stock.

    

    

    

The exhaust pipe is made from hydro-form, cone and stamped sections. Different pipes providing different powerbands were available. The silencer, although it looks very similar to the stock silencer, is actually very different and like the pipe, there were different ones to choose from. Bruce Hollingshead did a fantastic job saving the original as it was in pretty bad shape.

    

    

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