Motorcycle History: Yamaha YZ125 – The First Water-Cooled MX Bike


Category: Motorcycle History

Each week RideApart looks back at key milestones in motorcycle history from technical innovations to significant model introductions to racing successes and, of course, some of the disastrous things we’d rather forget. This week it’s the first ever water-cooled motocross bike – Yamaha’s ubiquitous and highly successful YZ125.

Liquid cooling is nothing new in the motorcycle world. Pre-war manufacturers had used the system on road bikes but it took until 1981 for a water-cooled motocross bike to be launched.


1974 Yamaha YZ125

Yamaha’s first introduced its YZ125 in 1974. It came with dual rear shocks and a 125cc, two-stroke, air-cooled engine. The following year it adopted Yamaha’s then revolutionary monoshock, which it has retained right up until it was discontinued in 2005.

However, the biggest change to the YZ125 didn’t arrive until 1981 when Yamaha’s engineers designed a cooling system for its engine making it the first dirt bike in the world of motocross to become liquid-cooled. (As an aside, the air-cooled Yamaha YZ125’s were the G models - VIN numbers start with a 3; the water-cooled models from 1981 became the YZ125 H with VIN numbers beginning with a 4).

There was initial skepticism from the media and Yamaha’s customers who couldn’t see a reason for Yamaha to make the YZ125 ‘wet’.

Principally it was done to increase the lifespan of the engine, especially the top end parts. On a regular air-cooled 125 the top end needs new rings after every race. With a water-cooled engine it could last as long as four races before any freshening up was needed and with a lower compression ratio just a single ring was used.


1981 Yamaha YZ125

Yamaha used a small aluminum radiator mounted behind a bulky front cowl for the YZ125’s water-cooled system. Rather than have long hoses going from the engine to the radiator, water was routed through the YZ125’s triple clamp and steering head. This effectively meant there were no hoses that could be twisted or stretched. The new water-cooled system, including the radiator, added around 9 lbs over the air-cooled version’s wet weight of 198 lbs.

Dirt Bike Magazine said at the 1981 launch of the water-cooled Yamaha YZ125: “The bike works well and the water cooling is a big plus for a rider at any skill level… Our first test riders were on the new bike and it felt plenty quick, but it didn’t overwhelm us.
After half an hour on the water-pumper, we slung a leg over the 1980 air-cooler. Wow! The difference was more than apparent. The water-cooler’s midrange is much snappier and throttle response throughout the range is more positive.”

There was no difference in power output between the air-cooled bike and the water-cooled version. Both developed around 33 hp. The top speed was the same, but the water-cooled version was a little quicker in acceleration with a broader power band. Maintenance was a lot easier on the water-cooled rather than the air-cooled version as you didn’t have to constantly keep taking the off the engine head. Other than normal nut and bolt tightening before using the liquid-cooled YZ125H, you just needed to check the tiny radiator’s fluid level. Then you were good to go.


2002 Yamaha YZ125

The Yamaha YZ125’s were produced between 1996 to 2002 and are regarded as some of the best dirt bikes of their generation due to their wide power band and terrific handling. In those years they were also ridden to five AMA National Motocross Championships and multiple regional Supercross championships.

Have any of you ever owned or been able to ride one of these early 1980s YZ125s? It would be great to hear if you thought they were as good as some people claim.

Related Links:
Motorcycle History: 25 Years Ago Today – BMW First to Offer Antilock Brake System
Motorcycle History: 50 Years of Full-Face Motorcycle Crash Helmets
Motorcycle History: 26 Years Ago Today – Radial Motorcycle Tires

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