Yamaha Rising, Part 2 - CLASSICS REMEMBERED

A look back at the Yamaha RD56 factory roadrace bike

Yamaha YDS1-R 250 static side view
Yamaha YDS1-R 250 in RiminiBy Midnight bird (Own work) CC BY-SA 3.0, via Wikimedia Commons

Yamaha engineers knew that to continue raising engine power, anything standing in the way of piston cooling had to be eliminated! The next step toward Grand Prix success was to remove one more heat barrier—the iron cylinder liner that was still being used by the most powerful two-strokes in European GP racing, the East German MZs. Yamaha's first try was to harden the surface of an aluminum bore by anodizing—oxidizing the aluminum into the super-hard ceramic aluminum oxide. With the low conductivity iron liner out of the way, piston heat could more quickly flow away into the high-conductivity aluminum cylinder and its generous fin area. The result would be a cooler-running piston whose crown temperature did not drive its fuel/air mixture into detonation and seizure.

The production-based YDS1-R twin (30 hp at 8,500, piston-port intake system) was upgraded to this technology in 1961, and race kits with anodized cylinders were offered to customers. The TD1-A on which the late Don Vesco in 1963 set a one-lap record at Daytona featured such cylinders. Despite this they were tricky; cooling was improved, but the moment piston pressure against the anodized bore exceeded some modest maximum, seizure resulted. Increasing the piston clearance just raised piston temperature by reducing piston-to-wall contact. More research!

In 1959 Yamaha had set engineer Noriyuke Hata the task of designing the company’s first 125cc GP bike. The real assignment was to take the powerful ideas behind MZ’s fast-but-unreliable rotary valve two-strokes and implement them in more durable form using Yamaha’s fast-growing industrial muscle: In 1960 the company would produce nearly 250,000 machines. Yamaha’s production-based Asama 125 racers, with simple and inexpensive piston-port inlet systems, had made 12 to 13 hp, but Hata’s rotary disc intake YX-18 was soon at the 18-hp level—approaching that of MZ.

Hiroshi Naito, later to become Yamaha’s Manager of Motorcycle Technology, made a trip to Europe, returning convinced that solid reliability was the key to winning GP races (which were the key to worldwide brand recognition).

For a fascinating review of two-stroke technology in the mid-1960s, read Naito's SAE paper #660394. It's interesting to note that among the audience when this paper was delivered, and speaking in the discussion that followed, were William J. Harley of The Motor Company and Gordon Jennings of Cycle World magazine.

Hata’s YX-18 featured rotary-disc inlet, a rear-facing exhaust port (because con-rod side-thrust pushes the hot side of the piston into better thermal contact with the cylinder), and the two large transfer ports proven in the Asama/Fuji singles and twins. The YX was Yamaha’s first “Autolube” engine, using Nakamura’s multi-port oil metering pump to send oil where it was needed—not just hoping that oil mixed into the fuel would do the job. This plunger pump was aluminum and itself featured an anodized bore. Another YX-18 feature that would become familiar to later Yamaha racers (TZ-750 and TZ250s from H-model on) was its dry-sump gearbox. In simpler designs the gearbox is filled to the shaft centerline and lubricates itself by the resulting oil-storm. Because this wastes between 1 and 5 hp in oil churning, Hata gave his engine a tiny pump supplying oil to jets for each ratio pair and to the shaft centers. The sump oil level was far below the gears.

YX-18’s cylinder was conventional, having a divided exhaust port and two large transfer ports. Intake was symmetrical by two rotary disc valves, one on either side.

Yamaha first raced in Europe in 1961, entering Naito’s new RD48 250 rotary-valve twin and the YX-derived RA41 125 single. Power was 35 hp for the 250 and 20 for the 125. Photos show that both bikes were built on a single top-tube/double-downtube swingarm chassis that is familiar to us because of its later use in the TD1-A, B, C series of 250 production racers. It also bore the signature red “watermelon” fuel tank of that time, emblazoned with the crossed-tuning-forks logo symbolizing the company’s origin in the musical instrument business. Bikes of this era seem tiny to us, accustomed as we are to the 1-liter behemoths of the present, whose engines have four times the displacement and six to seven times the power. Wheelbase was 50.4 inches!

Wheels were of traditional wire-spoke construction. The front brake was a single-sided 200mm (nearly 8-inch) full-width aluminum drum with cast-in (despite being coated with rust!) iron liner, with two leading shoes (meaning both shoes have self-servo action) of 25mm width. Rear brake had the same dimensions but with one leading/one trailing shoe operated by a single cam. Front fork travel was 4 inches on slender 30mm tubes and a 14mm solid axle. Rear travel with stock suspension units (and kidney-stimulating 125 pound-per-inch springs) was 2 inches. Each year that this basic frame appeared, it showed detail development in the form of increased swingarm bracing and strengthening gussets.

A two-stroke's all-important transfer ducts begin at the level of the tops of the crank flywheels and carry fuel-air mixture compressed in the crankcase up to flow into the cylinder through rectangular ports located on either side of the exhaust and aimed away from it. A look at a modern 125cc single shows these transfer ducts making gradual sweeping curves to arrive at their cylinder wall ports, but in 1960 the ducts rose straight up, parallel to the cylinder centerline, and then turned through an abrupt 90 degrees before entering the cylinder. A few moments spent moving an impact probe (a small tube connected to a sensitive pressure gage) over such a transfer port as air is blown through it reveals that the lower one-third of the port flows nothing at all; air can't make that sudden turn. Despite this, 1960s/early '70s two-strokes nearly all had this flow-restricting type of transfer duct. Why? The extra width of nicely curving, high-flowing transfer ducts conflicted with the need to squeeze the cylinders of twins or triples close together to limit engine width. Suzuki got around this by twisting the cylinders of its 750 triple enough that its curvy transfers fit past each other. Suzuki's TR750 with just two high-flowing transfers per cylinder always made more power than Kawasaki's H2-R whose four transfers were cramped by their sudden 90-degree turns.

fumio ito winning 250 gp race
Fumio Ito winning the first of Yamaha's 250 GP winsCourtesy of Yamaha Racing

At the 1961 Isle of Man TT the RA was uncompetitive but durable, finishing 11th (Ito) and 12th (Oishi), with the top five places taken by Honda fours, followed by an early Bultaco. The RD48 did better, Ito finishing sixth in the 250 TT (behind a wall of five Honda-fours). Problems were spark plug fouling, vibration, irregular carburetion (vibration does that!), limited power and poor durability.

Plug fouling afflicted all early two-stroke racers. Part of the reason was their magneto ignition, whose slow voltage rise-time could allow spark current to leak away across a film of carbon on the spark plug insulator. The rest of the reason was poor cooling, forcing engineers to run rich because rich combustion is cooler than combustion on a best-power mixture. Early aircraft radial piston engines were run rich for the very same reason. The extra fuel in rich combustion liberates free carbon, some of which makes its way onto plug insulators as a conductive layer.

Before WWII, DKW’s supercharged split single two-strokes had added a “blow-through” phase to provide extra cooling for that engine’s hard-working exhaust piston; the result was tremendous fuel consumption, requiring a ponderous car-size fuel tank to be carried.

Yamaha now had to find a way to (1) stop plug fouling without (2) having to run rich to make up for poor cylinder and piston cooling, yet without disappearing piston clearance that led to (3) seizure. This required intensive engineering—not hoping to get lucky with a just-right piston clearance that would work some of the time (as it did for MZ in this period).

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