Each piston, moving up and down in its cylinder, is connected to the rotating crankshaft by the aptly named connecting rod. The big end of the con-rod encircles an off-center crankpin that orbits in a circle, while the small end of the rod is joined to the piston by the wrist pin. The big end of the con-rod and its crankpin bearing rotate while the wrist-pin bearing at the small end only oscillates.

In a four-stroke engine, the piston is exposed to combustion heat during only one stroke in four, the power stroke. In a two-stroke, the piston is heated by combustion twice as often, every two piston strokes. This causes two-stroke pistons to operate hotter than those of four-strokes.

In a four-stroke engine under power, the wrist-pin bearing in the small end of the con-rod undergoes regular load reversals. As the rod stops the rising piston at the end of its exhaust stroke, piston inertia pulls up on the wrist pin, but when compression and combustion occur, gas forces push down on the wrist pin. This load reversal moves the wrist-pin bearing's clearance constantly from top to bottom and back, creating spaces into which fresh lubricant can find its way.

But in a two-stroke under power, the load on the wrist-pin bearing is always down, tending to squeeze all the oil out of that bearing, making it hard to lubricate. A two-stroke’s higher piston temperature, conducted to the wrist-pin bearing, can either gum or boil any lubricant present.

In a four-stroke engine under power, the wrist-pin bearing in the small end of the con-rod undergoes regular load reversals.

When Yamaha produced its first two-strokes, engineers surely encountered this problem, which explains why they tended to locate wrist pins quite low in their pistons, farther away from the very hot piston crown. In early models, this was evidently workable, for when I took after-hours work at a Yamaha dealer in 1966, rebuilding twins crankshafts for $10 apiece, I was fascinated by one of my first rebuilds, a twin whose con-rods were unusual. First, their oval beams did not have the normal I-beam cross-section. Second, their small-end bearings, through which the wrist pins pass, were plain bronze bushings. This engine may have belonged to a US serviceman who had brought it back himself from Japan. I never saw another like it.

For those bronze bushings to survive, I thought to myself, that engine must have been of moderate performance at best (meaning its hot parts ran fairly cool). Otherwise, those bushings would have had an unacceptable failure rate.

As Yamaha increased the power of each new twins model, even with the cooler-operating lower wrist-pin position I previously mentioned, enough extra heat would be conducted to the wrist-pin bearing to cause occasional overheating and seizure. “Bad effect!” as Mr. Yoshida used to say.

Engineers know that when lubrication is marginal, rolling bearings may survive longer than plain bushings.

Full-film oil lubrication requires that a dynamic oil wedge forms between the moving parts, but in a bearing that oscillates rather than rotates, no sooner does an oil wedge start to form but the rod swings back the other way, undoing that good work. No oil wedge can form, so metal-to-metal contact can occur. And when the bearing is also very hot, any oil in it loses viscosity, becoming watery and easier to squeeze out under load. Any remaining oil may even evaporate—boil away—leaving nothing to prevent bearing seizure. Bad for repeat sales.

Engineers know that when lubrication is marginal, rolling bearings may survive longer than plain bushings. Therefore, the con-rods with bronze-bushed small ends were replaced in the next model with a new design using a caged needle roller bearing. And if you examine any of the hundreds of thousands of two-stroke engines produced in Japan since the mid-1960s, that is just what you will find at the small ends of their connecting rods. And for much the same reason (bearings that oscillate rather than rotate), you will also find needle bearings inside of automotive driveshaft universal joints; bushings wouldn’t survive.

56.0mm x 50.0mm two-stroke parallel twin
The Yamaha YDS-2 was powered by a 56.0mm x 50.0mm two-stroke parallel twin with air-cooled, cast-iron cylinders and aluminum heads. Claimed output in 1962 was 23 hp at 7,000 rpm.Cycle World archives

Even four-strokes can have wrist-pin problems. When Ducati adopted pistons with very short wrist pins in its four-stroke V-twins, wrist-pin bearing temperature was increased because wrist-pin bosses in such pistons are like stalactites hanging down from the underside of the hot piston dome. With such a short heat path from hot dome to wrist pin, the wrist pin, the piston’s wrist-pin bosses, and the wrist-pin bushing in the small end of the connecting rod all operated at higher temperatures.

Eraldo Ferracci described how wrist pins began to occasionally pick up aluminum from the wrist-pin bosses because of oil evaporation, metal-to-metal contact, and local welding. This was just like the two-stroke experience! The temporary answer was to drill a small hole up through the beam of the connecting rod, conducting engine oil from the crankpin bearing (supplied under pressure) to the wrist-pin bearing, improving its lubrication and reducing its temperature.

Later, I suspect, Ducati improved piston cooling in a simpler way, by the use of oil jets located in the crankcase aimed upward at the undersides of the crowns. There may also have been a role for oils with improved additives for lubricity or anti-wear.

Engineering is not always a smooth process of carefully planned improvement. Rather, it can be a series of crises, each requiring a workable emergency solution that can be introduced into production with minimum disruption.