Keeping Oil Viscosity Where We Want It

[Hugh Janus]

Oil viscosity is affected by temperature—at both ends of the thermometer. (Cycle World Archives/)

I stepped off a train at wintry Westport station near New York City, pulled my suitcase off the rack, and went looking for my ride: an ancient black Buick straight-eight. I found it with other long-term cars in a snowbank. Eventually I made a way to the driver’s door and sat at the wheel. Key above the sunshade—there it was. Now the terrible contest—battery and starter versus the Pleistocene.

The starter turned, but so slowly. I’ll have to walk the 5 miles. I could hear the grr of tooth engagements of the starter pinion against the ring gear. Grinding. Surely it’s too cold for sparks to ignite gasoline. Grinding. I thought about the congealed oil on cylinder walls, in bearings, resisting all motion. But the battery kept turning the starter until there was a single jump: A cylinder had fired. Again. Shall I drive rather than walk?

Oil viscosity is both bane and benefit. Oil, thickened by low temperature, resists all motion. It has become glue. But oil’s viscosity in the fast-moving loaded zone of a crankshaft bearing is what keeps it from being squeezed out by the load. That resistance keeps enough oil moving through the bearing to support the load.

Had that Buick’s oil sump contained a modern multi-grade oil I’d have been spared that drama—the engine would have spun over and fired at once.

The point I’m making is that oil viscosity rises at low temperature and falls at high temperature. It’s a fight between the affinity of the long-chain oil molecules for each other and the vibrating energy of temperature. In the 1930s, Benelli in Italy borrowed from aviation practice by mounting an oil cooler on a bike. And it took a patent on the idea.

When Honda went GP roadracing, starting in 1959 at the Isle of Man, it quickly encountered severe oil viscosity loss at high temperature, and suffered some outright engine seizures. We’re talking four-stroke engines here; so what was happening was that in summer operation, oil in air-cooled race engines was losing so much of its viscosity that piston lubrication was occasionally breaking down completely.

In quick succession Honda tried the “English answer,” carrying the oil in a separate, remote tank, and it tried the “Italian answer,” which was to provide a long finned aluminum sump under the engine. Since neither was adequate, in 1965 it integrated small oil coolers into ducts in the bikes’ fairings, such that oil was actively, intentionally cooled rather than cooled by accident. That brought the situation under control, keeping oil from getting hotter and hotter until it “turned to water” and bad things happened.

I saw Honda’s oil coolers at the Canadian GP in 1967, dangling from their hoses after fairings had been removed. Success.

Oil acts as both lubricant and as heat-exchange fluid. Oil can go places where air and water cannot. It cools internal parts such as pistons, valve springs, and bearings, tending to level all temperatures.

Certain styling departments reflexively dislike oil coolers—rather like the vain young biologist I once encountered, who upon arriving at work invariably put his wallet in a desk drawer, explaining that “It spoils the line.” Okay, maybe even better than an oil cooler is a water-to-oil heat exchanger (the spin-on oil filter often mounts on such an exchanger) because the rapid warm-up of engine coolant sees to it that the oil warms up just as fast.

Why do we care how quickly the oil warms after starting? We care because any cycle shop service manager can name for you the older models whose sluggish cold-start oil circulation led to problems like premature cam or bearing wear. In an ideal world, every engine would have an electric pre-lube pump that pressurized the oil system just before starting.

Although liquid-cooling has solved many durability and running problems, cooling by internal oil circulation remains essential. In the case of air-cooled engines, which tend to run too hot in hot weather and too cool in cool weather, oil cooling has a long history of giving aid where needed. The large air-cooled radial piston engines that powered so many aircraft in WWII (all US bombers, plus the P-47, F4F, F4U, F6F, plus all ground-attack types) relied upon oil cooling for about 30 percent of their waste heat removal.

Therefore it is just continuing that great tradition when present-day builders of iconic air-cooled designs need to supplement what cooling fins can do by adding oil coolers plus strategic circulation through cylinder-head passages. This includes several Harley-Davidson models, the large BMW twins, and at least one of Honda’s large “cooling-fin revival” four-cylinder jobs, in which the official line is that the supplementary cooling is just for the spark plugs (nudge, wink).

Fins for cooling, on the cylinders and on the oil cooler on this Harley-Davidson Street Glide, keeps the oil in the proper temperature range so that the engine oil’s viscosity can do its job. (Kevin Wing /)

Let’s not kid ourselves. When oil cooling is necessary, the job is keeping oil in the temperature range in which its viscosity can carry engine loads. Putting the cooler in a desk drawer “because it spoils the line” may cut it at art school, but it’s poor physics.

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