We Expect Today’s Motorcycle Engines To Last Almost Forever

The engines in Top Fuel dragsters are rebuilt every five seconds, but the big fan-jet engines on today’s commercial airliners are now “on the wing” for 40,000 hours before removal and shop overhaul are required.

When we were running Yamaha TZ750 two-strokes in national roadraces, we figured a set of new crankshafts—each engine had two—would go 950 miles before needing to be rebuilt. Failure could take three basic forms: a) fatigue cracking of one of the flywheel discs; b) surface damage to the heavily loaded rollers in con-rod big-end bearings, leading to seizure; or c) crushing of the case-hardened surface of the outer crank main bearings’ inner races.

Such a short life would be unacceptable to riders of street motorcycles. The highest mileage I could document for a two-stroke streetbike crankshaft was 50,000. That was quite acceptable in 1971 because dealer cost for a new crank was less than $100. Most owners then never came close to such mileages; a seized bike would arrive on a truck at our dealership with its oil-injection tank always mysteriously full to the top.

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Why the difference in crank life between track and street? Bearing loads are roughly proportional to the square of rpm, so the 9,500-to-11,000-rpm operating range of the race engine imposed harsher loads than did the 3,000 to 8,000 revs used on the street. And there are no policemen on the racetrack.

The design features of engines that have been in production for many years are instructive. Harley-Davidson’s first Big Twin, the EL Knucklehead of 1937, had heads and cylinders of cast iron. Because most US roads then were two-lane and dirt-surfaced, opportunities to go fast were few; New York State’s 105-mile Taconic State Parkway was one of the nation’s first four-lane roads and opened in 1932. That made the EL’s claimed 37 hp plenty for riders of that time and cast iron perfectly adequate to deal with the engine’s waste heat.

After World War II’s strict rationing of gasoline and tires ended, a huge number of Americans took to the highways. More and more roads were paved and then came “the interstates,” aka the National Defense Highway System. As motorcyclists could ride faster, smoother roads, motels and roadside restaurants made it convenient to ride farther and engine duty cycle became more severe, generating higher operating temperatures. As the ability of iron to transfer engine heat to the surrounding air via cooling fins fell behind, piston temperatures rose, ring sticking—detergent oils were just coming into use in the 1950s—became more likely, and piston expansion and higher oil temperature increased the possibility of piston and ring scoring. Valve materials that had been entirely satisfactory before could now become distressed.

Everything necessary to uprate those engines—aluminum cylinder heads with shrunk-in hard valve-seat rings, more temperature-tolerant materials for exhaust valves, and iron or steel-lined aluminum cylinders—had been thoroughly researched for large air-cooled aircraft engines during the war.

The process of change was gradual: First the cylinder heads were switched to aluminum, allowing them to run cooler, making the abnormal form of combustion called detonation less likely. I have a magazine from the period containing a Harley-Davidson ad announcing exhaust valves made of Nimonic 80A, a material developed for jet-engine turbine blades during WWII.

“Head out on the highway. Lookin’ for adventure. And whatever comes our way…” For generations of motorcyclists, a national network of highways made riding to faraway places possible, pressuring manufacturers to produce more powerful and reliable machines. (Harley-Davidson/)

The Evolution engine of 1984 attacked the problems of gasket failure and oil leakage by holding down “the stack”—the heads and cylinders—by means of long, highly elastic through studs. Being more elastic than the previous short base and head bolts, they maintained more constant clamp loads on head and base gaskets. Since all parts in the stack were now aluminum, there was no longer the problem of differential heat expansion, causing an aluminum head to “scrub” against an iron cylinder, gradually wearing out the gasket.

The combination of warranty claims and factory testing identified problems, leading to a steady flow of durability upgrades and periodic redesigns. Those redesigns bear the names known to Harley owners: Panhead, Shovelhead, Evolution, Twin Cam (or “Twinkie”), and today the four-valve-per-cylinder Milwaukee-Eight.

Every make of motorcycle is similarly backed by its factory’s testing methods. Former American Honda race manager Gary Mathers described a 2,000-hour test that every new Honda design was required to pass. I have seen Ducati’s “40 miles of bad road” chassis tester in action, which horribly abuses chassis and suspension to find problems before buyers do. Racing engineer Erv Kanemoto saw Yamaha technicians drop-testing complete motocross bikes, just as wartime Navy fighters were drop-tested to establish durability to withstand repeated hard carrier landings. In motorcycling, this is just good business because happy customers become repeat customers.

In times past, numerous riders were dissatisfied with factory performance and turned to big-bore kits and other aftermarket soup-up goodies. Some experiences were good but, unsurprisingly, some builders found their projects short-lived. Consumers of hot or “big” cams could expect shortened valve-spring life for the same kinds of reasons as those that gave us only 950 miles from a TZ750 crank set, a duty cycle of higher severity. Riders of restored classics such as Triumph’s storied Bonneville cannot expect to routinely commute week in and week out in 85-mph rush-hour traffic; machines originally developed for country lanes have little margin left in today’s traffic. Yet when I took apart a recent 26,000-mile 600cc sportbike engine, I found almost no wear anywhere and bright unmarked friction surfaces. That’s because it was developed and proof-tested to withstand the harsher conditions of modern highways.

Today, we see fewer “little deuce coupes” and hot-rodded motorbikes. When, in the 1960s, Detroit saw that building more exciting cars could be profitable, it offered factory hot rods that are now sought after by muscle-car fanciers. The same factory re-engineering for high performance hit motorcycling in the 1980s, as big, heavy, and slow-turning 1,000cc air-cooled engines gave way to equally powerful but much lighter, more durable, and higher-performing liquid-cooled 600s and 750s. The technologies developed in this process have since been applied to motorcycles of all kinds, giving them the great engineered-for-the-conditions durability we have come to expect.

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