Exploring the Lost Language of How Engines Used to Fail

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Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

Today it’s rare to hear of engine blow-ups, but they were once a natural part of the motorcyclist’s experience. These days most causes of engine failure are either detected by sensors and made fail-passive by the ECU or are prevented altogether by improved materials.

It wasn’t always this way. In 1970 two tired-looking fellows walked past me in the Talladega infield, and I caught this snatch of conversation:

“We brought down seb’m motors for ‘im an’ ‘e’s already gone through fahve of ‘em…”

In that vein, we here present a review of language that was once common, yet seldom heard today:

Tied Up: This means seized; a piston which became hot enough from abnormal heat expansion to outgrow its bore. That overloaded the lubricant, allowing metal-to-metal contact, scoring, galling, and finally seizure. Riders of the two-stroke racebikes of 1965–1990 snatched in the clutch, pulled off the line, and hoped the engine would “free off” so they wouldn’t have to walk back.

Threw a Rod, or worse, Holed the Cases: Modern engines have rev limiters to mitigate their rider’s bad habits, like high-rpm downshifts or holding first gear too long because the shift pedal is on the ground. But in olden tymes a BSA 500 Gold Star (a universal motorcycle from the late 1940s to mid-’60s) could blow up comprehensively; the connecting rod could break near its small end, and the abbreviated shank would then stab holes in the crankcase as the momentum in its full-circle crank wheels carried it through several more very destructive rotations. One big-single rider in a long-ago Isle of Man TT suffered such a biggie and was treated to the sight of the crank hurtling ahead of his bike after bursting out of the crankcase altogether.

Spun a Bearing: When split-shell steel-backed plain bearings began to replace rollers in con-rod big ends and crankshaft main bearings, a common failure mode began with air getting into the oil pump; early Superbikes had shallow, flat sumps in which oil could surge away from the pump’s pickup. A gulp of air arriving in a hard-working plain bearing can blow oil out of it, allowing an instant of journal-to-bearing-shell contact that seizes the shell to the rotating journal, the former then spinning with the latter. Never mind the bearing’s careful installation with the right amount of “crush” or the little upset tangs intended to prevent rotation; when a bearing shell seizes to and turns with the journal, that’s a “spun bearing.” It doesn’t do much good for the crankcase saddle where the bearing was once seated. Designers quickly abandoned flat sumps for much deeper ones whose shape guaranteed that oil-pump pickups were reliably submerged. The “display wheelie” is a favorite way to induce this kind of failure—balancing on the rear tire until the oil-pump pickup “got air.”

Dropped a Valve: The classic air-cooled Triumphs and BSAs of the 1950s and ‘60s had overhead valves operated by pushrods and rockers. The English makers reckoned it took just about double the valve-spring pressure to make an OHV engine reliable at the same rpm as a previous side-valve model. In the old side-valve machines, such as the Harley KR, the valve itself made up most of the weight that had to be controlled by the valve spring. That made it pretty easy to keep the valve following the cam profile rather than “floating,” the phenomenon that occurs when the cam lobe at high rpm moves too fast for the spring’s stiffness.

But adding the extra mass of a pushrod and rocker arm required stiffer springs. Back in the 1920s, when OHV was first taking over from side valves at the Isle of Man races, pushrods and rockers operated in the open, giving the rider’s boots a perpetual coating of oil. A too-enthusiastic downshift could liberate a pushrod, so TT riders on OHV bikes carried a couple of spares down one boot top and a spring depressor down the other. It took but a moment to fit a new pushrod and push off again.

Once all those moving parts were enclosed, a departing pushrod could no longer act as a crude rev limiter, warning the rider to pay more attention to the tach. This led to valve float, sometimes culminating in contact between the exhaust valve and the piston. Legendary Italian auto racer Tazio Nuvolari, who got his start on bikes, is claimed to have said, “There is only one speed—that at which the valves bounce hard off the pistons.”

Valves, as hot as they operated in air-cooled engines, didn’t take to this. The vulnerable point was where the stem flared to form the head of the exhaust valve. Once the valve head broke off the stem, nasty ping-pong took place between head and piston with the valve head acting as the ball. Velocette’s racing manager Harold Willis, having seen many a valve head hammered edge-wise into a heat-softened piston crown, called it “penny-in-the-slot.”

Even today, one important step in building up a race engine is to check for adequate minimum exhaust-valve-to-piston clearance as the piston approaches TDC at the end of the exhaust stroke. If the exhaust valve fails to keep up with the cam lobe, or if the valve bounces off its seat one or more times during closing, it can be bent or broken by the piston.

Stuck a ring. Holed a piston. Dropped a valve. There are many terms to describe the myriad of failures possible in an engine, yet these terms are heard less often these days. (Mark Lindemann/)

Stuck a Ring: Again, this was most often seen in the air-cooled era. Four-stroke pistons typically have three piston rings each—a top ring to do most of the combustion-gas sealing, a second ring that provides backup sealing as well as a valuable extra path for heat to flow out of the piston and into the cooler cylinder wall, and one oil-scraper ring to keep engine oil out of the combustion chamber. Some race-only engines have just two rings (to reduce ring friction).

The top ring, being closest to combustion, operated at Thanksgiving-oven temperature. If for any reason the top ring ran too hot (hot weather, chronic detonation, over-optimistic engine tuning), the tendency of oil to oxidize—first into gum and then into hard varnish—could immobilize the ring in its groove. Thus robbed of its compression, the engine might continue to run, but only weakly.

Holed a Piston: Another oldie-but-goodie or, as Yvon Duhamel’s tuner Steve Whitelock had it, “Piston, golf-type” (hole in one). Today no one speaks of spark-plug heat range any more because no one ever sees a spark plug. But 40 and more years ago, when there was no benevolent electronic genie living inside your engine, racers and others checked fuel mixture by pulling the plugs and “reading” them for their mixture indications. If you accidentally replaced a spark plug of correct heat range with a “hotter” plug, its spark-gap electrodes just might run hot enough to act as an ignition source on their own. This was preignition, usually occurring close to bottom center after the intake stroke. Because such preignition forced your engine to compress expanding hot gas it overheated the piston in a cycle or two. What part of the piston would heat fastest? Why, the part farthest from help—the center of the dome! And that’s where the heat-softened metal would buckle inward. Holed a piston.

Or, if your engine began to detonate (an abnormal form of combustion other than preignition, leading to sudden pressure spikes and overheating) your piston could soften in the heat, combustion pressure forging down and/or breaking the top ring land. With the top ring no longer sealing, the action would then shift to the next ring down, eventually beginning to send jets of flame in through the oil-scraper ring’s drain-back holes. At any point in this process the piston could fail completely. One possible “exit strategy” was that the terribly overheated piston now seized in its bore, sometimes hard enough that crank inertia pulled the wrist pin right through its bosses.

My dad was in book publishing all his life and remained married to the same woman for 62 years. He once said, quite casually, that “Most novels could have been prevented.” That is also true of engine failures. As in the human case, there is tremendous drama in failure, and compelling stories can result. Human drama continues unabated. But rev limiters, detonation sensors, oil pressure monitoring, improved design, and better materials now prevent most engine histrionics.

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