Piston Ponderings

What will some future mechano-archeologist think when he or she unearths a piston while excavating the ruins of my shop? Is it some sort of religious relic, the default catchall for countless other objects we don’t fully understand? A tribal talisman perhaps? Let’s consider what we already know.

Piston power isn’t new. In fact, it predates the internal combustion engine. In a classic steam locomotive, each piston (there were typically two, one on each side) took the form of a simple disc mounted on the end of a round piston rod. The rod passed through a gland seal, allowing the engine to operate in double-action mode; steam was admitted alternately, ahead of and behind each piston, pushing first in one direction and then the other, giving two power strokes per revolution per cylinder.

To carry the heavy connecting-rod side forces (caused by con-rod angle), two slide bearings were located behind each cylinder, guiding the “crosshead” (imagine the letter “H” flipped on its side) at the rear end of each piston rod. Such crossheads are still employed in the giant 100,000-horsepower two-stroke diesel engines that propel container ships. While heavy, the extra weight of crosshead and piston rod are no handicap in such engines, since they typically develop peak power at 80 rpm.

They were a handicap, however, in the faster-spinning internal-combustion engines which began to emerge in the last decades of the 19th century. These were The Next Big Thing after steam, just as electric vehicles appear to some today.

The result was the so-called “trunk piston,” which combined the functions of both cylinder sealing and managing con-rod side thrust into the piston itself. I suspect now we’re getting into more familiar territory for some readers. Those original thin-disc steam pistons couldn’t carry side thrust; to enable these new internal-combustion pistons to do so, they were given a long cylindrical skirt that did a fair job standing in for a crosshead. That is, provided the piston and skirt: 1) received adequate lubrication, and 2) did not run hot enough to reduce the oil’s viscosity to the point where it was incapable of supporting the load.

This is why the pistons in my forty-dollar 1940 Chevy six looked like little upside-down pails or soup cans, cylindrical in shape, with their gas-sealing rings and an oil scraper ring located just below the flat piston crown. The top part of these pistons, the crown and seal rings, accomplished what the steam engine’s simple disc piston did. Meanwhile, this new long skirt section below (typically longer than the bore diameter) provided the bearing surface area, standing in for the steam engine’s crosshead.

Joining the piston to the connecting rod was a wrist pin. This passed through cylindrical inward extensions of the piston skirts, the wrist-pin bosses.

When I first disassembled the engine in our family lawn mower, a Briggs & Stratton single-cylinder flathead or “side-valve,” the first of many engines I would come to scatter in my time on this earth, that’s just how its piston looked. The same when I pulled the cylinder from a friend’s 500 Triumph twin and the one from my own AJS 500 single.

But I had been reading, and the contemporary motorsport magazines educated me: Modern engines ran pistons with shorter skirts. There was even something called a slipper piston, a shape quite new to me. These addressed two bothersome problems. One, the heavier the piston, the greater the vibration. Two, attempts to lighten pistons by making them thinner resulted in skirt cracking.

In a typical motorcycle engine, connecting-rod angularity drives the piston hard against the rear cylinder wall during the power stroke, and against the front cylinder wall during the compression stroke. Knowing that, designers realized that the sides of the piston need not touch the cylinder all at all. So, leaving enough arc-shaped skirt at the front and back of the piston, they joined those arcs with straight webs, one per side; viewed from the underside, the piston now was roughly box shaped. The design braced the thrust and anti-thrust sliding faces of the piston, and it saved weight because the shortest distance between two points, at least in our universe, is a straight line. It was through this area that the wrist pin passed.

Motorcycle engines started using these box-skirted “slipper” pistons widely in the later 1920s and ’30s. When England’s Bristol Aeroplane Company tired of the poor-quality cast pistons in its Jupiter radial aircraft engine cracking, it switched to forgings. While the five-piece permanent molds often used to produce cast pistons can produce elaborate internal shapes, a forging die must pull straight out of the piece. This made it impossible to remove the extra metal between the tops of the wrist-pin bosses and the piston’s crown.

Why not just leave it there? Let me count the ways:

One approach, taken by Kawasaki in its 750 H2-R roadracer and by Wright Aero in its 18-cylinder R-3350 aircraft engine, was to reach up into the piston’s underside after forging and machine away the extra material.

Bristol looked at the problem from the question of weight. In pistons whose wrist-pin bosses were part of the skirt, the pin had to be nearly as long as the cylinder bore was wide. But if the piston had a narrow “slipper” box shape, the wrist pin could be considerably shorter, and a fair chunk of extra piston material machined away around the outer ends of the wrist-pin bore. To isolate the wrist-pin bosses from heat coming down from the piston crown, we’ll just locate the pin lower in the piston. In the mid-1960s, Yamaha also adopted the low wrist-pin position for its two-strokes.

Piston side thrust is still very real, but it no longer endangers the engine thanks to liquid-cooling and better lubrication. This wasn’t always the case. In the 1920s, as each entrant in the Isle of Man TT roadraces made his push-start, he knew he must go easy at first because of the danger of piston scuffing before the oil reached operating temperature. Engine builders provided a special “Brae Hill lubricator,” tapped into the oil pump and discharging through the rear (thrust side) of the cylinder, in hope of forestalling scuffing and even seizure. And yes, we’re talking about four-stroke engines here.

In the 1920s, while trying to reduce overall engine diameter in their air-cooled aircraft radials, the US Army had used very short con-rods, and their greater angularity increased piston side thrust. The combination of air-cooling (hot cylinders and hot pistons) and increased side thrust caused these pistons to scuff and seize; you could see those cylinders visibly turn blue from frictional heating. And yet when designers in the Supercar era chose to give the Olds 455 V-8 similarly short con-rods they worked just fine. These folks were trying to make that big piece of iron fit between the front suspension structures; liquid-cooling made the difference.

By 1945, after the vast engine experience of World War II, aircraft powerplants incorporated oil jets aimed at the pistons to push temperatures down and reliability up. The greatly respected engine builder and cam grinder Tom Sifton either knew of this or invented it independently, and his oil jets eased distortion problems in the Harley-Davidson flathead racing engines he prepared for the legendary Joe Leonard. Today nearly every motorcycle engine, whether touring, enduro, or sportbike, ensures pistons durability by holding their temperature in the easy-to-lubricate range. Such oil jets are often part of the solution.

So long as motorcycle engines remained air-cooled, their pistons had to retain many antiquated features, such as long, full skirts. Why? In such engines the only cooling pistons received was by contact with the cylinder wall and from the small amount of oil thrown off by old-time rolling-element crankshaft- and con-rod bearings; the more piston contact there was with the cylinder, the more effectively the piston could rid itself of combustion heat. I learned this from Udo Gietl, who was at the time building air-cooled BMW Superbike roadracers. Driveshafts and all, his machines were fast enough for Reg Pridmore to win the AMA championship.

With better piston cooling and oil additives that slow oxidative gumming, the piston’s ring pack has moved closer to the hot crown. Higher engine speeds have meant thinner, lighter sealing rings. Their reduced inertia helps keep them from jumping up off the bottoms of their grooves as the piston decelerates toward TDC. Following the rings in their migration north has been the wrist pin: The higher an engine revs, and the more crucial it becomes to get rid of every unneeded gram of piston weight. Vibration! Power loss from excessive bearing loads! In some recent pistons, the wrist pin has moved so high that you’ll need to remove the (bottom) oil scraper ring to slide the short wrist pin out of its bosses.

Who cares how high the ring pack is? Here, this old air-cooled Ducati still runs just fine, and its top ring is 16mm down from the piston crown. So what? In today’s world, production engines must meet standards for unburned hydrocarbons (UHC emissions). Compression and combustion can push quite a volume of unburned mixture into the ring-land clearance and piston-ring crevice volumes. There it hides until the piston descends and falling pressure lets it come shooting back out to mix with the exhaust gas. Out the port it goes, and into the atmosphere. The higher you can move the rings, the smaller this space there is, and less UHC can come from this source. Pollution aside, even racers would prefer not to waste 2 percent of every cylinder charge in this way.

With cooler pistons and plenty of oil, kept out of the combustion chamber by highly capable three-piece oil rings, even short, narrow thrust and anti-thrust skirts can carry the load.

But there’s always a trade-off. Such short pistons tilt more in the bore, and this affects ring sealing. That calls for “barrel-faced” top rings. Not only do they make good contact with the wall despite some degree of piston tilt, they also offer faster break-in because their initial contact with the wall provides the high loading that break-in requires.

Now, here’s an age-old question: Should pistons be cast or forged? The answer, of course, requires deliberate thinkage. Each production method has its attractions. Forgings have high fatigue strength, but castings (found in most two-strokes) offer higher hot strength. Forgings bend; the ring lands on my 1970 Kawasaki H1R racer slowly deformed downward in use. Castings fail by cracking, as we found when rebuilding the engine in our Ford race van, where fully six or seven of the eight pistons had cracked. Today, heavy-duty four-stroke applications usually employ forged pistons, but Honda’s ultra-high-rpm air-cooled GP bike engines of the classic 1960s evidently needed hot strength more than fatigue strength. The choice fell to cast pistons.

As those early long, cylindrical “pails” have morphed into today’s short, flat “ashtrays,” piston weight has decreased, making lighter connecting rods possible. Decreased back-and-forthing mass has reduced shaking forces, and that’s allowed crank counterweights and bearings to grow smaller. Greatly superior piston materials have been developed; all that’s needed is for their cost to drop within reach of consumers. Each improvement is quickly exploited to its limit, causing a constant need for fresh ideas and new materials.

Or maybe the long-awaited SuperBattery—cheap, fast-charging, and energy-dense—will arrive in a magical rush, and our pistons become laughable archaeological curiosities. If so, then we can always repurpose them as ashtrays in fact and not just in appearance. But, of course, that’s only if people continue to smoke as well. Who knows what the future holds?