January 26, 2022

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Cylinder Bore Surfaces

engine cylinder bore surface

Back in the supposedly good old days, every auto engine had a cast-iron cylinder block and every motorcycle engine had cast-iron cylinder liners. What was supposed to be good about those old days was that when your cylinder bore wore out, it could be rebored .010” or .020” oversized and new pistons fitted, thus restoring original function.

Most bore wear takes place at the top of piston ring travel, and for three reasons;

۱) Piston rings are pushed out against the bore surface by combustion pressure.

۲) The piston moves very slowly near TDC, allowing more time for any oil film between ring and cylinder wall to squeeze out, possibly resulting in metal-to-metal contact.

۳) Because the top of the bore is its hottest part, any oil present there loses a lot of viscosity to this high temperature.

This was tolerable because car engines didn’t work very hard. They were big and slow-turning.

But in the two-stroke motorcycle engines that were being developed after 1960, conditions were much more severe–mainly because two-strokes fire every time a piston comes to TDC–not every other time as in four-strokes. Also, two-strokes had exhaust ports in their cylinder walls, so the supersonic outrush of very hot exhaust gas vigorously heated the piston crown and the port itself. With air cooling, there were limits to how fast this heat could be conducted away to cooling fins. We call this “cooling” but it was only comparative; if you happened to put a hand on the fins of an engine just in off the track you would instantly understand the short life of a burger on a grill.

Two-stroke pioneer Walter Kaaden responded to pistons that “swelled like a cake” and cylinders distorting by trying to overcome the problems mechanically. He provided a thick (1/4”!) iron liner inside the finned aluminum cylinders of his MZ race engines.

In the early 1960s, Japanese manufacturers Suzuki and Yamaha reasoned differently. Just as W.O. Bentley had found with the first aluminum pistons in 1911, aluminum can conduct heat so much faster than iron that it can remain relatively cool in conditions that make iron much hotter. Therefore the Japanese approach to improved cooling was to eliminate the poor heat conductor–iron–by somehow hardening just the surface of an aluminum bore.

Yamaha tried hard-anodizing cylinder bores (converting them to the ceramic aluminum oxide) but had indifferent results. Suzuki had just begun to test with hard chromium plating when new information arrived through the defecting East German MZ rider Ernst Degner; “No, no! Stop all that nonsense with chrome plating and instead adopt MZ’s thick, stable iron liners!”

And so Suzuki were delayed in solving this critical problem. Yamaha, with fast-improving chrome-bore technology, took two 250 World Championships over Honda in 1964 and 1965. The cooler you can make the pistons run, the more torque you can make with higher compression ratio. Conversely, the hotter the piston runs, the more it raises the temperature of incoming fresh charge, pushing it towards detonation.

Out in the “real world,” oil additives were proliferating. Of great importance were anti-wear additives. At points of local high pressure and temperature (such as between piston ring and cylinder wall near TDC), an anti-wear additive reacts to form a soft metal phosphide. Instead of local micro-welds between ring and cylinder wall, or between cam lobe and tappet, being broken to release wear particles, it is the phosphide which is sheared. If on the next cycle the process repeats, the soft layer is renewed from additive carried in the oil. In this way, wear was greatly reduced.

When I took apart 1940s auto engines, the wear ridge at the top of piston ring travel was so deep that it took considerable forceful rocking to get the pistons out. So-called cylinder ridge reamers were made to cut away the unworn material above the ring travel. But when I examined later engines, their wear ridges were much shallower; oil additives work!

In the emissions era, engine design changed rapidly. Because smaller engines use less fuel in highway cruise than great big ones giving the same horsepower, engines were quickly made smaller, and aluminum often took the place of iron in cylinder blocks and heads. But to retain the performance still important to sales, those smaller engines had to work ever harder. On two wheels, the “sportbike era” drank deeply of that stimulating beverage, horsepower. In both cases, piston temperatures rose and durability problems appeared.

The obvious answer was to improve heat flow from pistons to cylinder walls by applying the two-stroke solution–hard-plating. Some first tries were unimpressive, such as the Alloy 390 process used in some Chevy Vega engines (the block was made of a high silicon aluminum alloy, and the aluminum was etched back after bore finishing to leave a wear surface that was mostly hard silicon particles).

On the motorcycle side, engineers felt depression coming on as they stared at four great, thick, heavy, heat-insulating iron liners on a kilogram scale and yearned to scrap them and their excess weight (every year’s new model had to be lighter and more powerful!). But they knew that the old-timers out there would complain that now the only way to refinish worn cylinder bores was to install a new cylinder block (which in some engines was cast in one piece with the upper crankcase).

In two-stroke motorcycle racing, chrome-plated cylinder bores gave way in the 1980s to Mahle’s Nikasil, which is a layer of electroless nickel containing 5-percent super-hard silicon carbide particles. When diamond honed, this produced an oil-retaining textured surface that was extremely resistant to wear.

Now high-end automakers began to adopt aluminum cylinder blocks with Nikasil bores, and experience accumulated rapidly. The combination of oil anti-wear additives and hard-plating with Nikasil worked well.

In motorcycle engines–especially those designed for racing–bores were becoming larger and strokes shorter. Bigger pistons mean heat must travel farther from the hot center to the cooled cylinder wall. Rising rpm nixed the idea of improving piston heat conduction by just making their domes thicker. Cooler-running hard-plated aluminum cylinder bores were an important element in cooling such bigger pistons, supplemented later by use of upward-directed oil jets in the crankcase to carry away even more piston heat.

Necessity is the mother of invention.

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