Cylinder-Bore Ruminations

motorcycle piston and cylinder bore

Yes, in the case of engines whose cylinder block is cast in unit with the upper crankcase, a destroyed bore would require crankcase set replacement. In-line four-cylinder engines with separate cylinders (like Kawasaki's classic Z1 series) suffered base gasket leakage and failure when operated at rpm levels that caused crankcase flexure (An in-line four is "two dogs fighting in a sack" at high revs–each twin tries to rock back and forth, bending the crankcase back and forth, causing the cylinder base and crankcase mating surfaces to scrub against each other). Therefore in some sportbike engines a cast-in-unit cylinder block is used to increase the bending strength of the crankcase. Honda discovered this with its first four-cylinder race engine, the RC-160, back in 1959. It peaked at what now appears moderate revs–13,500. Every 600cc sportbike engine peaks higher than that!

The forces pushing design toward hard-plated bores and away from iron cylinder blocks or liners are powerful. In the case of motorcycle engines, the liners are reckoned to be responsible for 6 to 7-pounds of non-essential weight. In addition, with in-line motorcycle engines the use of hard-plated bores has allowed cylinder-to-cylinder spacing to be reduced, making engines narrower, more compact, and lighter. Bore life has benefitted from several things: (1) engine-mixture control has improved tremendously, preventing engines from running overrich during warm-up, leading to bore washing. (2) Not many things are harder than the 5-percent loading of silicon carbide particles that are in Nikasil. (3) the use of powerful anti-wear additives in engine oil had by the late 1960s greatly reduced the "wear ridge" that used to be worn into iron bores at the top of ring travel.

There was a furore some 20 years ago about rapid wear of Nikasil bore in some German auto engines operated on high-sulfur fuels, but that problem has disappeared.

In racing, I have seen chrome-plated bores wear through to the aluminum at TDC (Slide the tip of a steel scriber lightly along the bore. Where there is chrome, it glides easily. Where soft aluminum is exposed, it stops). I have never seen Nikasil worn through in the same manner, which is why it has replaced chrome as the hard-plating of choice.

Another pressure is that pushing toward lower fuel consumption and therefore lower engine friction. The harder the bore plating, the less likely becomes momentary micro-welding of ring to bore, with subsequent movement plucking wear particles from them. Permanently textured surfaces such as Nikasil (and there are others now) continue to be wettable by oil, leading to reduced friction. Under pressure from their governments, automakers adopt whichever affordable technologies allow them to reduce friction, and Mahle Nikasil is one of them.

A couple of summers ago I fitted an iron liner to a 175 two-stroke Kawasaki (its original bore was hard-plated), which has since done many hours of casual running. But when two-stroke racing engines were fast gaining power in the 1960s, they outgrew the ability of iron to keep their pistons cool. First bore hard plating and then water cooling allowed pistons to survive rising power. At the end of two-stroke GP racing, engines cooled so well that you could jet down until the engine slowed down–no seizures. Part of the reason for that capability is cylinder hard-plating, making it possible to remove the "woolen blanket" of an iron liner.

Iron, however, remains perfectly workable at the power levels of an earlier time.

I agree that plated bores reduce our options. After making many steel two-stroke racing exhaust pipes, I felt a similar sense of loss when factory teams switched to titanium pipes that looked as if made by The Great Creator (they were welded in glove boxes in an inert atmosphere). Changes like these make us dependent upon special processes that may be out of our reach, whereas in the old days a shop with boring bar and hone could refinish a cylinder in-house.

Another effect is that as manufacturing shifts to a new process, it finds or creates new economies. As an example, when digital fuel injection first appeared, people criticized it for its expense, and feared that "black-box failure" would strand them far from home with no hope of on-the-spot repair. Yet at that time carburetors had grown to amazing complexity with a great number of fuel circuits and mixture-correction systems, etc. As soon as fuel injection was tooled, the average throttle body contained less than 10 percent of the parts count of a carburetor.

Today, when I go out in sub-zero winter weather to start my econo-box, I sometimes think of the time I spent rebuilding Holly, Rochester, or Stromberg carburetors–of making sure not to drop tiny springs or check-balls, of bending stamped parts to templates in the rebuild kit to ensure proper choke pull-off during engine warm-up. Hooking up the warm-up line to the choke stove on the manifold (if it hadn't rusted completely away!). Then I turn the key. The engine cranks and fires. The stepper-motor controlling idle speed quickly moderates the initial fast idle as the engine warms. Mixture is controlled, not by furiously trampling the gas pedal as a pump, but by unseen sensors and software. I drive away–without coughing, spitting, or having to become for the moment the engine management system.