Building a Modern Motorcycle Engine


In the early 40s, engine cylinders were rough-bored in iron or steel by a single-point tool carried by a precisely supported rotating boring bar. They were then finish-ground by an orbiting, fast-spinning grinding wheel. Later, internal grinding was replaced by honing, a process that rotates an array of abrasive stones, pressed outward against the cylinder bore. The hone is moved up and down inside the cylinder as it rotates, producing a crisscross pattern of fine scratches.

This pattern, its lines typically intersecting at about 60 degrees, has several useful functions. First of all, in a new installation, the hone pattern acts as a fine-cut file, which during break-in shaves the piston ring into an accurate fit against the cylinder. Second, the existence of surface micro-roughness enables oil to wet and so lubricate the cylinder. And third, the hone scratches provide convenient 'ditches', into which the debris of any small, incipient local seizure can be harmlessly ploughed, restoring normal sliding.
Experienced engine builders are very interested in cylinder sealing, for any leakage of combustion pressure is a loss of power. Examination of cylinder bores that had been run reveled that making shop to a tolerance of .005-millimeter was not enough. It had also to be round and straight when assembled in the engine.

Ambitious builders created hardware that would both mimic the stress of head and main bearing saddles, bolted into place, but would also allow passage of the cylinder hone. Cylinders honed in this way were an improvement, but it wasn't yet enough. Running engines have a temperature distribution quite different from that found in a machine shop.

The next step was to circulate hot water through the coolant passages in the cylinder as it was honed. An improvement, yes, but not yet enough, for combustion heats the top of the cylinder much more than it does the bottom. Therefore some builders also 'choke' their cylinders, allowing them to taper very slightly near the top, such that in actual engine operation, that extra temperature at the top of the bore will expand it to a true cylinder.

In race engines, some designers have returned to a very old practice as a means of entirely avoiding the very large, distorting stress created by cylinder head bolts - screwing a separate cylinder liner into the head. AS a rule of thumb, peak, full-throttle combustion pressure in a high-performance engine, in atmospheres, is 6.7 times the compression ratio. Thus, in an engine with a 12:1 compression ratio, we may expect a peak combustion pressure of roughly 80 atmospheres.

Another of those rules says that the head gasket clamp load should be four times the pressure that combustion exerts on the head. In the case of a 100mm bore cylinder, combustion tries to lift the head with roughly 7000kilo of peak force, requiring four times that, or 28,000kg, of head bolt clamp force to keep the gasket in place. At the same time that our engine must withstand this very considerable force, it must also be light enough to find ready sale in a highly competitive market. Therefore cylinder distortion is a serious matter and engine designers who seek to relieve their engine structure of such forces are displaying common sense.

Engineers understood that as strokes have become very short and valve included angles very small, it has become practical to cast cylinder and head in one piece - and still be able to machine valve seat recesses directly, through the short cylinder. This eliminates not only the head bolt clamp load involved in merely holding the head and cylinder in place.

In the mad rush to produce this year's quickest 600, 100, or what-have-you, the lighter engines are made, the greater the distortions imposed by bolting as an assembly method. This drives a search for means of achieving precise parts alignment and dimension by seeking to eliminate any forces on engine structure that are not absolutely necessary.

In two-stroke engines, pistons run hot for they are exposed to heat twice as often as in four-strokes. This made the sluggish heat transfer of the otherwise very durable iron cylinder liner intolerable. An alternative layer of chromium directly onto the aluminum cylinder. This cut piston temperature and saved weight, but the chromium surface had to be patterned by some means to render it oil-wettable.

A more durable cylinder coating is Nikasil, a thin layer of nickel containing tiny particles of extremely hard silicon carbide. When finished, the diamond-honed surfaces of particles resist wear, standing slightly proud of the nickel, and thereby providing a surface texture that oil can wet. As racing and sport four-stroke engines were made lighter and harder-working during the 90s, they too could no longer afford the three or more extra kilos weight of iron cylinder liners. Plated-bore aluminum cylinders cannot be casually rebored, as in classic times, so cylinder damage on a multi-cylinder block requires replacement of the unit.

There is evidence that super-hard cylinder walls reduce not only wear but friction as well. As piston motion slows near TDC, piston rings may squeeze out their lubricating oil films, coming into direct contact with the cylinder. If ring and cylinder differ greatly in hardness there is less welding of the one to the other, less power is consumed in breaking suck welds, and less material is lost as wear particles.
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