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I’ M not going to teach you how to overhaul your motorcycle. Most modern motorcycles are too complicated for you to do much more than change the oil yourself, but you will need to become familiar with the essential parts of a motorcycle and how everything works together. If you already know these things, you might want to skip ahead to the next section, though it can never hurt to brush up.
    The engine, of course, is what puts the motor in motor cycle. Engines come in two basic types: four-stroke and two-stroke. Two-stroke engines haven’t been used much in the United States over the past several decades because of emissions standards. They’re called “two-strokes” because every two strokes of the piston comprise one complete cycle. The piston goes down and draws in the fuel charge; it goes back up and fires the fuel charge. Two-strokes are simple engines that don’t have internal oil-lubrication systems. Some of the oil lubricates the inside of the engine, and the rest is burned with the exhaust, which is why they pollute so much. The last full-sized street-legal two-stroke motorcycle sold in the U.S. market was Yamaha’s RZ350 from the mid-1980s.
    For several decades two-stroke engines dominated Grand Prix motorcycle racing because the engines are light and generate twice as many power pulses as a four-stroke engine, but they’ve been phased out over the past decade. In 2002 the top class switched from 500-cc two-strokes to 990-cc four-strokes, and in 2009 the 250-cc two-stroke class was retired, to be replaced by a 600-cc four-stroke class for the 2010 season. That leaves just the 125-cc class as the last of the two-stroke road racers.
    But because two-stroke street bikes are too old and too small to be used as practical transportation, we won’t be discussing two-strokes in this book. The day may come when we’ll ride around on electric motorcycles powered by hydrogen fuel cells, but for the foreseeable future we’ll be riding motorcycles powered by four-stroke gasoline engines.
    The basic systems of a four-stroke engine are the bottom end, the cylinder block, the piston, the cylinder, the combustion chamber, the cylinder head, and the fuel intake system.
    The Crankcase
    The crankcase is often referred to as the “bottom end” because it’s located at the bottom of almost every engine (though it’s at the center of opposed engines like those found on a BMW twin or a four- or six-cylinder Gold Wing—I’ll explain that later in this chapter). It consists of a crankshaft that rotates in a series of bearings. This rotation carries through the clutch, transmission, and final drive system, until it becomes the rotation of your rear tire on the pavement, which is what makes your motorcycle move down the road. Piston rods connect the crankshaft to the pistons.
    These days most motorcycles are so reliable that if you regularly change your engine oil, you can ride for hundreds of thousands of miles and not give any thought to the bottom end, but that wasn’t always the case. Before we had the advanced oils, oiling systems, and bearing materials we have today, spinning a bearing or throwing a rod was a common occurrence. These are catastrophic failures that can result in internal parts of the engine exploding through cases and cylinder barrels and becoming external parts. This can be a little like a grenade going off between your legs, so it’s a very good thing that modern bikes have such reliable bottom ends.
    To be fair, some of the methods we used to rely on for hot-rodding our engines, like “stroking” them (this refers to the practice of installing a different crankshaft that increases the length a piston travels up and down in the cylinder, effectively increasing cubic inches without making the cylinder itself any larger), improved performance, but they also put more stress on the parts and increased the likelihood that the engine would grenade between a rider’s legs. Modern motorcycle engines are