Big end bearings
At the "big end" of the connecting rods that join the piston to the crankshaft there's a large bearing. Normally a plain metal shell bearing, it works by having a "cushion" of high-pressure oil from the lubrication system between itself and the crankshaft. That cushion of oil prevents most metal-to-metal contact, reducing friction and wear. Older designs and two-stroke engines use large ball bearings to do the same job.
Main Bearings
The crankshaft runs in a series of large bearings. Like the big-end bearings, these allow the shaft to spin at high speed without causing damage or a lot of friction. Modern engines use plain shell bearings and high-pressure oil, older engines and two-strokes use ball or roller bearings.
Alternator
Your bike needs a lot of electricity to keep all those fuel injectors, fancy ECUs, ABS and the like all running. So there's usually a mighty big alternator to generate the power needed. High school physics should have told you that when a coil of wire spins inside a magnetic field, it generates electricity. And that's what an alternator on a bike does: it spins a magnetic flywheel around some coils of wire, and that produces the electricity to charge your battery and run the bike's electric system.
Primary Drive
The primary drive is what transfers power from the spinning crank to the rest of the engine. It usually sits between the crank and the clutch, and most modern designs use gears. The outer edge of the clutch has gear teeth machined in it and a matching set of teeth on the crankshaft engage with them. Some older designs use a chain between the crank and an intermediate shaft, and belts have also been used.
Gearbox
Engines run best at a fairly narrow band of speeds, so if you want a wide range of possible road speeds you need a range of gears and a way to easily change between them. Bike engines almost all use sequential gearboxes, which is to say you have to change through every gear in order (you can't go from first to fourth gear directly like you can on a manual car).
Your bike's gearbox looks simple but has a few tricky concepts. Essentially, there are pairs of gears inside mounted on two shafts: the input shaft is connected to the clutch, and the output shaft goes to the gearbox sprocket or the drive shaft. Some of the gears are locked to the shafts while some are free to spin. The changing mechanism slides the gears along the shafts so they are locked onto the shaft in various orders, producing the six different
gear ratios.
Toothed wheel
Fuel injected bikes need to know precisely where the crankshaft is in its rotation so the ECU injects the fuel and makes the spark at the right time. There's a toothed wheel on the end of the shaft with a sensor mounted close by. As the teeth pass the sensor they create a pulse of electricity that the ECU counts. At Top Dead Center the wheel has a missing tooth, so there's a gap which tells the ECU that the crank is at that point, and it can time its fueling and sparks from there.
The toothed wheel also gives the ECU information it can use to implement traction control - if the crank starts spinning too quickly, the ECU can work out that the bike has lost grip and can act accordingly to reduce torque.
Oil filter
As the engine runs, the moving parts wear tiny particles of metal off each other. The oil collects all these tiny bits of shavings along with carbon soot and other combustion byproducts. Running the oil through a filtering unit helps cut wear and extends the life of the engine and the oil. Most modern bikes use a disposable cartridge filter that simply screws into the side of the crankcase. A series of metal and paper filter elements removes particles and holds them inside. When service time comes around the old filter simply goes in the trash.
Starter motor
Kick-starting a bike is a nice cliche from the 1950s, but we prefer a button here at SSB. It's basic technology (though modern starters are tiny, super-efficient beasts compared with older designs). A small electric motor drives through a series of gears to turn the engine over fast enough for it to begin combustion. A one-way clutch mechanism allows the starter motor to disengage drive once the engine starts.
Sensors
Modern bikes, with their complex engine management systems, have a variety of sensors to tell the ECU "brain" what the engine is up to. Most of these live in the intake system and the top end of the engine, but there are some in the bottom end. A gear position sensor tells the ECU what gear you're in, so it can select a suitable fueling map and perhaps reduce the torque slightly. The crank position sensor is the most important sensor, and it tells the ECU where the pistons are in their travel, and hence when to inject fuel and set off the spark plugs. Without a crank sensor, most bikes won't run. Finally, the good old fashioned oil pressure and neutral light switches are mounted down here too.
Slipper Clutch
This special type of clutch disconnects drive when the rear wheel is turning faster than the engine. Usually there are a series of one-way ramps inside the clutch basket. When the engine is driving the back wheel normally, the clutch acts like a normal clutch. But when you change down gears suddenly and too much torque feeds back into the clutch from the rear wheel, the ramps push up against the clutch springs, releasing pressure on the clutch plates and allowing it to slip.