If you've ever spent any time checking out the tech specs on a new bike, you might have come across a few terms that left you scratching your head. And to be honest, we sometimes use jargon without explaining ourselves too well either. So put an end to perplexing specs with our handy guide to the bottom end of your engine.
Oil pump
Engines need a constant source of high-pressure oil to lubricate all the bearings and cool the internal parts. A mechanical pump is normally gear-driven by a shaft off the clutch or primary drive. It sucks oil up from the sump (or oil tank) and pumps it through the oil filter and into oil galleries that feed the crankshaft bearings and the top end of the engine.
Balance shafts
In 90-degree V-twins (Ducati) and flat-sixes (Gold Wing) the forces generated by the pistons moving up and down are cancelled out by each other. But most bike engines don't have this characteristic, and they often use balance shafts to counteract the vibration this lack of balance creates.
The shaft is gear or chain driven off the crank and has off-center weights. As these weights spin around with the shaft they create forces which, hopefully, cancel out the forces from the pistons moving up and down. The result is a smoother engine that can be made lighter, and also be used as a stressed part of the chassis, thus increasing stiffness without adding mass. It's also much more fun to ride.
Crankcases
This is the general term for the biggest cases of the engine which house the crankshaft as well as balance shafts, clutch, primary drive and transmission (except in old designs with separate gearboxes). Normally in two parts, the crankcases clamp together horizontally around the crankshaft's central axis, but some designs use three-part split crankcases. V-twin and single-cylinder crankcases often have a vertical split.
Crankshaft
In many ways it's the heart of the engine. The crank converts the up-and-down (reciprocal) motion of the pistons into the rotary motion needed to turn the back wheel. Like the crank on a pedal bike, the shaft spins on a central axis and has offset crankpins where the rod connects from the piston. The piston is pushed down by the expansion of the hot, burning fuel in the combustion chamber, and this pushes the crankpin down and around, turning the crankshaft around its central axis.
Clutch
Internal combustion engines really do need a method of disconnecting the drive from the back wheel - just disconnect your clutch cable and see how you get on with pulling away, stopping and changing gears. Therefore the clutch is a major part of what makes your bike stop and go. There are a couple of basic designs but the multi-plate unit is the most popular and used on most Japanese bikes. Multi-plate units have two parts: an inner drum and an outer basket, each of which has its own set of plates (one set plain and one set friction). The outer basket is driven by the primary drive from the crankshaft and has the friction plates fixed in slots, while the inner drum has plain steel plates fixed in grooves. The plates are interleaved, alternating between friction and steel plates.
This "pack" of basket, drum and plates is bolted together with a set of springs behind the bolts. These springs clamp the plates together so the basket and drum turn as one unit. An actuation mechanism then either pushes or pulls the pack apart slightly against the spring pressure, the plates are freed, and the basket can turn without moving the drum.
Multi-plate clutches normally run in a bath of oil, which cools the plates and makes them last longer. They can run dry though, and often do in racebikes and high-end exotica like Ducati's superbikes. Dry clutches are lighter and have less drag, but they are noisier (that's the "rattle" you've likely heard coming from a Ducati) and don't last as long as a wet clutch.
Sump
The sump sits right at the bottom of the engine and holds the oil. The oil pump inlet sits in here with a strainer, and the drain plug you use when you change the oil resides here. Dry sump designs are used by Aprilia, Triumph and a few others, and these use a secondary pump to move oil from the bottom of the engine into a separate oil tank.