Engine Design & Basic Theory Part 3 (The Bottom End)

Engine bottom end consists of crankcase, crankshaft, main and rod bearings, and connecting rods. These parts convert the up-and-down linear motion of the pistons into round-and-round motions needed to power the transmission. The crankshaft is connected to the pistons by connecting rods, which has bearing on each end.
Types of crankcase designs

  • wet-sump engines – oil is stored in the crankcase
  • dry-sump engines – oil is stored in a separate tank
  • unit-construction engines – many modern motorcycles also house transmission and clutch in the crankcase
  • non-unit transmission engines – as you guessed it, tranny and clutch are in a separate case

The backbone of the crankcase is the crankshaft. Crankshaft converts the linear motion of the piston into a rotary motion that’s use to drive the motorcycle.
Types of crankshaft

  • pressed – can be made of several separate pieces bolted together
  • forged or unit – made from single chunk of steel alloy

The connecting rod connects to the piston to the crankshaft. The end connected also sometimes called the big end to the crankshaft is bigger, because it has a larger diameter than the end of the connected to the piston. It should be light and strong and able to transmit the heavy loads imposted on the piston without deflecting.

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