Big End

The big end is the larger end of the connecting rod, which wraps around the crankpin of the crankshaft. The small end at the other end connects to the piston via the gudgeon pin. The big-end bearing carries very high loads as it turns the piston's up-and-down movement into rotation.

Most two-strokes and many single-cylinder off-road four-strokes use a one-piece rod with a needle roller bearing on a pressed-together crank. Multi-cylinder four-strokes usually use a split rod with plain bearing shells fed by pressurised oil.

A worn big end makes a deep knocking noise that rises with revs and often shows as vertical play at the rod. Causes include low or old oil, overheating and over-revving. On race engines the rod and bearing are replaced at set hour intervals.