A crankshaft has to be stiff, straight and balanced. It carries the connecting rod, drives the primary or clutch, the alternator and sometimes the cam drive. Most crank problems announce themselves as a knock, a vibration or metal in the oil, and by then the engine is usually coming apart.

SystemEngine bottom end
Typical materialsForged steel, cast iron or a pressed assembly
What to checkBig-end play, runout, journal wear and oil passage condition
Technical schematic of Motorcycle crankshafts
Fig. 01 — working principleIndoBikes drawing

What it does

The crankshaft converts the up-and-down motion of the piston into rotation. The crankpin carries the connecting rod, the main journals sit in the crankcase bearings, and the ends drive the clutch, alternator and timing system. In a multi-cylinder engine the throws are arranged to balance firing forces.

It also stores energy between power strokes, which smooths idle and helps the engine pull from low speed. The oil passages drilled through the crank feed the rod bearing. A blocked passage or low oil pressure destroys the bearing quickly.

How it works

Combustion pushes the piston down, the rod pushes on the crankpin, and the offset between the crankpin and the main axis creates torque. The rod bearing rides on a film of oil. The main bearings hold the crank in line and take the load from the rod.

In a pressed-up crank, the crankpin, webs and main shafts are pressed together and trued. In a one-piece forging, the crank is machined as a single part. Pressed cranks can be rebuilt; one-piece cranks usually cannot be reworked beyond bearing replacement and polishing.

Types and variations

  • One-piece forged crank. Common in modern single-cylinder and many multi-cylinder engines. Strong and stiff, but not usually rebuildable at the big end.
  • Pressed-up crank. Common in two-strokes and some twins. Can be disassembled, fitted with a new rod and bearing, then trued.
  • Stroker crank. Changes stroke and displacement. Needs matching piston, rod, deck and clearance work.
  • Counterweighted crank. Uses bob weights to reduce vibration. Balance factor matters when piston or rod weight changes.
  • Keyed and splined ends. Drive the primary, alternator or cam. Spline and keyway wear causes timing and drive problems.

Wear and failure signs

  • A deep knock under load that changes with engine speed. big-end or main bearing wear.
  • Metal particles in the oil or on the magnetic drain plug.
  • Low oil pressure at idle, especially when hot.
  • Excessive runout at the crank end, measured with a dial gauge.
  • Vibration that appears after a rebuild. balance factor or mismatched parts.
  • Worn splines or a damaged keyway causing a rattle or loss of drive.

Choosing a replacement

  • Identify the exact engine code and crank type before ordering. Visual similarity is not enough.
  • Inspect the rod and main journals for scoring and measure them. A polish may be enough; a regrind needs matching bearings.
  • For a pressed crank, use a specialist with a jig and a press. Check runout after assembly.
  • If the engine is tuned, discuss balance factor and rod selection with the machinist.
  • Replace bearing shells as a set and use the correct grade. Do not mix worn and new shells.

Fit and compatibility

  • Main bearing clearance and end float are measured with the correct tools and the manual's figures.
  • Rod side clearance and big-end axial play matter on pressed cranks.
  • Runout is checked between centres or in the crankcase with a dial gauge.
  • The oil passages must be clean and the plugs secure. A loose plug can end oil pressure.
  • On a pressed crank, the rod must be centred and the webs trued before the engine is assembled.

Inspection and care

  • Change oil and filter on time. Bearing life depends on clean oil and steady pressure.
  • Do not lug the engine at low rpm in a high gear. High cylinder pressure with low oil flow is hard on rod bearings.
  • Warm up before revving. Cold oil is thick and slow to reach the rod.
  • Listen for a new knock and check oil pressure rather than turning up the idle.
  • After a rebuild, check oil pressure and look for metal after the first run.

Safety and limits

A failed rod bearing can throw the rod through the cases and lock the rear wheel. A heavy knock is a stop-riding fault.

Crankshaft work needs precision measurement and a clean assembly area. Grit in a bearing destroys it in minutes.

Do not reuse bearing shells, circlips or rod bolts that are specified as one-time parts.

Questions

Can a crankshaft be repaired instead of replaced?

Pressed cranks can often be rebuilt with a new rod and bearing. One-piece cranks are usually replaced if a journal is badly damaged. A specialist can measure and advise.

What causes crankshaft runout?

A drop, a seizure, a bad press job or uneven bearing wear. Runout is measured, not guessed.

Do I need to rebalance after fitting a lighter piston?

Often yes. Balance factor changes with piston and rod weight. A machine shop can advise.

Is a knock always the crankshaft?

No. A loose clutch, cam chain, piston or exhaust shield can sound similar. Use a stethoscope and check oil pressure before condemning the crank.