A camshaft is a shaft with lobes. Each lobe pushes a valve open against a spring, then lets it close. The shape of the lobe, the timing and the lift are the engine's breathing plan. Changing a cam without changing the rest of the system usually moves the problem rather than solving it.

SystemEngine top end
Typical materialsChilled cast iron, steel billet or a reground original
What to checkLobe wear, journal scoring, follower contact and timing marks
Technical schematic of Motorcycle camshafts
Fig. 01 — working principleIndoBikes drawing

What it does

The camshaft converts rotation into valve motion. It is driven by a chain, belt, gear or pushrods at half crankshaft speed in a four-stroke engine. The intake lobe opens the intake valve to let the charge in; the exhaust lobe opens the exhaust valve to let burned gas out.

Lift is how far the valve opens. Duration is how long it stays open. Overlap is the period when both valves are slightly open. Those three numbers, plus the lobe centreline, shape idle quality, low-speed torque and top-end power.

How it works

As the cam turns, the lobe nose lifts the follower or bucket. The valve opens against spring pressure. As the lobe recedes, the spring closes the valve. The ramps at the start and end of the lobe control how gently that happens.

In a direct-acting bucket head, the cam acts on a shim and bucket. In a rocker head, the cam acts on a rocker that multiplies lift. In a pushrod engine, the cam acts on lifters, pushrods and rockers. Each design has its own clearance and adjustment method.

Types and variations

  • SOHC and DOHC. Single or double overhead cam. DOHC allows more valve area and independent intake and exhaust timing.
  • Billet and reground cams. Billet cams are machined from a blank. Reground cams reuse an original core and weld or grind the lobe. Both need correct follower and spring matching.
  • Stock-profile replacement. Restores original timing and lift. The safest choice for a standard engine.
  • Performance profiles. More lift or duration moves power higher in the rev range, usually at the cost of low-speed manners and idle quality.
  • Variable valve timing. Some modern engines vary cam timing. The phaser and its control system are part of the cam system.

Wear and failure signs

  • A worn lobe reduces lift and duration, so power drops and the engine may idle poorly.
  • Scored journals or a worn cam bearing cause low oil pressure and metallic noise.
  • A flattened follower or bucket shows as a dull, worn contact face.
  • A broken cam chain or belt can bend valves and stop the engine instantly.
  • Incorrect timing causes hard starting, backfiring, poor idle or valve-to-piston contact.
  • Excessive clearance in a rocker or bucket makes a ticking noise that changes with temperature.

Choosing a replacement

  • Decide what you want from the engine before choosing a profile. A cam that makes top-end power can make traffic riding worse.
  • Match the cam to the compression ratio, intake, exhaust and ECU or jetting. A cam is part of a system.
  • Check valve springs, retainers and seals. A higher-lift cam may need more spring pressure and shorter installed height.
  • Confirm the cam fits the head. bearing journals, drive type, timing marks and follower style.
  • For a stock rebuild, a stock-profile cam is usually the best value. Regrinds can be fine if the core and follower are matched.

Fit and compatibility

  • Align the timing marks exactly as the manual shows. Count chain pins or teeth if the manual specifies it.
  • Check valve clearance after fitting and set it cold or hot as specified.
  • Check valve-to-piston clearance with clay or a borescope if lift or timing has changed.
  • Use assembly lube on lobes and followers. Dry starts destroy new camshafts.
  • On a shim-under-bucket engine, measure and fit the correct shims. Do not sand a shim.

Inspection and care

  • Change oil on time and use the grade specified. Cam lobes rely on a good oil film.
  • Check valve clearance at the stated interval. Tight clearance can burn a valve; loose clearance makes noise and reduces lift.
  • Do not let the engine idle for long periods when cold. Low oil flow at the cam is a wear risk.
  • After a cam change, check timing and clearance again after the first heat cycle.
  • Listen for a new tick or rattle and investigate it before it becomes a valve problem.

Safety and limits

A cam timing error can cause valve-to-piston contact and destroy the head. Turn the engine by hand before using the starter.

A broken cam chain or belt can lock the engine. Replace timing components at the recommended interval.

Cam work requires clean assembly and correct torque. If you are not confident with timing marks, use a workshop.

Questions

Will a performance cam add power without other changes?

It can move power, but it rarely adds much on its own. The intake, exhaust, compression and fuel system all limit the result. A cam that is too large for the rest of the engine can make it slower everywhere.

How often should valve clearance be checked?

Follow the manual for your model. As a habit, check it at major services and whenever the top end sounds different.

Can I regrind my original cam?

Sometimes. The core must have enough material and the followers must match the new profile. A specialist can measure it.

What happens if the cam timing is one tooth off?

The engine may start poorly, idle badly, backfire or make contact between valve and piston. It is not a small error.