Cams and Valve Timing: Open Early, Close Late

'Degreeing' checks that a camshaft is timed as its maker intended. 1. Fix a degree wheel (a large protractor) to the crankshaft nose and a wire pointer to the block. 2. Find TRUE top dead centre: bring the piston up against a positive stop screwed into the plug hole from each direction, note both readings, and set the wheel so TDC is exactly half-way between them. 3. Put a dial indicator on the inlet valve's lifter or retainer, parallel to the valve. 4. Turn the crank forward and note the crank angle at maximum lift. Better: note the angles at 1 mm of lift on each side of the peak and take the mid-point — the lobe centre. 5. Compare with the cam card. A timing chain or belt a tooth out shows as the whole curve shifted by a fixed angle.

A practice rung: poppet valves and camshafts go back to the earliest four-stroke engines, and early valve timing was found by trial. The ideas here — early opening, late closing, overlap — are how every four-stroke is timed. Variable valve timing shifts the lobes while the engine runs, to get the best of both at every speed. **Honest limits.** The timing figures in the notebook are an example, and the lift curve is generated, not a real cam's. Real cams have ramps to take up clearance gently; real clearances are measured with modelling clay on the piston crown.