How do I replace my car's spark plugs?

The combustion of petrol inside your car's engine is a controlled process relying on the precise timing of a spark to coincide with the movement of a piston and the induction of a fuel 'charge'.

And the subset of electrical components that produces this spark and times it in such an exacting way is the ignition system. The ignition system comprises spark plugs, leads and a coil (or coil packs). Over time, the constituent components of the ignition system, including the contact points in older cars, will break down. Symptoms of this include hard starting and rough running.

Most modern cars are fitted with electronic ignition systems, so contact points are very much a thing of the past. If you do have an older car, however, you will need to replace the contact points inside the distributor at around the same time as the spark plugs.

To do so, use a feeler gauge to set the 'gap' with the engine stopped at 'top dead centre'.

You may have to turn the crankshaft over by hand (remove the spark plugs first so you're not fighting compression in the cylinders) for the number-one piston to stop at top dead centre. You can see the piston at this point through the spark plug hole.

Unclip the distributor cap and unscrew the existing points – usually held in place by one small screw. Remove the old points and replace them with new points in roughly the same position.

Tighten the screw enough to hold the points mounting plate in place, but not so tight that you can't move the plate around to adjust the timing. Use the appropriate feeler gauge (as per the manufacturer's service information) to set the gap at the right distance. Tighten the screw.

Replace the spark plugs, refit the distributor cap and the engine should be ready to start.

Use a timing light to adjust the ignition timing, but you will need to beg, borrow or steal one if you don't already own one.

Tighten the contact points in place, but loosen the distributor to turn it and adjust the timing so that the strobing from the timing light synchronises with timing marks on the engine's harmonic balancer, which runs off the crankshaft.

When the timing marks indicate the engine's ignition timing is correct, simply tighten the distributor so it won't move around freely anymore.

To replace spark plugs – which happens less frequently these days, with long-life plugs now available – remove the plug lead from the top of the plug by gripping the 'boot' at the spark plug end and firmly but gently pull it back.

Using a socket spanner, a long bar and a plug socket – a longer socket than usual, specifically intended for removing spark plugs – unscrew (anti-clockwise) the spark plug. Once loose, the plug should withdraw in the grip of the plug socket.

To fit the new spark plug, make sure the gap between the central electrode and the lateral electrode is set correctly as per manufacturer's specifications. Most plugs are 'pre-gapped' these days.

Fit the plug snugly in the plug socket and screw it (clockwise) into the cylinder head, a reversal of the procedure to remove the plug. The plug only needs to be tight enough to form a seal – not much more than finger-tight. Don't overtighten the plug.

Over a longer period – some years, typically – spark plug leads from the distributor can break down and the high-tension current to the spark plug can diminish. Replacing the leads is a simple process. Grip the lead near the spark plug between thumb and forefinger and gently tug it away from the spark plug. Repeat that at the other end of the lead, where it connects with the distributor.

To replace the lead, remove the new lead from its packaging and reverse the removal procedure: gently push the lead over the head of the spark plug at one end and into the distributor connection at the other end.

Make sure the copper sheath inside each boot closes over the terminal of the spark plug at one end and slots into the correct distributor connection. Replace the plug leads one at a time so that you don't become confused with which plug connects where on the distributor cap.