Humble plugs deserve special care

The least expensive item in your car's engine is one of the most important - the humble spark plug, which you buy at the spares shop in the familiar little cardboard box.

The least expensive item in your car's engine is one of the most important - the humble spark plug, which you buy at the spares shop in the familiar little cardboard box.

Spark plugs and their high-tension wires are the critical last pieces in the ignition system, which every petrol engine needs in order to operate. Maintaining these items can be done without sophisticated test equipment. Usually, you just need a few of the hand tools that are found in most tool boxes.

On traditional ignition systems, the high-voltage pulses come to the plugs from the distributor through the plug wires and any spark-plug service should start with these wires.

Problems in the plug wires are rare. But problems in the end-fittings, where the wire goes into the distributor tower at one end and onto the plug at the other end, are not so unusual. There you find copper connectors intended to provide a firm contact.

These are prone to corrosion and can come loose from the attached wire. Inspect them regularly by pulling the rubber boots off the distributor tower and plug terminal, and check that the copper fittings are clean and firmly attached.

Never yank on the wire. Use an ear-bud and switch-cleaner from a spray can to clean off any corrosion.

If plug wires have to be replaced, do them one at a time to prevent mixing up the firing order. Wipe some rubber grease on the boots before pressing them back on. This will make their removal easier next time.

When it comes to removing the spark plugs themselves, remember that if the plugs are fitted to an alloy cylinder head, the engine should be allowed to cool before you unscrew the plugs. That will reduce the risk of damaging the threads in the relatively soft alloy.

Be careful when loosening the plugs, especially if they have become tight in their threads. It could be due to corrosion if the plug had been undisturbed for a long time, and in such a case the threads in the head will be fragile.

Stripped threads or, worse, a snapped-off plug, is an infuriating and expensive calamity. It is preferable to use a proper plug socket and ensure it is securely seated before you apply torque.

Always inspect the old spark plugs for any obvious signs of engine malfunction. There was a time when "reading" spark plugs was a highly developed art, but on modern fuel-injected engines, on which the fuel-air mixture and cylinder temperatures are maintained within a narrow range, this has become almost obsolete.

Still, it would be unwise to ignore obvious warning signs on the plugs. Take an old plug with you when you go to buy a new set to make sure that you get ones with the same thread length and the correct heat range. The heat range tells you how fast heat will be conducted away from the central electrode, normally the hottest part on a working plug, to the cylinder head.

The heat passes first through the ceramic insulator surrounding the central electrode to the metal shell of the plug and from there into the cylinder head by way of the plug threads. On a cold plug, heat is conducted away quickly, whereas on a hot plug it happens more slowly.

Each engine needs spark plugs of a specific heat range, as determined by the engine's manufacturer. If the tip of a plug gets too hot when the engine is operating at full power, it can cause pre-ignition, detonation and engine damage. If the tip remains too cool when the engine is idling, deposits build up on the insulator and earthed electrode, causing a weaker spark or a short in the spark current.

The aim is to avoid both these dangers, so a hot-running engine needs a cold plug and a cool-running engine needs a hot plug.

If the electrodes are not too badly eroded, a plug can be re-used after being cleaned and re-gapped. It helps to file the top of the central electrode, which becomes rounded in use, flat again with a thin file because a spark jumps more readily from sharp edges.

The gaps on new plugs should be checked as well because the gap set by the factory is not necessarily the same as the one prescribed for your engine. To check the gap, it is best to use a plug gauge, which is basically a set of wires of different diameters.

The final step in the service is to refit the plugs and this is where mistakes are often made because there is always the temptation to tighten them too much.

Here is the procedure recommended by a highly respected plug manufacturer. Screw the plug by hand into clean threads as far as it will go. I always put a thin smear of high-temperature grease on the plug threads. Then you apply a plug spanner and, if you are fitting new plugs, you turn them an additional 90 degrees after you first encounter resistance, whereas if you are re-installing used plugs, you turn them an additional 30 degrees after first encountering resistance. That is all: 30 degrees corresponds to five minutes on a clock face.

Like just about everything else on a car, spark-plug technology has advanced rapidly in recent years.

Next week we take a trip to the wonderland of metallurgy and look at the latest spark plugs.