Spark plugs recommended by Land Rover were carefully chosen for the operating conditions present inside the combustion chambers of their engines. Generally, the operating temperature inside a classic petrol combustion engine ranges from 500C to 850C. This range has been found to be sufficiently cool to prevent pre-ignition of the fuel/air mixture, yet sufficiently hot to burn off any carbon deposits if they occur.
If the spark plug electrode becomes too hot, it can glow and then ignite the fuel mixture before the spark occurs. When this happens, we get a characteristic knocking sound from the engine and damage to the pistons, rings and crankshaft bearings can occur. If however, the spark plug does not reach a sufficiently high temperature, the carbon deposits that form on the electrode (under engine idling and heavy load conditions) cannot be burnt off. Carbon deposits on the electrode can then glow and also be a cause of pre-igniton of the fuel/air mixture.
Spark plugs are manufactured with a range of operating temperatures - a 'hot' spark plug will work well for town work and heavy load conditions whereas a 'cold' one will work best at motorway cruising speeds. Manufacturers use numbers in the name of the spark plug to indicate its temperature range: Champion, Autolite and Bosch, use a higher the number for a hotter plug e.g. (for Champion) N12YC runs hotter than N11YC. For NGK, the higher the number, the colder the plug.
If you change your spark plug heat range, it is best to err on the side of too cold a plug, because using a plug than runs too cold can only cause it to foul, whereas running one that is too hot can cause serious engine damage.
Vehicle manufacturers also recommend particular spark plugs according to their thread reach. If the thread is too long, then exposed spark plug threads can build up a carbon deposit and lead to pre-ignition or even contact of the electrode with the piston head - causing damage. If the thread is too short, then cylinder head threads will be exposed and this can also lead to carbon build-up. Using spark plugs with the wrong reach can be the cause of engine damage, poor starting or idling.
To remove the spark plugs you risk letting damaging particles fall into the combustion chamber. Always clean around the plugs carefully before you remove them.
Some people recommend using copper-based grease on spark plug threads. But the reduction of frictional forces on the thread will reduce the torque needed to tighten them; over-tightening can then occur if you are not careful.
You should always wire brush the threads before re-inserting the plugs into the cylinder head. A torque of 30 lb.ft is typical for a cast iron cylinder head, but otherwise finger tight and then a firm half turn with a spark plug spanner will do (you may not get a full half turn).
The ideal working spark plug should have both electrodes showing a light brown coloration. If there is a dry, black deposit then the carburettor is giving a fuel/air mixture that is too rich; you may have noticed a slight smell of petrol when the engine was idling and a sooty deposit inside the exhaust pipe. If the black deposit is wet then oil is getting into the cylinder via worn piston rings or worn valve stem oil seals. If there are black particles on and around the electrodes then the cylinder head and pistons may need decarbonising. Decarbonising needs the cylinder head removed but some fuel additives are available that may help to decarbonise. As a temporary stop gap measure you could use a hotter rated spark plug, but this is not a cure.
If the spark plug has been running consistently too hot you may see the insulator take on a white or greyish cast and may also appear blistered. The electrodes will wear more rapidly also.
Check that the insulator is not cracked and replace the spark plug if it is; the heat conduction will be affected.
Good electrical conductivity is required across both parts of the metal electrode, but if you are cleaning the electrode surfaces with emery paper, make sure no emery particles are left inside the plug before re-installation; they will scratch the cylinder wall and lead to loss of compression. Good thermal conductivity is also required through the porcelain surrounding the central electrode and into the cylinder head. So, when cleaning a spark plug, it is necessary to clean inside the spark plug hole to remove heat insulating carbon deposits. If this porcelain cleaning is not done, then the spark plug will run hotter than intended and may lead to pre-ignition problems. It is not easy to clean effectively inside the spark plug without a commercial spark plug cleaner
Spark plug manufacturers pre-set the gaps to the most common settings recommended for the vehicles which use them. For Series Land Rovers, this is 0.030in. You should of course always check the gap before fitting new spark plugs. If adjustment is required you can close down the gap by carefully tapping of the curved electrode but you should not open the gap by levering against the central electrode as this can cause damage or mis-alignment to the surface of the central electrode.
If you have raised engine compression by altering the cylinder head then you may find that reducing the gap to 0.025in gives a denser spark that can better ignite the denser air/fuel mixture now present.