Long-reach Plug Used where the cylinder-head section is very deep. In a thinner head, such a plug would project too far into the combustion chamber. Short-reach Plug Used where the cylinder-head section is thin. In a deeper head, this plug's electrodes would be too far recessed for efficient ignition.

Side-gap Plug Often used in highly tuned engines. Three-gap Plug Has three earthed electrodes ( one obscured in drawing ) to give longer plug life. Tapered-shoulder Plug Used instead of a plug with a gasket. The conical shoulder is tightened into a recess to provide a gas-light seal.

Cold Plug Has a short insulator and therefore a short heat-flow path. Used on high-performance engines to prevent pre-ignition through overheating. Hot Plug Has a long insulator and therefore a long heat-flow path. Used for low-performance engines. A hot plug also burns off any deposits.

Spark plugs vary in design to suit a number of applications, such as loading and speed range of an engine, compatibility with the combustion chamber design, tolerance for varying mixture strengths, compression ratios and operating temperatures.

Spark plugs are basically classified according to heat range, ie, the ability to transfer heat from the firing tip of the spark plug to the central electrode and to dissipate this heat to the engine's cooling system. This classification divides spark plugs into two categories, namely 'cold' plugs and 'hot' plugs.

A 'cold' plug has the advantage of quicker heat transfer. It has a shorter insulator and hence heat has to travel a shorter distance. Ideal applications are in highly tuned engines and for continuous speed running conditions. In a cooler engine, it would get fouled up.

A 'hot' plug has a longer insulator, thereby heat travel to electrode from firing tip takes longer. This enables it to operate at higher temperature to compensate for the cooler running engine. Unsuitable for a highly tuned engines as the high temperatures involved would cause overheating and pre-ignition.

Spark plugs are made in differing ranges from 'hot' to 'cold' to ensure dependable performance in any engine.

Another base for classification depends on the length of the threaded portion. This varies according to the thickness of the cylinder head above the combustion chamber. The length of the thread is known as the 'reach' of a plug. A spark plug with longer threads should never be fitted onto a short reach head as the excess length could damage the valve or piston. Similarly a short reach plug should not be fitted in a long reach head as the exposed threads of the cylinder head can get damaged during combustion and make it difficult to fit on a correct reach plug later.

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