Spark plugs are the most fundamental ignition consumable, and the right plug in the right heat range is essential to a properly-running engine. Classic-MG specifications for spark plugs are well-documented in the original workshop manuals, and the standard plug for each application has typically remained in production from Champion, NGK, and the other principal manufacturers. This section gathers the standard specifications and the performance alternatives in the appropriate heat ranges and reaches, with the right plug matched to the application at the point of order, as fitting a plug of the wrong heat range or reach causes fouling, misfire, or in the worst case pre-ignition that can damage the engine.
Heat Range & Reach
Spark-plug heat range refers to the rate at which the plug transfers heat from its insulator nose to the cylinder head, colder plugs transferring heat more rapidly and running cooler, hotter plugs transferring heat more slowly and running hotter, with the right heat range for a specific engine determined by the engine's combustion-chamber temperature characteristics, a cooler-running engine needing a hotter plug to prevent fouling and a hotter-running engine needing a colder plug to prevent pre-ignition and detonation. The MGB was fitted with the Champion N5 from launch in 1962, superseded in early 1963 by the equivalent N9Y which remained the specification through the production run, with the resistor-cored RN9Y appearing on some of the final cars, and the firing order was 1-3-4-2 on all four-cylinder cars. The plug gap was originally specified at .024 to .026 inches, increased to .035 inches from 1976. The reach is the threaded length of the plug body, too short and the plug does not reach into the combustion chamber, too long and the plug protrudes too far and can foul the piston crown, with the standard reaches well-established across applications and matched at the point of order.
Champion & NGK Standard Specifications
Champion has been the established spark-plug supplier for British classic cars for decades, with their N-series plugs covering most classic-MG applications, while NGK is the established Japanese alternative with cross-references available between Champion and NGK part numbers for most specifications. Either brand is appropriate for classic-MG use, many owners staying with Champion for period-correct visual specification while others prefer NGK for slight improvements in cold-start behaviour and longer service life, and the wider Bosch, Beru, and Denso ranges cover the alternatives for specific specifications. Standard plugs are stocked in grades covering the range of engine applications and states of tune, with a harder-grade plug designed to withstand higher running temperatures without pre-detonating, the correct choice for performance engines being driven hard in track-day conditions, and the right plug for a specific engine and ignition setup matched to the application at the point of order.
Platinum & Iridium Performance Plugs
Platinum and iridium spark plugs use precious-metal centre electrodes, and sometimes side electrodes, that wear far more slowly than the standard nickel-alloy electrodes of conventional plugs, the headline iridium specification being the NGK iridium range, the exceptionally high melting point of iridium allowing for a much smaller centre electrode than conventional plugs, producing a more concentrated spark, a cleaner burn, crisper throttle response, smoother idle, and improved cold-start behaviour. The service life is dramatically extended, typically tens of thousands of miles compared with the roughly 10,000 miles of a standard plug, and the more consistent spark gap across the plug's life produces more consistent ignition performance. For modern MG applications such as the MGF and MG TF, platinum and iridium plugs are the standard specification rather than an upgrade, while for classic-MG applications they are an option for owners who want extended service intervals between plug changes, the original-specification nickel plugs working perfectly well with the choice principally about service interval rather than performance. A 90-degree suppressor cap is available for applications where a straight cap creates clearance issues against the bonnet or surrounding components, and the technical team is available to advise on the right plug specification for a specific engine, intended use, and service interval.