Ignition leads, the high-tension HT wires that carry the spark from the distributor cap to each spark plug, are wear items on any classic MG. The lead insulation hardens with age and temperature cycling, the inner conductor breaks down internally over time, and the connector ends corrode at the connections to the distributor cap and the spark plugs, so a tired set of leads produces weak sparks, misfires under load, and the various intermittent running issues that defy diagnosis until the leads themselves are renewed. A complete lead set is one of the lowest-cost reliability improvements available, restoring strong, consistent spark delivery to every cylinder, and quality silicone leads carry a far longer service life than the original-pattern rubber-insulated leads.
Silicone-Cored Construction
Modern ignition leads use silicone insulation across the lead's working length with the appropriate inner conductor specification matched to the application, silicone insulation being dramatically more durable than the older PVC or rubber insulation used in original-equipment leads, resisting heat far better, maintaining its insulation properties across decades of service, and not cracking or hardening in the way conventional rubber leads do. The inner conductor is typically either carbon-fibre for resistive cores that suppress radio-frequency interference, appropriate for original-points and standard-electronic ignition systems, or stainless-steel wire for performance applications where minimum resistance and maximum spark energy are the priorities. Carbon-fibre cores are the standard specification for most applications, while stainless-steel cores are used in competition and high-performance specifications where the additional spark energy is valuable. MGOC stocks pure red silicone leads for maximum spark and starting reliability, and performance high-tension blue ignition leads using stainless-steel wire winding over a ferromagnetic layer and a Kevlar fibre core, providing enhanced RF suppression alongside low electrical resistance for maximum spark energy delivery, the choice for cars with electronic ignition or sensitive audio equipment.
Lead Length, Connectors & Distributor Type
Each MG application has its own specific lead-length requirements, the layout of the distributor relative to the spark plugs varying between the four-cylinder cars, the six-cylinder MGC, and the V8, so quality lead sets are supplied cut to length and pre-fitted with the appropriate connectors at each end. The distributor-cap end uses the connector style appropriate to the cap type, and the cap type itself varied during production, the MGB originally using a side-entry distributor cap with the cap changed to the top-entry type from the start of the MkII in late 1967, which necessitated fitting plug leads approximately four inches longer than previously. Sets are therefore stocked for the 25D top-entry distributor, the 45D top-entry distributor, and the V8 engine, with the correct lengths and connectors for each. The spark-plug end uses the appropriate connector for the plug type, typically a 90-degree boot connector for most applications with straight boots for some specific layouts, and a 90-degree suppressor cap is available where clearance is an issue against the bonnet or surrounding components.
Colour choices cover black as period-correct for most original applications, red as a popular performance-look choice, and other colour options used by some owners as a styling feature, with special suppressed cables having been fitted from the factory to certain export markets and standardised across MkII production.
Renewal & Routing
A complete HT lead renewal is straightforward, removing the old leads one at a time and fitting each new lead from the distributor cap to the appropriate spark plug with the correct routing, confirming the firing order matches the engine's specification, 1-3-4-2 for the B-Series and A-Series fours, 1-5-3-6-2-4 for the C-Series six, and the appropriate firing order for the Rover V8. Routing matters, as leads should be kept apart from each other where possible to prevent the inductive coupling that can cause cross-fire between adjacent cylinders, and should be routed away from heat sources, the exhaust manifold being the obvious concern, with an ignition lead clamp set available for correct routing. With a quality set of silicone leads the renewal interval is typically 10 to 15 years rather than the 3 to 5 years typical of original-pattern rubber-insulated leads.
For owners making up or repairing their own leads, an HT lead end kit and copper-core lead by the metre are available, and the technical team is available to advise on the right specification for a specific car and intended use.