The factory MGB GT V8 distributor is a Lucas item driven from the engine camshaft via the distributor drive gear. The distributor performs two functions: it provides the timing trigger for the ignition coil (via the contact points), and it distributes the resulting high-tension spark to the appropriate spark plug for each cylinder via the rotor arm and distributor cap. Both vacuum advance (responding to engine load via inlet manifold vacuum) and centrifugal advance (responding to engine speed via internal weights and springs) are incorporated, optimising ignition timing across the engine's operating range.
Distributor Cap and Rotor Arm
The distributor cap covers the rotor arm and provides the connection points for the eight HT leads (one per cylinder). The cap and rotor are wear items: tracking, carbon deposits and physical damage develop in service. Renewal at routine service intervals is good practice, a worn cap or rotor causes ignition misfire that is difficult to diagnose. The Red Rotor Arm is a popular upgrade specification offering improved durability over the standard rotor arm.
Contact Points and Condenser
The contact points open and close as the distributor shaft rotates, switching the primary current in the ignition coil to generate the high-tension spark. Points wear in service, both the rubbing block (which wears, retarding the timing) and the contact faces (which pit and carbon up). Routine renewal at service intervals is required. The condenser fits in parallel with the points to suppress arcing, a failed condenser causes severe points burning and misfire.
Vacuum and Centrifugal Advance
The vacuum advance unit on the side of the distributor responds to engine load via a hose to the inlet manifold area, advancing the timing under part-throttle conditions for improved fuel economy. The centrifugal advance mechanism inside the distributor uses weights and springs that swing outward at higher engine speed, advancing the timing for high-rpm running. Both mechanisms can fail, the vacuum diaphragm splits, the centrifugal weights stick, causing fuel consumption and performance issues.
Electronic Ignition Conversion
Electronic ignition conversions replace the contact points with a hall-effect or optical trigger, eliminating the points wear and providing more consistent ignition timing. Electronic ignition is a popular upgrade, see the Performance Ignition sub-section in Upgrades and Alternatives.
MGOC Spares
MGOC Spares supplies V8 distributors, caps, rotor arms, contact points, condensers and all distributor service components for the MGB GT V8.