Tuning Guide

How a distributor works

By Nathan Devonshire

The distributor is the brain of a classic ignition system. It does two jobs at once — triggering the spark, and sending it to the right cylinder at the right instant. On the Stag’s high‑revving V8 it works especially hard, which is why Triumph gave it a twin‑point Lucas distributor.

Two jobs in one

A distributor does two distinct things. First, it triggers the coil to make the spark, by making and breaking the coil’s low‑voltage circuit at precisely the right instant. Second, it distributes that spark, routing the coil’s high‑voltage output to the correct spark plug in the firing order. The first job is about timing; the second is about direction.

How the spark is made — points and coil

An ignition coil is a transformer: feed a modest current through its primary winding and it builds a magnetic field; interrupt that current suddenly and the collapsing field induces tens of thousands of volts in the secondary winding — the spark. The switch that makes and breaks the primary current is the contact breaker points, opened and closed by a cam on the distributor shaft. Beside them sits the condenser, a capacitor, which soaks up the surge that would otherwise arc across the points and sharpens the spark. Here the Stag differs from a four‑cylinder: its Lucas distributor carries two sets of points, working together to keep the coil charged for long enough between sparks. A V8 revving past 5,500 fires eight times every two turns of the crank, leaving very little time to build each spark — the twin points share that work and hold the spark strong at high revs. The catch is that there are two sets to keep clean and correctly set, not one.

How the spark is delivered — cap and rotor

The coil’s high‑voltage output arrives at the centre of the distributor cap, passes down through a spring‑loaded carbon brush, and onto the rotor arm spinning on the shaft. As the rotor turns, its tip sweeps past the cap’s eight outer terminals — one per cylinder — without quite touching them; each time it lines up, the high voltage jumps the gap, races out along that cylinder’s HT lead, and fires its plug. Spin the rotor in step with the engine and each cylinder gets its spark in turn.

Why it turns at half engine speed

The distributor turns at half crankshaft speed, because a four‑stroke engine fires each cylinder once every two turns of the crank. On the Stag it is driven not straight off a camshaft but by a jackshaft, itself turned by the timing chain — the same jackshaft that drives the water and oil pumps. One full turn of the rotor serves one complete firing cycle of all eight cylinders.

The clever part — the advance mechanisms

A distributor does not fire at a fixed moment. The spark needs to happen earlier as revs rise, and can be nudged earlier still under light load for economy — and it does both automatically.

Centrifugal (mechanical) advance

Hinged weights on the shaft, held in by springs, swing outward as revs rise and twist the cam a few degrees ahead, advancing the spark up to a built‑in limit.

Vacuum advance

A diaphragm connected to carburettor or manifold vacuum advances the timing under light load for economy and backs off under hard load to guard against pinking. Together with the base setting, these serve the right advance at every speed and load — the subject of the ignition‑timing page.

What goes wrong, and upkeep

Being mechanical, a distributor wears: points burn and drift, the condenser fails, the cap and rotor track or corrode, and worn shaft bushes let the timing wander. On the Stag there are two sets of points to stay on top of, which is one reason many owners fit an electronic ignition conversion — a trigger that replaces the points inside the same distributor, doing away with their wear and adjustment for a stronger, steadier spark, while keeping the advance mechanisms. On a hard‑worked V8 it is one of the most worthwhile reliability upgrades.