The ignition uses a Lucas coil and distributor with combined centrifugal and vacuum advance, providing automatic timing across the engine's range. Several production changes during the TD's run, the polarity sensitivity of the coil, and the chassis-specific distributor all require careful identification before ordering.
Ignition Coil
The coil must match the car's polarity. On positive earth, the SW terminal goes to the distributor and CB to the ignition switch; on negative earth these are reversed. A wrong-polarity coil does not stop the engine but reduces spark energy by ~25%. The coil mounts vertically on the bulkhead, connecting to the distributor cap centre tower via a single HT lead with acorn nut each end.
A Lucas decal, copper washer, and rubber waterproof cover complete the installation; a stainless cover is an alternative. The RB12 sports coil was fitted to some Mark IIs as Special Tuning Stage 1A.
Distributor, Three Variants by Chassis Number
Three distributor variants exist, identified by Lucas i.d. on the body. Early TDs to chassis 24488 used the Lucas 40162 (base-plate condenser, two-bush body, specific springs). From 24489, the standard TD used the 40368 (screw-on condenser, single bush, different springs). The TD Mark II and all TFs used the 40367 with revised advance curve for the higher compression and H4 carburettors, also screw-on condenser and single bush but its own springs.
All three are reconditioned on a customer's own unit basis. Mark IIs had a manual advance/retard control.
Distributor Service Items and Mounting
The distributor cap (with integral lead nuts), rotor arm, and contact set are routine service items common to all three variants. Five acorn nuts secure the HT leads. The distributor is camshaft-driven via a drive gear with locating pin, with the body in a mounting clamp on the block.
Early TDs to engine XPAG/TD2/20941 used a lock bolt clamp; from XPAG/TD2/20942 (October 1952) this changed to a cotter bolt, no change to the distributor itself. Drive gear, pin, clamp, and hardware are catalogued individually.
Contact Breaker Points
The points gap changed at engine XPAG/TD2/24489 (January 1953) from 0.010-0.012" to 0.014-0.016". This coincided with the revised camshaft and new oil suction filter, verify against the engine number before setting points, as incorrect gap affects ignition timing and dwell angle.
HT Leads and Plug Caps
A complete HT lead kit comprises coil-to-cap, four cap-to-plug, and four plug caps. Bulk copper-core HT lead is available by the metre for bespoke sets. A four-way spacer keeps leads routed and prevents cross-firing. A number decal kit marks each lead by cylinder.
The HT lead specification was modified during TF production at engine XPAG/TF/31857.
Spark Plugs
Two Champion spark plug specifications are catalogued, the change at TD engine 22735, with the round-coolant-hole cylinder head (XPAG/TD2/22735, November 1952). Specify by engine number: TDs to 22734 use one; TDs from 22735 and all TFs use the other. The head requires the matching plug reach.
Performance and Tuning
The Mark II Special Tuning Stage 1A fits the RB12 sports coil for the higher compression and H4 carburettors. Stage 3 racing tuning (up to 12:1 compression with bespoke pistons) requires a special distributor with custom advance curve. Performance coils and electronic ignition modules (fitting inside the original Lucas distributor body, replacing points and condenser) are catalogued for modernised or tuned engines. A Gunson Advance Xenon stroboscopic timing light is catalogued for dynamic timing.
Service and Diagnosis
Cap, rotor arm, points, and condenser are routine service items, typically renewed annually. Inspect the cap for carbon tracking between terminals, a hairline track causes intermittent cross-firing that is difficult to diagnose. Check rotor tip erosion and points pitting. The condenser prevents arcing across the points, a failed condenser is one of the commonest causes of sudden ignition failure, often appearing fundamental when the cure is a low-cost replacement.
Ordering Considerations
Ignition coil specified for polarity (positive or negative earth). Distributor, condenser, advance springs, and bushes specified by chassis number, early TD (to 24488), late TD (from 24489), or TD Mark II/TF. Distributor cap, rotor arm, contact set, and HT lead components common across all variants. Spark plugs specified by engine number (change point 22735).
Distributor reconditioning offered on a customer's own unit reconditioned basis.