Spark plugs and ignition on the AC Light Six
One easy answer, one useful contrast with the Bristol engine, and one genuine dispute that this page reports rather than resolves.
Which engine this covers
This page covers AC's own Light Six, chassis prefix AE. If your car has the Bristol 100D2, prefix BE, see the Bristol spark plugs page instead. The chassis guide explains the prefixes in full.
The plugs are conventional, and that is worth saying
The AC Light Six uses ordinary 14 millimetre spark plugs. The plug recorded for this engine is a Lodge C14.
That is worth stating plainly because the Bristol engine, covered elsewhere in this section, famously uses 10 millimetre plugs, and owners moving between the two cars or reading across between the two sets of pages could easily carry the assumption over. They are not the same. If your car has the AC engine, the plug problem that makes Bristol owners buy motorcycle plugs is not your problem.
A caution on cross references. The archive found one supplier listing a Champion plug set described as being for the AC Ace and Aceca, which on closer reading is qualified as the Ford 2.6 application. It is not for this engine. A single online cross reference offers a modern equivalent for the Lodge C14, but that site states its own results are for general reference only, and the heat range appropriate to a triple carburettor Ace is not necessarily that of a saloon. The archive is therefore not naming a modern equivalent, and would rather say so than guess at a heat range.
Firing order and distributor
The firing order is 1, 5, 3, 6, 2, 4. That rests on a single source, although it is also the conventional order for a British straight six, so it is at least internally consistent.
Ignition is by Lucas distributor, though earlier engines in the family used a magneto before the change to a distributor. Saloon units are stamped 40143A, B or D on earlier cars and 40143E on later ones fitted with high lift cams. The specialist distinguishes between what it calls PVT and post war distributors and rebuilds both.
Contact breaker gap
| Distributor | Gap |
|---|---|
| Lucas 40143A, B or D | 0.012 inches, which is 0.3 mm |
| Lucas 40143E, later, with high lift cams | 0.018 inches, which is 0.46 mm |
Period service data for the 1954 to 1955 Ace and Aceca independently gives 0.012 inches, consistent with the earlier distributor group. Establish which distributor you have before setting anything, because the difference between the two figures is large enough to matter.
Ignition timing, and a conflict this page will not resolve
Two published figures exist and they are far apart.
| Source | Figure |
|---|---|
| AC two litre saloon material | 20 degrees before top dead centre, static |
| Period service data, 1954 to 1955 Ace and Aceca | 12 and a half degrees before top dead centre, indicated on the flywheel |
These cannot both be right for the same engine, and the archive is not going to average them or pick the one it prefers. There is a specific reason for suspicion about the second: the same data table also gives the inlet valve as opening at 12 and a half degrees before top dead centre, the identical number, which raises a real possibility that one figure has been transcribed into the other. That is a suspicion and not a finding.
Anyone setting timing on this engine should take the figure from AC Cars service data or from the club rather than from this page. The archive can tell you that a conflict exists, which is itself useful, and it can tell you not to trust a single number found online for an engine whose data is this thinly published.
Plug gap, where the sources also differ
| Source | Gap |
|---|---|
| AC two litre saloon, with Lodge C14 | 0.015 to 0.018 inches, which is 0.38 to 0.46 mm |
| Period service data, 1954 to 1955 Ace and Aceca | 0.020 inches |
This may well be a genuine difference between the saloon and the sports cars rather than an error, but it is unconfirmed, so both are given with their source.
One piece of owner practice worth recording separately, and clearly labelled as such rather than as specification: an owner of an AC engined car reports that opening the gap to 0.025 inches cured a misfire on modern petrol. That is one person's experience on one car, not a recommendation, but modern fuel is a real variable on engines designed around what was available in the 1950s.
Service intervals for the ignition
From the AC two litre saloon schedule: lubricate the distributor every 3,000 miles and clean the contact breaker every 6,000. No Ace or Aceca specific schedule was found.
What the archive could not establish
The dwell angle, for any of these distributors. The correct modern spark plug equivalent, for the reasons given above. The plug type for the Ace and Aceca specifically, as opposed to the saloon's Lodge C14. The coil specification, the condenser part number, the plug torque, and the plug reach and heat range. And anything specific about plug access on this engine, where nothing was found and nothing will be guessed. The AC Owners Club runs a dedicated Weller engine forum board which the archive cannot read, and which is the most likely place these answers exist.
Related reading
- AC Light Six valve clearances0.020in hot, the roller rockers and the timing chain.
- AC Light Six oil & filterGrade, oil pressure, the bypass filter and the zinc question.
- Cooling the AC Light SixCoolant choice, the water pump fault and reading the gauge.
- Servicing hubWhere each task fits in a full service.
- The AC Light SixBackground to AC's own two litre six.
- Parts and specialistsWho supplies parts for this engine, AC6 among them.
This guide is not a substitute for the official AC Workshop Manual. If anything here is wrong or can be improved, please get in touch.
About the author
A lifelong classic-car enthusiast in North Yorkshire, near Thornton-le-Dale, who owns and maintains an AC Aceca, writing the single-model reference he always wished existed. Read the full bio.