for AC's post-war
2 Litre Saloon

Page 4

Engine (Ignition System)

Every 3000 miles (4800km) the distributor needs lubricating. The cam should receive the slightest smear of grease, and the rotor-arm should be removed and a few drops of light oil applied (for the shaft bearing). Similar oil should also be applied through the hole in the baseplate to lubricate the centrifugal weight mechanism. A tiny amount of grease or engine oil should be applied to the contact breaker pivot. Every 6000 miles (9600km) the contact breaker should be checked and cleaned if necessary (with an extremely fine abrasive and then washed in petrol). For the earlier distributors (stamped 40143A, B or D), the contact breaker gap should be set to 0.012 inch (0.3mm) with the contacts fully open (the series 3 handbook states 0.011"). For later distributors stamped 40143E, which have a high-lift cam, the gap should be 0.018 inch (0.46mm). The toolkit's screwdriver comes with a suitable feeler gauge. If you have the full toolkit in your car, then you are very lucky!

The static timing is 20 degrees before top-dead-centre. If the engine has been uprated with higher compression pistons (which is very likely now), then changes to the timing, both static and dynamic, are desired. This is explained in some detail in Leo Archibald's book "AC 2 Litre Saloons and Buckland Sports Cars" (Veloce - 2002).

The distributor illustated here has a radio suppressor (coloured red) on the lead from the coil. Suppressors on the individual spark-plug leads have little effect, but some suppression on the coil lead is recommended.

The surfaces of the high-tension leads, distributor-cap and the top of the coil should be kept clean, especially when the weather is turning damp and cool. In particular, the spark-plug leads are bundled together into a tube, where dirt can accumulate. These can be cleaned using WD-40, but this should not be sprayed directly onto the ignition parts themselves, because it is a good insulator and so it should be kept away from any connectors and contacts.

Spark-plugs are specified in the manuals as Lodge C14, 14mm. The gap is specified as 0.015 to 0.018 inch (0.38 to 0.46mm). When low octane 2 star petrol was phased out in the UK (in 1989), I found that slight mis-firing occurred on partial throttle, and I followed advice of another classic car owner to widen the spark-plug gap. With a gap of about 0.025 inch (0.64mm) the trouble was solved.

Data from Lucas literature gives the following regarding the distributor:

Contact-breaker spring tension = 20 to 24 ozs (measured at contacts).

Condenser capacity = 0.18 to 0.23 micro-farads

Centrifugal advance commences at 200 to 400rpm distributor speed (or 400 to 800rpm crankshaft speed)

Maximum advance = 16 to 18 degrees at 1600rpm (presumably distributor speed).

Engine (Dynamo)

The dynamo is gear driven, and is mounted transversely on the right-hand side of the engine near the flywheel. Every 1000 miles (1600km) add a few drops of engine oil to the dynamo bearing via the lubricator. Every 12000 miles (19200km), remove the felt pad from the lubricator and half fill with petroleum jelly. Also at this service interval, the brushes and commutator should checked. These can be seen after removing the metal band from the outer end of the dynamo. The spring-loaded carbon brushes should slide easily in their holders, but can be cleaned with a tiny amount of petrol on a cloth if necessary. The same cleaning process can be done to the commutator (i.e. the copper segments that the brushes run against) while cranking the engine over. The starter motor should receive similar attention. If brushes have to be replaced, then they need bedding in before the dynamo or motor can be used.

Data from Lucas literature gives the following regarding the dynamo:

When cold, the cutting in speed should be 900 to 1050rpm at 12.5 volts.

Current output should be 13 amps at 1500 to 1700rpm, at 13 volts (test on a 1 ohm resistor without the regulator).

Brush tension = 36 to 44 ozs.

Resistance of the field windings (i.e. the static windings within the casing) = 6.8 ohms.

Engine (Starter-Motor)

Data from Lucas literature gives the following regarding the starter-motor:

Lock torque = 17 lbs-ft (approx.) with 450 amps at 7.2 volts.

Brush tension = 32 to 40 ozs.

Ignition Coil

Data from Lucas literature:

Current consumption = 0.95 amp when running (2.5 amps stalled).


Car batteries have changed somewhat since the 1940s! The old maintenance procedure specified checking/topping up (with distilled water) the cells, every 1000 miles (1600km). Battery terminals should be smeared with petroleum jelly. Occasionally, check the state of charge of each cell using a hydrometer to test the specific gravity of the electrolyte. Readings of 1.28 to 1.30 would indicate a fully charged cell, in mild weather. Readings will be lower in cold weather.

Replacement hard rubber batteries are available from some specialists, but it is difficult to find any with the mounting flange at each end (at the top). Long threaded rods with nuts held the battery down via these flanges.


A new head gasket is recommended whenever the cylinder-head is refitted. Unfortunately, these gaskets are expensive! The head is very heavy, so remove exhaust, carbs etc. The timing wheel needs to be removed and that has its own perch to rest on. I found it easiest to stand astride the engine, lift up the head and rest it on the wing (wing protected by cloths and a plank of wood). If the head is stuck to the block, then refit the timing wheel and crank the engine over to use the compression to break the seal.

In the block's water jacket, you may find that debris has almost buried the rear two cylinders. This is a laticework of debris that accumulates because the water flow is stagnent in that rear area. Check carefully the condition of the top surface of the alloy block, as a perfect joint is required. Corrosion or old repairs may cause trouble. Also check that the liner top flanges are all level with each other. These flanges should be higher than the surrounding block, typically by 10 to 12 thou (although some people prefer a greater height). Also check how flat the mating surface of the head is. Not forgetting the condition of the cylinder-head studs, as these may corrode near the water passages through the gasket towards the rear.

When refitting the cylinder-head, AC recommended a gasket sealant. Also make sure that the gasket is the right way up so that the water passages line up with those in the head. As the head is lowered into place, the top of the chain tensioner needs to be pushed inside the rear of the head, so that it is not trapped underneath. The tightening sequence of the 14 head nuts, is to start from the centre pair and work your way outwards, but leaving the 4 longer studs until last. Some heads have all studs equal in length, so the final 4 to tighten are either side of the centre pair. The torque setting is 40 ft-lbs, and they should be retightened after the engine has run and warmed up and again after 100 miles running. The reason for retightening is that the gaskets are soft. I don't believe it makes any difference whether the engine is warm or cold for the 100 mile retightening. When analysing this topic, some articles state that the alloy block expands more than the iron cylinders, because of the alloy's greater rate of thermal expansion. This is incorrect, because the cylinders become much hotter than the alloy block. The timing wheel has offset driving studs to ensure that it goes back onto the camshaft the right way.

Tappet clearance is 0.020" when hot.

Be sure not to try running the engine without the rocker-cover in place, because there is an oil jet supply to the timing chain which will spray out if not covered.

Torque Settings for Engine Bolt/Nut Tightening

Cylinder-head: 40 ft-lbs

Big-end bolts: 25 ft-lbs

Main bearing caps: 40 ft-lbs

Flywheel: 28 ft-lbs.

Engine Theory

I hope to include information on mechanical over-hauls at a future date. A bit of theory can be useful, especially since I've seen articles over the years with incorrect theories about the design of the AC engine.

Head fastening

In this photo of a headless AC engine, you can see that the water-jacket is exposed and the cylinder-liners are only fixed into the block at their lower ends. They are an interference fit, but also sealed by copper-asbestos gaskets. The top flanges of the cylinder-liners are set slightly higher than the top of the block, usually by 10 to 12 thou (0.254 to 0.305mm). This design raises a few issues worth considering:

1) It's sometimes pointed out that aluminium expands at a greater rate than iron, but over-looked that the cylinders are considerably hotter than the block when the engine is running. This means that the heights of the block and cylinders increase more or less equally, although unequally at certain stages of warming up or cooling down. It also means that the cylinders probably remain an interference fit in the block bores, except maybe when the engine is cooling down.

2) When the cylinder-head nuts are tightened, the block is placed under tension and the cylinders are under compression, thus reducing the protrusion of the liners. The amount of this deflection is larger than one might expect, because the loads are sent down the full height of the water-jacket (approx. 4 inches/100mm).

3) The AC engine's construction was already a bit out-dated in that it used soft gaskets and stiff studs. This is why the head nuts need re-tightening after the engine first warms up, and again after 100 miles, as the soft gasket settles. The soft gaskets do have the advantage of allowing for cylinders that are at very slightly different heights (although hard gaskets under the liners would improve matters provided that all component dimensions were to a high accuracy). At one time AC changed to harder gaskets, using a series of gaskets in layers, each of a different material.

4) Copper gaskets in contact with the alloy block causes corrosion of the latter. This is all the more reason to follow AC's instructions to use liquid gasket sealant with the head gasket. Note that graphite (used in some modern gasket designs) is even worse than copper for causing electrolytic corrosion of the aluminium alloy.

Non-pressurised cooling system

The AC's cooling system is not sealed. It has an over-flow pipe from the radiator which allows water to escape when the temperature is high and slight pressure builds up. The real reason engines changed to pressurised systems was to raise the working temperature of the coolant. This increases the heat transfer from coolant to air via the radiator due to the higher temperature difference. The reduced temperature difference between cylinders and coolant is much less of a design problem. The AC's coolant operates at 70-75 degrees C. AC later fitted pressure type radiator caps to ACs that came in for servicing, simply to provide a quick release cap. The original caps are brass with a fine thread and take a long time to screw on!

Oil filtering and by-pass filters

Old engine designs had no filters other than a gauze to prevent sludge blocking oilways. Removal of fine metal dust was achieved by providing a large capacity oil sump which acted as a sediment bowl. The stagnent oil permitted heavy metal dust to sink, forming thick sludge which would be flushed out during regular oil changes.

In the early 1950s, AC added a by-pass filter. This causes some confusion nowadays. Note that this is not a full-flow filter. As its name implies, it is on a separate circuit from the main oil circulation. Oil is pumped to the by-pass filter and returned back to the sump. The idea is that this filter can be very efficient at removing impurities, without it affecting oil flow to the bearings. All the oil will pass though this filter, but it takes several hours of running to achieve this. Hence, it is not as effective as later full-flow filters, but is an improvement on the old system.

The amount of oil filtration required depends upon the bearings fitted, which in turn tend to depend on the power output of the engine. Bearings use soft white-metal alloys to allow metal dust to become embedded in them. Higher engine performance requires a less soft white metal - or else a thinner layer of white metal as found on shell bearings. Harder white metal means that metal dust is less likely to sink in, and may then abrade the shaft (crankpins and journals), and so the crankshaft then requires surface hardening to resist this. It also increases the need for good oil filtering. Therefore, by-pass filters helped when shell bearings were introduced for main bearings. After shell bearings were introduced for big ends, it then became a good idea to upgrade to full-flow filtering. The final development of the AC engine also featured surface hardening by nitriding (the "N" of the CLBN engine number prefix refers to nitriding).

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Website started 29th December 2006