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.

Cylinder-Head (de-coke/light attention)

For cylinder-head removal, you need to take off the carbs, manifolds and thermostat. Unbolt the timing sprocket and place it on the resting bracket provided. If the head is stuck to the gasket/block, I would recommend making wooden wedges to tap into the gap between the 2 gasket layers. Also, you can try using the engine compression by cranking it over (with timing wheel attached). I know of a head that cracked after more brutal methods were employed, so take great care.

I used to lift the head off, by standing astride the engine, and then resting the head on a wooden panel resting on a front wing (protecting the paint with a cloth). It's better to winch it up if you have the facilities.

With the head and gasket removed, you might find that alloy corrosion debris has built up around the rearmost cylinders. You might also find corrosion around the studs and maybe some old repairs? Light corrosion to the top surface of the alloy, can be sanded off. Check that the liner flanges are standing at equal heights. This was typically about 10 thou above the block. This figure tended to be increased to mitigate issues arising from aging engines (see further down this page).

When refitting the head, a new gasket is required. Originally, these were copper asbestos (or modern equivalent later on). This type of gasket needs a liquid gasket sealant. Make sure the gasket is the right way up, so that the water passage holes align with those in the head. The timing sprocket can be remounted onto the offset dowels that maintain the valve timing. Head tightening sequence works outwards from the centre pair of nuts, but leaving the 4 longer studs until last. Final torque setting is 40 ft-lbs. Re-tighten after the engine has been run and warmed up, and again after about 100 miles. The above assumes that the original style gasket is being used. Different tightening procedures might be specified by makers of new improved gaskets.

More detailed information will be provided at a later date in the overhaul section.

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.

AC 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 a light 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) The head studs extend deep into the block, and are threaded towards the bottom. In theory, this should mean that tightening the head nuts will place the upper part of the block under compression, and there will be a nice long length of unthreaded stud to stretch. This stretchiness should reduce stress range, and thus less risk of fatigue. Unfortunately, once corrosion sets in, the stud may become fused into the block. That will leave a short free length of stud above the block, placing it at greater risk of fatigue. Also, when the head is next tightened down, the block will be under tension. That will reduce the liner protrusion. That might explain why some have increased the liner protrusion beyond the old figure of 10 thou?

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.

5) If you have to remove old liners, I would recommend a gentle heating up of the entire block. A few owners have reported that liners come out easily by heating the block with electric heaters.

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