This is one part of the AC that does receive its fair share of acclaim. Originally designed by John Weller with the early prototypes appearing from 1919, it was very advanced indeed. It featured light-weight aluminium alloy crankcase/water-jacket and sump, cast iron cross-flow head and over-head camshaft. Initially the camshaft was driven via a vertical drive-shaft and skew-gears, but this was quickly replaced by an inverted tooth chain (later on replaced by a duplex chain-drive) with Weller's own patented tensioner (a simple spring-steel strip). It was a straight six cylinder engine with wet cylinder-liners and a 4 bearing crankshaft (a 5th bearing was subsequently added to the rear).
Auxiliary drives were well designed. Camshaft, distributor and dynamo all take their drives from the rear of the crankshaft. The rear of the crankshaft has a lower level of torsional vibration, and so a smoother drive is provided. This is important for gear-driven auxiliaries, and the AC engine features a skew-gear set driving a cross-shaft for the dynamo and distributor.
It entered commercial production in the early 1920s as a 2 litre, and by the 1930s was sporting triple SU carburettors and developing up to 80bhp (60kW) or 90bhp (67kW) super-charged. For the post-war saloon, the cooling system was modified. To obtain a lower bonnet level, the water-pump was moved to the left-hand side of the crankcase, and pumped coolant into the side of the engine. This has the added benefit of maintaining some circulation even if the water level has dropped (a high mounted pump would become dry if water level dropped a little). The post-war engine also featured a thermostatically controlled cold-start carburettor, rather than a choke. This fed an air-fuel mixture into the carburettor balance pipe, and had a mixture needle-valve controlled by the level of vacuum in the manifold (which tended to stop the engine from stalling). A thermostat in the radiator bypass pipe switched this device off at a coolant temperature of 35 deg.C (95 deg.F).
Liners and Gaskets
Cylinder liners were an interference fit in the bottom of the crankcase, and sealing was looked after by copper-asbestos figure-of-eight gaskets (i.e one gasket serving two adjacent cylinders). The cylinder-head gasket was also copper-asbestos. The combination of iron, aluminium and copper was not good for avoiding corrosion! The alloys used for the block seemed to improve over the years for corrosion resistance.
The cylinder head was very well laid out. Part spherical combustion chambers with slightly inclined valves. Cross-flow layout, that is, inlet and exhaust manifolds on opposite sides for good gas-flow. Cam-followers were of the roller type, operated by the single over-head camshaft. For easy removal of the head, a rest was provided for the removed timing sprocket.
Circulation of coolant through the water-jacket was less than ideal. Entering low down towards the front, the main flow was uphill towards the rear and into passages in the head. This left a wedge shape of stagnant coolant, particularly around cylinders 5 and 6. A build up of debris around these cylinders was a symptom of this lack of water movement. Ideally, this should have been updated in later years to feed coolant to the rear of the water-jacket.
Bolts and Studs
One area that did not appear to keep up with the times was the design of bolts, studs and gaskets. Broken head studs can be rather a nuisance in that alloy block! Studs had cut threads, and little thought of stress relief at the ends of the threads. Rolled threads are more desirable for highly stressed fasteners. The modern trend was towards harder and thinner gaskets, and longer, more stretchy bolts or studs, and AC was not following this trend. Ironically, I understand that the earliest versions of the AC engine (early 1920s) were clamped together by long bolts in the manner that became popular in post WW2 engine designs.
An important factor in keeping the AC's cylinders nicely sealed, was to make sure that the tops of the liners were all the same height. The top flanges need to be above the level of the alloy block. How much above? 10 to 12 thou (0.25 to 0.30mm) used to be recommended. Many people later suggested higher and higher figures, although this could then lead to leaks around the oil-feed to the camshaft. More important, in my view, is to get the cylinders at the same level. More modern gaskets under the cylinders may make height settings rather more precise. The reason for having the liner tops higher than the surrounding water-jacket, is partly that there are higher pressures to seal in the cylinders. But also, when the cylinder-head nuts are tightened, the alloy water-jacket is under tension while the liners are under compression - and the depth of material is about 4 inches, so the actual stretch and compression should be measurable.
Another feature of this engine was the fly-wheel damper. The fly-wheel was in two parts separated by rubber damping bushes. This was abandoned during the 2 Litre Saloon's production run, in favour of a damper mounted on the front of the crankshaft.
By the post-war years, this engine's main bearings were separate shells, but the big-ends still retained the traditional direct white-metalled bearings. Separate shells permitted thinner layers of white-metal (white-metal being a general term for low-melt alloys). Thinner bearing metal means a greater load could be sustained. The principle behind using soft metal bearings against hard metal shafts, is that metal dust from the cylinders, etc. can become embedded in the soft metal. The harder the bearing, the more likely it is that metal dust protruding will abrade the shaft, so the need for surface hardening the shaft arises as power demands rise. The need for oil filtering also increases, if bearing wear is to be kept in check.
When the 2 Litre Saloon was introduced, there was still no filtering of oil. There was a gauze filter just to prevent anything large from blocking oilways, but this had nothing to do with dealing with the fine metal dust in the oil. That was dealt with (to an extent) by the large capacity sump acting as a sediment bowl to allow metal dust to settle. It would then be flushed out when the oil was changed. By the 1950s, AC added a by-pass filter mounted on the exhaust manifold. The filter itself, is highly effective, but it takes some time for all the oil to pass through it.
Compression ratio had to be kept low in those early post-war years, because of the low octane petrol available in Britain. The ratio was initially 6.5 to 1, but was later increased to 6.75 to 1 when pistons were changed to Wellworthy (from Specialloid). Engine numbers were given the suffix letter "W" to indicate this. This piston change increased maximum engine torque from 95 to 105 lbs-ft (130 to 140 Nm), but power only increased from 74 to 76bhp (55 to 57kW). Maximum power was attained at 4500rpm.
In conclusion, this engine should be considered to be a fine, reliable and durable unit, provided that it is maintained meticulously. It is what I would term "bodge sensitive"! Bodge it, and it will be a huge source of trouble!