Incredible though it may
seem, within a few years we shall be celebrating the centenary of the AC
Light Six engine.
remarkable fact merits a moment's reflection.........it means that an
engine designed at the end of the Great War enjoyed a production run of
forty three years and is still in use in the 21st century and in forms
recognisably deriving from that long ago original.
of its time...
What is it
that makes the engine so special and so long-lived? Essentially, the
design of the engine was so advanced for its time that it retained its
technological lead for very many years. Even in the post second
world war period it was unusual to find engines similarly specified.
In fact, many family cars and a good number of those with sporting
pretensions were still using engines with side valves, a far cry from
the AC's single overhead camshaft and inclined valves.
beyond its time...
of course, even the brilliance of its design could not disguise the fact
that the engine had reached the limits of its potential for development
and had become too long in the tooth even for the AC Factory
to perpetuate its use in their cars. Thus enter the Bristol engine and
eventually the Zephyr.....but
that is another story.
still not out...
their abandonment of engine manufacture, the Factory supported their old Six
years with an excellent spare parts supply.
Few manufacturers can have been so loyal to their customers for so
long. Inevitably though, time caught up with even this lifeline; the
Company changed hands and premises more than once and rationalisation made
supplying spares in small quantities for such an old engine an economic
nonsense and a distraction from the serious business of producing high
powered sports cars for a very changed world market. Also, many of
the small companies that had supplied parts to AC were themselves going
out of business as the realities of economics overtook them.
lifespan of the engine however, was far from over and indeed, is brighter now that at any time
since production ceased.
And that is what this website is all about!
by John Weller under the leadership of SF Edge in 1919, the AC Light Six
engine remained in production until 1963.
there were many improvements to the configuration of the engine during
this astounding production run, the last engines were clearly identifiable
with the very first, a tribute indeed to the soundness of the original,
highly innovative design, with all engines sharing the same 1991cc
capacity, a bore of 56mm and stroke of 100mm and wet liners in a light
aluminium block. One feature
of the engine, initially derided but later adopted by other makers, was
the use of a long timing chain to drive the single overhead camshaft,
chain tension being maintained by the Weller sprung blade. In
the first engines that same camshaft also drove the water pump.
magneto and the dynamo were driven by either end of a cross shaft itself
driven by a helical gear mounted behind the fourth main bearing of the
crankshaft. The same helical
gear also drove the shaft that turned the oil pump mounted low in the
forged iron crankshaft was initially carried in four white metal main
bearings, the rearmost of which had to support the massive weight of a
huge flywheel. The clutch was
separate from the engine and mounted in the centre of the car, the
three-speed gearbox being unitary with the rear axle. A
single, updraught Stromberg carburettor provided the fuel.
brilliant design notwithstanding, the engine did have some faults, notable
amongst which was a tendency to vibrate at certain RPM.
Lightening of the pistons, rods and crankshaft alleviated but did
not cure the problem. Partial
salvation came when Sydney Smith modified the rear end of the crankshaft
by adding an adaptor that carried both an extra, fifth main bearing and
the chain sprocket but the vibration was not fully cured until 1927 when
Smith modified the flywheel by splitting it into two parts, the outer
being mounted on the inner via a series of rubber rings, thus adding the
function of a damper to the flywheel.
the vibration issue behind them, AC were free to gradually, if somewhat
conservatively, increase the power output of the engine.
And it was this conservatism, coupled with poor trading conditions,
that brought AC to its knees in 1929 when, refusing to update the by now
hopelessly outmoded chassis and transmission of their cars, the receivers
were called in.
owners, new engines...
life came when the Hurlock brothers took the company over, mostly for the
premises but, from deciding that they might just as well complete the
unfinished cars that they acquired, they eventually resumed car
manufacture and introduced a range of supremely elegant and high
the new cars were a great advance on their forbears as the ancient
transaxle finally disappeared and the gearbox was now attached to the rear
of the engine, thus necessitating changes to the rear of the crankcase
casting. Although a
conventional clutch was now employed in place of the earlier, centre
mounted type, the split damper flywheel was retained and remained a
feature of the engine for many more years.
The single Stromberg carburettor layout was relegated to the status
of an option whilst most cars were fitted with triple 11/8”
SUs. The drive to the water
pump was now via a conventional belt driven by a pulley at the front of
the crankshaft and the magneto was replaced by a distributor.
The new engines, of steadily increasing power, were numbered in the
UMB prefix series. Some
engines were supercharged. During the Thirties the inverted tooth
timing chain was superseded by one of the Duplex type.
WWII the AC engine was perpetuated in the Two Litre Saloon, the low bonnet
line of which necessitated the moving of the water pump to the left hand
side of the block, whilst engine mountings, formerly forward pointing, now
the production run of the saloon, the UMB engine remained largely
unchanged, the most significant modification being the final abandonment
of the split damper flywheel in favour of a conventional, front mounted
Metalastik vibration damper.
was the introduction of the AC Ace and Aceca that spawned the last changes
to the AC engine. Needing more
power, the UMB engine was finally phased out in favour of the increasingly
more powerful CL, CLB and CLBN series of power units, all having shell
main and big-end bearings and 11/4” carburettors.
Spurred on by lack of space in the Aces and Acecas, the dynamo migrated from its
traditional position at the rear of the engine to a more normal, belt
driven siting at the front of the engine.
Power output had
increased from an initial 40bhp for the very first engines to a claimed
110 for the last.
not modern enough...
although the AC engine had shrugged off some of its anachronisms
almost conventional in detail, it remained at heart a vintage engine that
had lived beyond its time, a dinosaur amongst emerging leaner, fitter and
faster mammals. It
could have been brought up to date. It could have been redesigned in
line with contemporary trends with a shorter stroke and larger bore but
such would have necessitated the completely new manufacture of just about
every part of the engine. It just didn't make economic sense,
especially for a company as small as AC. It was far more cost-effective
to outsource than re-design. Nonetheless, it was not until
1963 that the faithful old AC Six finally went out of production.
many are left?
is believed that a total of 2,750 cars with AC engines were
produced. Allowing for spares (from which complete engines could and
have been built), this could mean that about 3,000 engines altogether were
made. Best estimates suggest that about 1,000 of these survive
today, either running, under repair or salted down for a rainy day and
ranging in condition from very good to total wreck, mostly the latter.