Background to the MGA
When the MGA arrived in 1955, it must
have come as quite a shock to MG aficionados who had become used to the
pre-war look of the company's sports cars. Even the revamped TF left
nothing to doubt about its 1930's-style design. The MGA was a complete
departure in styling for MG.
Its beautiful streamlined body was right up to the minute in terms of
appearance, and it was powered by a new engine, as MG had decided that the
old XPAG unit had had its day. The MGA was powered by the much more modern
B-series engine that had made its debut in the recently announced Magnette
MG enthusiasts had been given a hint of
what was to come as early as 1951 when George Phillips drove a re-bodied
TD Midget in the Le-Mans 24 hour endurance race. The car had been built
for him by MG following his successes with his own TC, which had been
fitted with a lightweight two-seat race car style body. So different was
the appearance of his new TD racer, that it must have been difficult to
believe that it was actually a venerable TD Midget underneath!
It was a road-going version of the Phillips car which had been proposed to
BMC in 1952 as a replacement for the TD Midget, but which had been turned
down because of the corporation's decision to build the Austin-Healey 100.
MG had gone as far as building a full prototype of the MGA by using TD
running gear, the 1250 XPAG engine, a re-designed chassis, and the MGA
When it eventually became clear that the TF Midget was a bit of a lame
duck and that it would have to be replaced, the new MG sports car was
finally given the chance it deserved. The delay in production had one
advantage in that it allowed MG to refine the design and install the much
newer 1489cc four cylinder B-series engine and its transmission from the
Before the official launch of the MGA in
1955, three aluminium bodied prototypes of the new sports car, coded
EX182, were entered into Le Mans. Fortunately for MG, they acquitted
themselves well, finishing fifth and sixth in their class.
Thus, when the MGA was finally announced shortly after this, the car
already had a competition background as testimony to its pedigree. No
doubt this did much to ensure its acceptance by enthusiasts who were
reluctant to say goodbye to the old fashioned traditional looks of the MG.
A Change of Style
The chassis for the MGA was a
development of the TD Midget's unit, but with more widely spaced side
rails to allow for a lower seating position to fit in the new sleek
bodywork. This not only put the driver and passenger in a more sensible
position in relation to the proportions and height of the body, but it had
the added advantage of lowering the centre of gravity, thus improving the
cornering ability of the car.
The two-seat open body was unmistakably
a development of the earlier Philips racer design, being of the full width
type, the wings blending into the bodywork and each other to produce a
beautiful and aerodynamic design.
Other than a shortened, stylised and widened version of the now familiar
MG grille, there was very little about the MGA which bore the slightest
resemblance to any of its predecessors. From the scuttle, the body fell in
one constant curve to the radiator grille, blending into the full swept
front wings on each side. The line of the front wings was taken back past
the cockpit with its cutaway doors, to where it merged into the rear
wings. These tapered almost to a point at the rear and were blended into
the rear portion of the bodywork that curved down from the back edge of
A pancake-style bonnet provided access
to the engine, and a separate boot for luggage was able to provide a
reasonable amount of space despite the fact that the spare wheel was
mounted to the boot floor.
The car had bolt on steel disc wheels as standard, but centre-locking wire
wheels were available as an optional extra, along with a removable
hard-top with rigid sliding windows to replace the soft-top's sidescreens.
The MGA Twin-Cam
The MGA continued in these open and
closed forms until 1958 when another high performance version was added to
the range. This was the MGA Twin-Cam, which was essentially aimed at
competition use rather than everyday road use. In appearance, there was
very little to distinguish this car from the other standard MGA models,
apart from its special centre-locking steel disc wheels. However, there
was a lot more to this car than met the eye...
engine was a
development of the B-series unit which was being used in the standard car.
Essentially, the cylinder block and bottom end were strengthened B-series
components, but the cylinder head was a new aluminium unit incorporating
twin overhead camshafts. Twin SU carburettors were fitted as standard,
giving a power output of around 110bhp which was sufficient to propel the
Twin Cam to a maximum speed approaching 115mph. At the same time it
slashed acceleration times by a considerable amount. With all this power
available, it was considered that the old drum brakes inherited from the
TD Midget were no longer up to the job, so they were dropped in favour of
four-wheel disc brakes.
MG A twin
The original twin-cam engine was the
idea of Gerald Palmer, the designer of the 1953 MG ZA Magnette, it's
sister the 1952 Wolseley 4/44; the 1955 Riley Pathfinder and Wolseley
6/90. The ZA Magnette was offered to the public using the then new 1489cc
BMC 'B' series four cylinder ohv engine. This engine produced 60bhp, later
improved to 68bhp. The ZA was a fine sports saloon of its day, but Palmer
tried to convince the new BMC bosses to build a twin-camshaft engine of
his design, so as to be able to offer a 'GT' version of the ZA as well. MG
were being starved of cash, as BMC would not even sanction the MGA at
first, let alone another engine. Things did not lay idle for long though,
as both the Austin Design Office,(ADO) and Morris Engines Branch, were
permitted to design their own 'Twin Cam' engine.
Meanwhile the MGA was introduced to an
excited public in 1955, using the ZA 68bhp ohv engine. This was improved
to 72bhp in 1956 in the MGA, but the following 1956 ZB Magnette kept to
the 68bhp unit.
Palmer had penned an outline of the
engine he saw as suitable, but the two halves of BMC set about their own
designs. By July 1958, a Twin Camshaft engine had been chosen from the
designs. That from the ADO had won. The unit from the Morris office had
been designed from first principles, on a clean sheet of paper. That from
Austin was more relevant, in that it used the cylinder block of the MGA/ZB
1489cc bored out to 1588cc. This would easily fit the current MGA 1500
gearbox and engine bay. Breathing was always the limit on any engine's
power output, so the double overhead camshaft ( dohc) cylinder head was
ideal. It was an aluminium alloy casting, of the cross-flow design.
To fit the dohc head to the 'B' series
cylinder block, the studs were slightly repositioned. So successful was
the design in improving the engine, it would rev quicker than some people
could take their foot off the throttle pedal. The engine was easy to
over-rev, and hence damage the internals, such as a piston hitting a
valve. A cure that MG never used, existed even in those days, a
rev-limiting rotor arm.
The Twin Cam required 100 octane fuel,
or it pinked, or worse, overheated. For fast driving the correct,
cooler-running spark plugs must be fitted, N7YC, (were N5RS.) In normal
road use, Champion N3 were used with 100 octane fuel. If fuel less than
100 octane was used, Lodge RL47 were recommended, but the engine would run
on after. The unit got itself a bad name for melting piston crowns, but
this may have had more to do with inexperienced drivers, and mechanics,
who did not understand the meaning of accurate ignition timing, correct
plug grades, or RPM red lines. The original 9.9:1 compression ratio did
not help, and the last few engines were found to last longer and behave
better when this was dropped to the normal MG 8.3:1 ratio, ( from engine
number 2251.) The 8.3 ratio dropped the power figures from 108bhp to
100bhp @ 6700rpm.
Zero to 60mph was in 11.5 seconds (
factory figure,) and in 1958 that was fantastic in a car of the MGA Twin
Cam's price range. Zero to 100mph took just 30 seconds. Motor magazine
testers said 0-60 was in 9.1 seconds; Autocar magazine said 13.3 seconds.
The car prepared for the Autocar testers was obviously a poor one. The oil
consumption of 1500 miles per UK gallon was poor, mirroring that of the
Jaguar XK150, ie awful. The Twin Cam did an official 21mpg of petrol.
MG issued a small pink leaflet entitled
'Getting the best from your MGA Twin Cam'. It has, in large letters, "
PLEASE READ BEFORE DRIVING". No doubt this was because the car was being
used by owners who did not understand the requirements of, what after all,
was a road-going racing car.( Leaflet is BMC publication ADK1412.)
The engine used a normal 1489cc cylinder
block, with the water passages re-cored to allow bigger bores of 75.414mm.
With the 88.9mm stroke this gave 1588cc capacity; about the maximum a 1489
block can be bored to, without moving the bore centres about; ( as MG did
with the later 1622cc MGA 1600 Mk2; and again to get 1798cc on the MGB
block.) It was fitted with a ribbed, cast aluminium alloy sump, cylinder
head, and front casing carrying the new position for the distributor. The
'camshaft' of the old block now fulfilled the use of a jack-shaft, (
called a half-speed-shaft by MG,) to drive the oil pump only.
This shaft is gear driven, not chain
driven as on the ohv engines, so rotates in the opposite direction to the
old camshaft. The engine has fully floating gudgeon pins, ( wrist pins,)
in solid skirt pistons with a compression ratio of 9.9 to 1. The head
carries two cast-iron camshafts, with 20;50;50;20 valve timings, and
0.375" lift. The inlet valves are 1.6" diameter, and the exhaust 1.44" dia.
Both are set at a 45 degree angle in the head giving a hemispherical
combustion chamber, ( hemi-head.) Double valve springs are fitted. For
racing use, the exhaust valves are filled with sodium inside the stems.
This leads the heat away from the head quicker, and was a common fitting
to WW2 piston engined figher aircraft. Both cams are identical, appart
from the timing slots; and both have hexagons cast in to enable them to be
turned when timing.
The cylinder head, with renewable steel
valve seat inserts, was of efficient cross-flow configuration. The exhaust
manifold was on the normal 'B' series side, but with two huge SU H6 1 3/4"
carberetters, (carberetors,) with pancake air filters and a cold air feed
duct, on the other side. On cars meant for racing, the SU's grew to H8 2"
carbs. Interestingly, on the SU inlet manifolds, one securing nut is in
the INSIDE of the port. Owners would Locktite and peen the threads, so as
to not lose this nut into the engine. On cars fitted with after-market
Webers and relevant manifolds, for racing, did not have any nuts inside
their manifolds. To feed fuel to the two carbs, there was a high-capacity
double-ended SU electric pump fitted, similar to that fitted to the Jaguar
To hold the solid skirt aluminium alloy,
convexed head pistons to the crankshaft, 'H' section forged 55 ton,
alloy-steel connecting rods were fitted. These were the normal angled
split big ends type, but without the pinch-bolt little end arrangement of
the normal ohv unit. No oil spray hole was drilled in the rod end either,
for bore lubrication. The fan was of five blades, not the normal two or
four of the other cars. The engine was a 'BC16GB' in BMC engine numbering
terms. All you could see under the open bonnet was two massive, polished
alloy camshaft covers, it must have been a swine to work on in situ. Later
cars had the inner wing splash panels capable of being removed,( inner
fender panels,) to give better access to those hidden components, with a
front wheel off.
Early cars used the MGA/ZB Magnettes
4psi cooling system, but later this rose to 7psi. Again, the distributors
had a vacuum advance from the MGA/ZB, ( type 40510B,) but this was soon
deleted due to ignition timing wander. Not something such a cammy, fussy
engine known to be prone to burning out pistons required. The later
distributor, ( type 40718A,) with no vacuum advance, was often
retro-fitted to many earlier cars. From engine number 1523 the ignition
markings were stamped on the distributor, ( 22-26 degrees advance at
3000rpm.) Distributor drive gears would wear rapidly as it was only
supported one side, especially if a magneto was fitted.
At engine number 1587 cast-iron tappet
bore liners were introduced, due to the bucket type cam-followers picking
up on the original aluminium bores of the head, they ran in. These iron
liners were secured by a grub screw. MG had some fun with the tappets (
followers,) themselves, as the original 1 1/4" long versions could tilt in
their bores at high rpm, and would lock-up. This broke the camshaft, and
led to the engine blowing up, often with the piston hitting a locked open
valve as well. From engine number 1087 the tappets were lengthend to 1
1/2" long. The camshafts were chain driven off the half speed shaft,
itself driven by a gear on the crankshaft nose.
This was a long chain, and could mis-behave
if not tensioned/timed correctly. MG went to great lengths to ensure
owners/mechanics knew this, and issued a four page 'service memorandum' on
the procedure, written by Bob Seymour, MG's Twin Cam expert in the field.
Bob travelled the world 'fixing' Twin Cam problems, for MG. The chain
would stretch in use, and regular adjustment was very important. The
tensioners were aluminium and the threads inevitably pulled out from the
constant whipping of the chain, after high mileage. This can be cured
using a helicoil thread insert, or fitting aftermarket steel tensioners.
The little end bushes were modified at
engine number 710, to improve the oilways. MG did not trust the
'pinch-bolt' little end arrangement of the ohv 'B' series. ( The wrist-pin
to con-rod clamp bolt.) Engines up to number 445 had rough turned chromed
top piston rings, with a cast-iron oil-scraper ring, with oil drain holes
directly underneath in the skirt. From engine number 446 to 605, the oil
control ring was improved to a twin-segmented, ( twin-rail,) oil ring,
with no drain holes. From engine number 606 a new type piston was fitted,
with a smooth chromed top ring, and the segmented oil control ring, but
the spacings were wider.
From engine number2060, expanders were
fitted behind the oil control ring segments. All this was to try to
control the high oil consumption. Today we know it was half the fault of
the oil technology of those days, too thin when very hot. Progress has
improved oils beyond recognition. At engine number 315, the ratio of the
gears on the half speed shaft to the oil pump drive was altered, to speed
up the pump. It was 10-11 teeth, changed to 11-10 teeth.
There were quite a few niggly things
with the Twin Cam engine, other than the above problems. On early cars,
numbers 504 to 531, the starting handle dog on the front of the crankshaft
could foul the steering rack. Dynamo brackets would crack, so MG issued
stronger cast-iron items. The gearbox ran very hot, and if its breather
blocked up, the expansion of the oil could cause leaks.
If, after suffering too much, MG would
fit a normal ohv 1600 ( 1588cc) engine for you. The Twin Cam production
ended in April 1960, after 2111 cars. About 50 went to the USA. A 1588cc
developement Twin Cam MGA beat a 2639cc 'C' series engined prototype
Austin Healey 100/6 in the USA, in August 1956, in a record attempt. The
MG, EX179, did 170mph, the Healey just managed 152mph.
Experience with racing the Twin Cams
over the years has led to the possible cause of the piston problems. After
all it was not the timing or plugs, but simply fuel starvation at certain
rpm, giving a very hot, weak mixture. The cause was fuel frothing that
leaned out the mixture, and then burnt out the pistons, the fault the
engine became notorious for. This occurred at two specific engine resonant
frequencies that coincided with about 2500 and 5500rpm.
The first would pass by quickly, but the
second was within the rpm range often used. The vibration had a tendancy
to jam the fuel float on the centre pin, resulting in fuel starvation at
those rpm. All SU cars had this problem, but those fitted with Webers did
not. The solution, use Webers, or have the SU manifold machined to take
the Weber rubber 'O' ring from the 45 DCOE. This fits the SU H6 bolt
pattern. The slightly flexible joint that results cures the vibrations.
The engine was still fantastic for its
day, and like many highly strung machinery, demanded constant attention.
It was not a car to pose in, but to drive.
1958 MG MGA Twin Cam
dimensions & weights
8 valves total
2 valves per cylinder
Bore × stroke
75.40mm × 88.90mm
2.97 in × 3.5 in
(96.906 cu in)
2 SU carbs
109.5 PS (108.0 bhp)
142.0 Nm (105 lbft)
1.11 bhp/cu in
0-50mph (80 km/h)
rack & pinion
Dunlop RS4 5.90-15
Dunlop RS4 5.90-15
Brake ∅ F/R
Top gear ratio
Final drive ratio