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Development History

Introduced 1958 – 1960. Number built – 2,111

THE MGA Twin Cam was announced in 1958 and was the product of a lengthy and involved development.

This car is somewhat a rarity with only 2111 cars being produced between early 1958 and early 1960. The twin cam was to utilise an engine that was never fitted to any other car and was developed from the B series unit.  For ease of production, economy and reliability, the standard pushrod type of engines had been used by MG, but it was evident that performance suffered as a result and it was when the Abingdon engineers became frustrated at trying to extract more than 60 bhp from the B series Austin-derived engine, that was currently powering the MGA, that attentions were turned to the production of an overhead cam unit. The Company was also keen to get the name of MG back into serious competition, particularly racing, as this had been shelved some years previously during the Nuffield rationalisation.

It was Gerald Palmer, a design engineer at the Cowley plant, who originally set the wheels in motion for a twin cam conversion of the B series engine. In his plans he set the two lines of valves at 90 degrees symmetrically opposed and it was intended that as many of the existing B series engine components as possible should be utilised. The basic designs were then handed over during 1954 to the Morris Engines Division at Coventry to be developed further. It was not until the summer of 1958 that any production units were available for fitting into the MGA and therefore the development work was quite a protracted affair. There was however a prototype twin cam engine which appeared in an MGA that competed in the Dundrod Tourist Trophy Race in Northern Ireland as early as September 1955.

At that time there was also another prototype twin cam engine that was under development by the Austin engineers at Longbridge and this too was scheduled to power an MGA in the same race, but due to rev limitations and carburation problems the car never appeared. The Austin unit was a totally new design and not based on any other existing engine. With the valves inclined at an angle of 66 degrees it proved to be a very smooth engine that also fitted well into the MGA engine bay. This particular engine however was very short lived and apart from running EX 182 at Le Mans it was never to be seen again. The Morris twin cam was not to achieve any success in Northern Ireland due to mechanical problems although it was considered promising enough to form the basis of a new production car and development was started more or less immediately. Further development engines appeared in EX 179 and EX 181 record cars during 1956 and 1957 before the final production version was ready in the summer of 1958. These units were, as Gerald Palmer had intended, based on the B series block, albeit with many changes. The unit was bored out from 73.025 mm to 75.4 mm giving a capacity of 1588 cc, this was to take advantage of certain competition regulations, the bores being siamised. The crankshaft had narrowed main bearings and an extended nose to carry the timing chains, whilst the con rods were considerably strengthened. Heavily domed pistons were fitted to these con rods to allow a very high compression ratio of 9.9:1 which meant that the engine had to run on high octane 5-star fuel in order to get the best performance.

A special light alloy cross-flow cylinder head carried twin overhead camshafts with valve operation via Coventry Climax style inverted bucket-type tappets. The front of the engine displayed a very complex looking alloy casing which housed the drive gear and duplex chains for the camshafts and distributor. Two smart looking alloy cam covers adorned the top of the cylinder head whilst a large finned aluminium sump helped to keep the lubricating oil cool. The carburation was by 1 ¾ ” twin SU’s with flexibly mounted float chambers and they appeared on the left-hand side of the engine as opposed to the right hand side on the standard pushrod engine. New manifolds were produced with separate downpipes for each cylinder making the engine unit look very business like.

At the end of the day the impressive results achieved from all this lengthy development on what was basically a B series block made the exercise worthwhile. A very healthy 108 bhp @ 6700 rpm together with a maximum torque figure of 104 Ib ft @ 4500 rpm meant that the new MGA Twin Cam was to be no slouch. There were other obvious changes that were made to the MGA in producing the Twin Cam. The Chassis did differ slightly to that of the 1500 MGA and there were important changes to the brakes and wheels. In view of the 113 mph performance attainable, Dunlop 10 3/4″ disc brakes were fitted both front and rear together with Dunlop centre lock disc wheels carrying Dunlop Road Speed tyres. Wire wheels were not available as an option.

The brakes were different to any other type of system used on MG’s previously and due to the large braking surface area of the discs they were most efficient and did not require servo assistance. One drawback of the system however was the relative inefficiency of the handbrake which worked on the rear discs by means of a separate calliper with small pads and a pivot system that readily seized up without regular maintenance.

Beyond the wheels, externally there were virtually no visible differences in the body compared to that of the MGA 1500. It was only the wheels and the discreet Twin Cam badges that were fixed to the top bonnet surround adjacent to the air intake grille and on the boot lid below the MG Octagon that betrayed its identity. The instrument layout was almost the same as the MGA 1500 but with a tachometer that read 7500 rpm and a speedometer that took account of the top speed of 113 mph. The instrument fascia panel was given a face lift, being tastefully covered in leather, whilst leather was also employed on redesigned, better padded seats of the bucket type.

In the short production life of two years only 2,111 examples were produced, 1,801 of which were roadsters. Today the car quite rightly has its niche in the MG history book and is a much desired classic.

Brief technical specifications…

Engine: Four cylinder, in line, cast iron block, alloy head.
Capacity: 1588cc
Bore & Stroke: 75.39mm x 88.9mm
Main bearings: 3
Compression Ratio: 9.9:1 initially, then 8.3:1
Valve gear: Twin overhead camshafts with shim adjusted bucket tappets.
Carburation: Twin semi-downdraught 1 3/4” SUs
Power output: 108 bhp @ 6,700 rpm
Maximum Torque: 104 lbft @ 4,500 rpm
Transmission: Four speed manual gearbox with synchromesh on 2nd, 3rd and top.
Clutch: Borg and Beck dry plate.
Suspension: Front – independent by coil springs, wishbones and Armstrong lever arm dampers. Rear – Live rear axle, half elliptic leaf springs, Armstrong lever arm dampers.
Steering: Rack and pinion.
Wheels and tyres: Dunlop pressed steel centre lock disc with 5.90-15 Dunlop cross ply tyres.
Brakes: Dunlop disc brakes front and rear 10.75″ diameter discs.
Length: 13’0″
Width: 4’9.25″
Height: 4’2″
Wheelbase:7’10”
Performance: 0-60 mph; 9.1 secs
Maximum speed: 113 mph
Number built: 2,111 between 1958 and 1960
Price new: Roadster; £843. Coupe; £904