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 XK's.

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.

Written by Neil Cairns, with lots of help from Bob Seymour's brother, (and editor of "Morris Bull Nose Engines" book ,) Peter Seymour; and Bill 'S' who racers MGA Twin Cams in the USA.

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