Why couldn't MG fix the MGA twincam?
Discussion
I have been interested in the MGA twincam for quite a long time, and was doing some more internet "research" recently.
Paraphrasing from http://www.mg-cars.org.uk/MGA/mgatwine.html :
It should have been a great engine, and lifted MG well clear of its rivals, but instead was a costly failure.
From my reading, a properly sorted twincam is indeed a peach of an engine.
Surely they had the engineering skills to analyse and solve the problems, so why all the half-arsed attempts to fix things by trial and error?
Paraphrasing from http://www.mg-cars.org.uk/MGA/mgatwine.html :
mgatwine said:
The oil consumption on early Twin Cam engines was extremely excessive. This also caused fouled spark plugs. The piston rings were changed on several occasions with the latest changes being late in the production period at engine 2057. The main changes were the replacement of the top chrome ring with a cast iron type and a new scraper ring with an expansion ring. This resulted in a normal oil consumption.
...
The unit got itself a bad name for melting piston crowns and the company mistakenly attributed this to over advanced ignition timing. The distributor was changed to a positive stop mechanical advance type without vacuum advance. This did not solve the problem and so it was decided to reduce the compression ratio to 8.3:1 which prevented the pistons from melting, but had not solved the problem, only masked it. The problem was actually caused by the SU carburettors becoming lean due to the effects of engine vibration at certain RPM. This problem was already well known to other engine manufacturers e.g. Coventry Climax. It was even documented by BMC themselves in the Special Tuning booklet for the pushrod MGA if 10:1 compression was used.
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Some engines suffered from fracture of the tappets. This would normally lead to follow-on engine destruction. It was mistakenly believed that this was due to the tappets tilting and locking in their bores and the length of the tappet was increased from 1.25” to 1.50” at engine 1087. However this did not cure the problem and a second incorrect diagnosis was that the tappets were picking up aluminium from their bores and seizing, and so cast iron sleeves were fitted to the tappet bores at engine 1587. The fractures continued until it was finally realised that the tappet material composition was incorrect. At engine 2211 molybdenum was added to the tappet material. All fractures ceased and many engines were retrofitted with the new tappets.
So why did it take them so many attempts to fix the oil consumption and tappet problems, and why did they never solve the carburetion problem? ...
The unit got itself a bad name for melting piston crowns and the company mistakenly attributed this to over advanced ignition timing. The distributor was changed to a positive stop mechanical advance type without vacuum advance. This did not solve the problem and so it was decided to reduce the compression ratio to 8.3:1 which prevented the pistons from melting, but had not solved the problem, only masked it. The problem was actually caused by the SU carburettors becoming lean due to the effects of engine vibration at certain RPM. This problem was already well known to other engine manufacturers e.g. Coventry Climax. It was even documented by BMC themselves in the Special Tuning booklet for the pushrod MGA if 10:1 compression was used.
...
Some engines suffered from fracture of the tappets. This would normally lead to follow-on engine destruction. It was mistakenly believed that this was due to the tappets tilting and locking in their bores and the length of the tappet was increased from 1.25” to 1.50” at engine 1087. However this did not cure the problem and a second incorrect diagnosis was that the tappets were picking up aluminium from their bores and seizing, and so cast iron sleeves were fitted to the tappet bores at engine 1587. The fractures continued until it was finally realised that the tappet material composition was incorrect. At engine 2211 molybdenum was added to the tappet material. All fractures ceased and many engines were retrofitted with the new tappets.
It should have been a great engine, and lifted MG well clear of its rivals, but instead was a costly failure.
From my reading, a properly sorted twincam is indeed a peach of an engine.
Surely they had the engineering skills to analyse and solve the problems, so why all the half-arsed attempts to fix things by trial and error?
I had an MGA 1500, I rebuilt a 5 bearing 1800 with a gasflowed head, bought a 5 speed box and taller diff, made it a fabulous car.
Ten years later I took an MGA Twin Cam in part ex for a Gordon Keeble, the Twin Cam is an absolutely fabulous sports car, a real delight, much under rated.
I've had them all, sprites, A's, B's, Alpines, Healey's, E types, 911's and so it goes on, the Twin Cam is miles ahead.
Ten years later I took an MGA Twin Cam in part ex for a Gordon Keeble, the Twin Cam is an absolutely fabulous sports car, a real delight, much under rated.
I've had them all, sprites, A's, B's, Alpines, Healey's, E types, 911's and so it goes on, the Twin Cam is miles ahead.
MG did get the twincam engine sorted towards the end of the production run of 2100 cars, but by then they had suffered so much bad press that the model's reputation was virtually irretrievable. The good news is that 58 years later a twincam that has been rebuilt using pistons, rings and tappets made from modern materials is a fabulous, reliable and fun classic. Their relative rarity (there can't be many more than 1200 or so still inexistence) must make them a good investment too. Dunlop centre lock wheels and discs all round coupled to a very free revving power unit make the twincam a great MGA to drive....mine is coming out of the Airchamber this week for a summer of fun!
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