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The seized big-end bearing was on cylinder 4 so I could chose between keeping 2 and 3 och 1 and 2. If I had kept piston 2&3 the engine would be a more common twin with "symmetric" firing order of 360 deg. On the other hand it would also get the same problems with mechanical balance as any single cylinder engine. In that case I would have to add some counterweigths at the crankshaft but would still have had vibrations at any speed.
I also have a smaller Fiat 126 with a 600cc paralell twin in the rear. The only reason this engine SEEMS to run smooth is because it rests on a big soft coil-spring. The engine itself is constantly rattling itself to pieces. It´s air-cooled with at lot of sheet-metal that cracks everywhere if you push it to the maximum. (which you mostly do because it only have23 hp...)
A single cylinder och 360 degree P-twin simply can´t get good balance unless a balance-shaft or similar solution is added. This is why I chosed to go for the 180 degree configuration on my Fiat 127. The two counter-acting pistons doesn´t give perfect balance either but the need for counter-weight is less critical I think. Since the 1050cc straight four is quite long i block and it´s crankshaft have five bearings the weight of the block will reduce the rocking vibrations. I guess the front-drive gearbox at the end of the engine also add some extra momentum.
Anyhow, my somewhat unusual cylinder arrangement gave a well balanced engine at any speed, but due to the long separation of 540 degrees every second firing the engine´s power-strokes felt a lot at low rpm. I also guess that the car would have been slightly easier to drive if I had chosed the more common 360+360 config but I´m sure the vibrations at ANY speed would have driven me crazy and perhaps killed the engine.
It would be verry interesting to try the same modification but saving piston 2 & 3 to get a comparison.
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