Re: Broken front spring
I don't know if 166s are very similar to 164s as far as front springs go but I failed this year's MOT on a broken offside front spring on my 164 too! I replaced only the broken spring but I replaced it with one from a scrapper - so any argument about a new one not having "settled" wouldn't have been valid.
I don't agree with the comparisons drawn between springs and brake linings, tyres or cam belts either. The latter are "service" items that are supposed to wear and be replaced periodically. Alfa don't have any specified service interval for road springs - they're supposed to last the life of the car. It would be more like me also changing the passenger side window because someone broke the driver's side window!
Of course, a badly designed / manufactured spring will sag or break sooner or later and if the springs are marginal in either of these respects, then changing them as a pair would be sensible. On the other hand, as far as a difference in handling goes, I didn't spot any difference at all on the 164. Even immediately after changing the broken spring I didn't notice anything either! The other thing to note here is that the weight (on a 164 at least) is not evenly distributed from side-to side anyway. I weighed mine and was staggered at the difference in front corner weights from side to side. As a Left Hand Drive, the car would be pretty well balanced side-to-side with a 75kg driver in the left hand side. The extra weight of the engine on the right offset this. On a RIGHT hand drive 164, however, the car was already about 75 kg heavier on the right hand side BEFORE I got in! With an inherent inbalance like that, I very much doubt if ANYONE would be able to notice a (say) 5% difference in spring rates from side to side!