Not yet mentioned as a reason why the wider 5/6 speed chains of the1970s and early 1980s (which often measured at about 7.8mm pin length) is the chain ring center to center amount. I have replaced those old spec chains with "modern" 7.3/7.1mm wide ones and have found that when shifting off the big ring the narrower chain can end up riding the tops of the smaller ring teeth. As though you shifted into "neutral". For a rider not expecting this loss of pedal support (the force to move the bike forward) can cause a fall down at worst and a stalling on a hill. The wider "classic" 5/6 speed chains wouldn't do this to anywhere the same amount. But good luck finding a new one. Andy