Originally Posted by
FBinNY
It's not about the outer plates being thicker or thinner. It's the width of the inner block ---- 2 plates rollers ---- that drives the bus here.
Connectors vary in terms of the usable length of their pins to match the inner links that they fit.
As Kontact noted, many chains are marketed as 6s/7s/8s meaning they're narrow enough for 7s & 8s and will also we work for 6s which is wider.
So, you need one says 6s, or 5s/6s. Better yet, measure the outside of the inner link, and buy based on that.
Just to clarify, the inner width of the chain where the cog teeth go are the same on 5 through 11 speed. Chains got narrower largely because the plates of the inner and outer links got thinner. So the shaft of a 10 speed quick link will not pass all the way through the inner link of a 9 speed chain because the 9 speed chain's inner plates are too thick - the little groove in the pin that the link locks on to won't be exposed.
"Need one says 6s" - one what? I don't think there are any true 6 speed quick links, and no one 'needs' a 6 speed chain to operate 6 speed freewheels. If you want to use old 6 speed chains, you'll be using a chain tool to put it together, not a special link or pin. And if you want to use a link, buy a 6/7/8 speed chain that comes with one and use that. There's no other practical solutions.