Originally Posted by
Jeff Neese
Responding so that your post doesn't confuse newbies. There are no differences between 6 and 7 speed chains, at least in the Shimano world.
Surprised me too, but KMC differentiates between them although not consistently on their package, website or by vendors. For example the Z33 chains I bought to splice together a couple of weeks ago for a long wheelbase bike (needs 122 links) were marked as 18 speed on the front (and on their shop chart in PDF version), but 6-12-18-21 on the back. Some vendors list it as 5-8 speed; others as 5 or 6 speed only. It's an ordinary looking chain, and appeared identical to the brown/grey Z7 chain originally supplied with the bike, but without a micrometer I couldn't measure it to determine any minor differences in pin length, etc. Some KMCs are 7.1, others are 7.3, and while it's usually marked on the boxes that info is not consistently supplied on KMC's website, so ordering online can be a crap shoot.
And I'm not sure whether it's critical with a bike using Shimano's RevoShift twisty grip shifters.
But the main reason for the skipping on the two smallest sprockets appears to be wear, so I'll replace it. Both the new Z33 and relatively new Shimano HG50 chains skipped on the two smallest cogs, but were fine under pressure on the less worn gears.