I've bought the Shimano HG40 chain which is advertised as a 5,6,7 and I think 8 speed chain. I've just received it and it also says 15,18,21,24. (Obviously including the front gears). My bike is a 2x6 so it's got 12 gears, why isn't that listed and will it still work? Surely it has to since it seems to be one of the only chains for 6 speed. Thanks, Harvey.
It will work adequately on your 6 speed rear cluster. Its basically 8 speed chain which is identical width to 7 speed chain, and its "good enough" for 6 speed.
Curiously, on a supplier's website for HG40 I notice this line:
Shimano Altus HG40 6-7-8 Speed Chain
7.3mm chain width, optimised for 7- and 8-speed HG drivetrains
6 speed is 0.5mm wider but given the tolerances of the day, that should work adequately. You have to move the derailleurs a tiny bit further to make a shift if its friction shifting. The FD cage may need to be closed up slightly.
Finding 5/6 speed chain will be interesting - even KMC's own chain checker only lists 8 speed chain as the best option. http://www.kmcchain.eu/chain-road_cross-multispeed-5_speed
Apparently "clarks CL20" is 5/6 speed chain and may be found at various on-line suppliers.
6 speed is a comparatively old standard and is way less common now, so not worth printing on the packaging?
6 speed – 7.8 mm (5⁄16 in) (all brands)
7 speed – 7.3 mm (9⁄32 in) (Shimano HG), 7.1 mm (9⁄32 in) (SRAM, Shimano IG)
8 speed – 7.3 mm (9⁄32 in) (Shimano HG), 7.1 mm (9⁄32 in) (SRAM, Shimano IG)