I have just installed a new 7-speed mega cassette (MF-HG40) and the old IG chain seems to hit high up on the teeth of the cog (rather than between teeth) as the link meets the cog causing a grinding noise under load.

Is this just a simple prob of chain stretch? Could the derailer need tuning?

Shimano recommend a HG50 chain for the MF-HG40 cassette. Can I use a HG40 chain which is cheaper?

I am about to fit 9-speed cranks (FC-M440) can I expect any problem with an 8-speed chain. I know this has been covered a bit but looking for wider opinion.