In largest cog, an expander. Only under lots of power. New chain and more worn cogs, that the reason?

by ‘cogs’ do you mean the chainrings? (the ones by the pedals) or the cassette? (the big load at the back)

either way, probably.

Ha ha! This is because I went out with my newbie mate today, my terminology has dropped!
I mean the cassette, it’s got a 42t expander on that’s done a lot of mileage.

yes, I think it is generally regarded as good practice to replace chain and cassette at the same time. new stuff with worn stuff does usually mean slippage ime. get yer wallet out!

Replaced chain and cassette last week. On a ride at the weekend there was a slight grinding noise, had to change the chainring as well. Now all is silent. 😆

Having similar issues myself. Been switching between 3 different chains and let them all get to .75mm of wear before deciding to go to a new chain and cassette setup. New chain ran OK on the large chainring but was noisy on the small chainring under load due to mismatched levels of wear.

(Old casssette has a lot of wear and won’t accept a new chain)

Only options for me are a new small chainring or put the old stuff back on and wear the lot out, then start from scratch with all new rings. (and jockeys)

Went with option B as i can get a lot more miles out of my setup, switching between 3 chains. Then I get a full new drivetrain which is better than having differing levels of wear on the chainrings.