chain slipping, new chain and cassette

I'll make this as simple as I can.

Bought a bike which had 'apparently' never been used as the previous owner managed to snap the chain before even riding (how do you do that?)

Came with new chain and fitted it, all hunkydory except 9th gear (11t) caused the chain to slip. Shortened chain thinking that might have been the issue but no change. Put up with it and just avoided using 9th gear as thought they must have damaged it when they snapped the chain (although looked ok)

Just bought new cassette, identical to the one fitted and problem is exactly the same. Mech alignment etc is ok. All gears fine except 9th gear. didn't change the chain as only done about 200 miles on it. Is it a case of bite the bullet and fit another new chain to match the new cassette or should it be ok after 200 miles?

Any advice welcome as only bought it as a temp until I can afford one of my dream bikes but can't sell it for anything near what I paid like this.

Thanks everyone

Is it on all front rings that it has a problem?

Chainrings?

Anything fouling the chain under load at the cassette end?

Re:

It's definitely the cassette it's slipping on, if you're really gentle it's fine but as soon as you put pressure down it slips only in 9th. Chainrings are like new and only done about 200 miles. Nothing fouling the chain that I can see

Just thinking chain line not condition.

8spd width chain on a 9spd cassette?

Re:

Nope. Human error (aka my fault). Completely didn't even notice I put the quick link on the wrong way. (First bike I've ever used a quicklink on) Just swapped it around after noticing some writing on it and all is good :facepalm: (will fit a new one to be safe as I know you shouldn't re use them)

I'm an idiot.

Not an idiot. Just an error. Bet you never do it again so all good learning experience.

Glass half full.

Re: Re:

MarinMartin":267gw6p2 said:

I'm an idiot.

Click to expand...

I wouldn't worry, everyone's an idiot sometimes (some more than others ).
At least you found the problem
I was going to suggest it was a 12T cassette lockring on an 11T sprocket.

<tuts quietly whilst walking off into the middle distance awaiting the shot>