Originally Posted by
Clem von Jones
4.8 mm vs 4.34 mm center-to-center
I'm afraid you're right.
https://sheldonbrown.com/cribsheet-spacing.html
The outer chain plates will probably rub against the larger sprocket. I'll try it and see, then report back here in case someone else does a search on this subject.
No need to report back, anyone with half a clue knows already it is a bad idea. Why are you even considering doing it?
Have you tried googling “8 speed chain with 9 speed cassette”? There are some specific combos that work out on occasion but most don’t.
There is absolutely zero benefit to running an 8 speed chain in a 9 speed cassette. Only downsides.