I thought I had read that with Shimano ten speed chains, older chainrings may be too fat for the narrower chain to fit. I really thought ten speed was the cutoff.

With my most recent bike, I threw on a used chain I had sitting in a bin that looked in decent shape. When I saw that it wasn't engaging with the teeth fully, I assumed it had maybe stretched too much. I got a replacement chain without even measuring it, but then the new chain was exactly the same. I went back and measured the old chain with a chain checker and sure enough, it's just fine.

The chainrings also look to be barely worn.

Was I wrong this whole time about 9 speed chains and old chainrings? Not too thrilled to have to order new chainrings for this project, especially since the nine speed is going to be just temporary.