104 mm BCD 4-bolt Chainrings from Harris Cyclery
For more technical detail see the B.C.D. entry in Sheldon Brown's Bicycle Glossary
There is a lot of confusion about the compatibility of narrow 9-speed chains with older cranksets. Shimano says you should replace the inner chainring(s) with specially designated 9-speed ones, but then they're all too eager to sell you stuff, whether you need it or not.
Shimano is also concerned about clueless users. The worst-case scenario is that you will be riding along with the bike in its highest gear (large front, small rear) and then for some bizarre reason shift down in front before downshifting in the back. (There is no shift pattern in which it is reasonable to shift in this sequence.) If you do shift this way, there's a small chance that the chain might "skate" over the edges of the teeth for maybe half a turn.
In practice this "problem" almost never materializes. Many, many cyclists are using 9-speed chains with older cranksets and having no problems whatever.
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