Bicycle Mechanics - Chain width compatabilities: which work, which don't

There seems to be ALOT of questions lately about modernizing or retrofitting old bikes to new parts. Invariably, chain width and cogs spacing issues come up.
Older vintage 5 and 6 speeds chains are the same. When 7 speed first came out, those chains were just a scoosh thinner. But they'd still work on the older models, and older chains would usually work with the new freewheels. NOTE: This is for FRICTION shifting only.
These older chains will typically NOT work with the newer, narrower cassette cogs. To get more gear options (or index shifting capability), you'll have to upgrade everything.
For a more thorough treatment (and alot of stats and figures), visit Sheldon Browns website.

5-speed and "standard spaced" 6-speed freewheels and 6-speed cassettes (both intended for 126 mm dropouts) used the same chain width. It's what we would now call a wide chain.

Sun Tour made Ultra-6 6-speed freewheels that were narrower and allowed a 6-speed freewheel to be used in a 5-speed frame/hub (120 mm dropouts). These Ultra-6 freewheels needed a "narrow" chain that is the same width as current 7 and 8-speed chains.

CRANKS
Your chainrings really don't care what chain goes on them; the difference between an 5-6-7-8sp chain and a 10sp chain is only the outside width. This means that you can put a 10sp chain on just about any crankset you want.

There is a (small) potential that the spacing between the chainrings could be too big and allow the chain to fall between them, but that is very unlikely.

This does not necessarily mean that your front index shifting will work flawlessly. You will just have to try it and see.

Interloc Racing Design has 5, 6 and 7 speed freewheels

http://www.interlocracing.com/freewheels_steel.html

As a test, I tried a Campy Record Ultra Narrow 5.9 mm chain on my wife's 9-speed Ultegra triple. It work fine in the back but would not shift in the front at all. The chain would slide on the tips of the chainring teeth instead of dropping in place on the rings. The outside width of the chain does effect shifting in the front because the FD pushes the chain into position.