My wife's 8 speed specific chain needs to be replaced. Can I use one of my 9 speed chains? She is running a shimano indexed drivetrain with barcons if that matters any...

Similar question...can I use a 10 speed chain on a 7 speed cassette?

Yes, no.
There are no absolute rules about this. Generally you can run a chain that is one or maybe two sizes narrower than intended. What works on a double crankset may or may not work on a triple crankset.

Al

I would use properly width chain. It is the length in links that is more important though.

i dont see any reason not to at least try the nine speed chain on the bike. if it works it works right? its not like its some kind of major waste of time and money if it doesn't work. provided you have a chain tool.

I would bet a ten speed chain is to narrow to shift properly on a 7 speed drivetrain. do i know for sure? no. my bikes are all 6 or 7.

The inner width ( the width of the rollers ) on chains intended for use with derailleur systems has been 3/32" since the days of 5 cog freewheels. What has changed is that the chains have been re-engineered to be narrower over the outer dimension to fit between the narrower spaced cogs of 8 and 10 (11 now) speed systems. So you can use chain intended for more rear cogs than your system has, the only possible problem is that the chain may fall between the chain rings during a shift. The spacing change is really quite small so that is pretty unlikely. The other reason for not using higher speed chain is that they get more expensive as they get narrower.

My LBS only has 8 speed chains. I restore old vintage road bikes with (5-6 speed free wheels) 10-12 speed. The only thing you may have to do is take our a few links if you can't take up all the slack properly. They come with a master link, so that's not an issue. I basically adjust the length such that the idler just clears the derailer in the smallest gear. Hope this helps. Best regards, Doug

thanks for the responses... I just its time to do some trial and error.