hyunelan2

I'm in the process of converting over my 8-speed Sora/Tiagra-based Specialized Sequoia, to 105 components. Everything I've ordered is the 9-speed variety (as opposed to the 10). I was going to keep my Sora Shifters and rear cassette for now.

I don't know if I need to use an 8-speed chain (to match rear cassette) or a 9-speed chain (to match front chain rings and rest of components). This is the last stumbling block and my project will be complete... well, ordered anyway.

DMF 

I'd use the 8-speed. 9-speed tooth is - if anything (and I don't think there's a difference) - a little thinner. Too-wide chain is better than too-narrow chain.

robhunterx

The various chainrings are pretty indifferent...while the spacing is definitely different on the various cassettes. Match your chain (and shifters) to the cassette.

Al1943

I think you could use either. If the crankset is a triple, or if the front derailleur is a 9-speed, I'd use a Shimano 9-speed chain to reduce chainrub on the front derailleur.
I would not run an 8-speed chain on a 9-speed cassette but I think you can run a 9-speed chain on an 8-speed cassette just fine. This is based on posts made by others. YMMV

Al

hyunelan2

It will be a double, with a 9-speed FD. Cassette and shifters are 8 speed. I think I'll just hold off on the new chain temporarily and try to set it up with my existing 8-speed chain. If there is rubbing on the FD, I'll try a 9-speed chain.

My first worry was that the narrower 9-speed chain would get stuck 'between gears' on the 8-speed rear cassette. Is this a legitimate possibility?

bonfree

if u are using the 8 sp cassette it is advisible to use the 8 speed chain.A new 9 speed chain might not match a used cassette.

Al1943

I don't think that would be a problem. Your plan to try the 8-speed chain first is good, unless the chain is stretched.

DMF 

If there is chain rub, adjust the dérailleur.

Good question. I doubt that the width is that much different, but I was wrong once.