OK-This shouldn't be this confusing. But, I bought a DA group without chain that is a combination of 8spd & 9spd. 7400 8spd derailer & cassette with 7700 Octalink crank. I go to the LBS for a chain and they say it's either a 6,7,8 spd chain or a 9,10 spd. I say, "No, the previous owner assured me that these things work together". So, what's the difference between the 8spd cassette and the 9spd crank?