I am building a fixed gear bike out of an old ten speed frame for the first time and after feeling like an idiot at bike shops today I am wondering if anyone can enlighten me on chains and cogs.

Taking Sheldon Brown's advice I purchased a SRAM PC-58 3/32 inch chain off the internet for my fixed conversion. I bought a Dura-Ace 16 tooth track cog from a local bike shop and mounted it on my hub and started to install the chain. The chain did not mesh on the cog, it seemed like the links were too long and the rollers did not line up with the teeth. The chain did fit on the Rocket chainring and the Shimano BMX freewheel on the other side of the flip flop hub.

I went to a different (much closer but no fixed gear parts) bike shop with the cog and tried a 1/8 inch chain. This chain did not mesh on the cog either. The wrench then tried a 3/16 inch chain and this one meshed fine. He looked up the cog in his catalog and it said it was compatible with 3/32 and 1/8 inch chain.

Thinking I had a mislabed cog, I then went to the original bike shop and told them the cog only fit a 3/16 chain. They said no, the 1/8 inch chain would work, and they took a 1/8 inch chain off their shelf and it meshed with the cog.

Can anyone explain what happened? Are derailer, BMX, and singlespeed chains different? When I get home with this new 1/8 inch chain will it fit my Rocket ring that the PC-58 chain fit?