I'm riding a '71 Le Juene converted to SS. The rear hub is a flip-flop with an ACS Claws freewheel (20t). I just put a Surly track cog (20t) on the flip side, and find that the track cog is about 3mm inboard compared to the freewheel cog. So with a perfect chainline as a SS, my chainline is too far inward at the rear if I run it fixed. I don't understand how this could be, I thought a SS freewheel would be symmetrical with a track cog for chainline. Otherwise it couldn't be run as a flip-flop.

I know for a fact that Surly is redesigning their track cogs, but I wouldn't think a dimension this critical could have been screwed up.

Thanks for any advice. This is my first SS conversion and I'm anxious to try it fixed too.