I recently put a 130mm 52t SuginoSR ring on my bike.
I didn't have an EAI cog with 18 teeth, so I put a cog made by another manufacturer on the hub.
As I spun the pedals with my bike on the workstand, I could see the chain loosening and tightening as everything went round and round.
I thought I might have somehow put the chainring on the spider off center, and so I tried Sheldon Brown's method of centering the chainring.
I spent half an hour trying to center the chainring and didn't get much improvement.
Anyway, I rode with this combination for about two weeks, listened to the chain noise which came and went rhythmically, and eventually decided I had an out-of-round chainring.
Today I decided I had sufficiently habituated to the 75.9 gear inches that 52t X 18t X 23mm provided and I might find it interesting to play around with 80.5 gear inches.
I rummaged through my parts box, found a 17t EAI cog and put it on my hub.
As I spun the pedals my chain did not go up and down.
As I rode my bike with the EAI cog, I could hear nothing from the chain.
All along I had had an out-of-round cog, and not an out-of-round chainring.
I won't name the manufacturer of the out-of-round cog.
Suffice it to say that EAI has made at least one very round 17t cog.
I have found that with SuginoSR chainrings from Sheldon Brown on the outer position of the spider, if I use an EAI cog with the flange against the hub, I have a perfect, silent chainline with no loose or tight spots in the spin.
With this bike, a 2005 Pista, I have decided to buy and use only EAI cogs.