It's a surly cog.

I just got this bike (steamroller complete) like a month or two ago. I commute 10 to 15 miles a day, sometimes ride 6 or 7 miles after work, have gone on a few 20 or 30 mile rides. Maybe 600 or 700 miles max.

The reason for swapping chains is I put some bigger tires on here, and there was barely any clearance between the rear tire and seat tube. So ordered a half link and another 2 chains just in case. Couldn't get the half link on the old chain, I got it on, but couldn't close the chain. It's the kind where it has the 2 half links with 1 peg in each, connect and pulling the chain tense locks it in place. The side with the half link just wouldn't close. So was like screw it, lets try the other chain. The single speed chain closes by putting a plate over the link and then another piece slides on to lock it. Went on no problem.

I have read that it's best to change the chain, chainring, and cog all at the same time for this exact reason.

I can feel it in the pedals too, kinda like a vibration. It's subtle, but can feel it.

I guess I'll order a new cog, probably should get a new chainring too? Thanks for the help.