Why is my drivetrain so noisy after replacing the chain? I ride an SE Lager fixed. On the 30th of May this year I did a bit of an overhaul, replacing the rear cog, chain, wheels, tyres and brake pads.

I measured the chain last weekend and discovered that it was already over .75 (which I asked about here), but less than 1.0. So I decided the replace the chain with a new KMC Z410 chain. I use a Surly track cog 1/8 inch, 17t on the back and since it was only a few months old and with a single chain I decided not to replace it. The chainring is also not old and looks to be in fine condition.

Here is a photo showing the old cog (on the left) and a brand new cog (on the right). I couldn't see any appreciable difference which I took as reinforcing my decision that it didn't need replacing.



Unfortunately the next day I went for a ride the drivetrain was incredibly noisy. Clunking and grinding. I could even feel roughness through the pedals. I was thinking maybe the bottom bracket (the only drivetrain component that has not been replaced yet) might have completely fallen apart. What is causing this noise?

Fair warning: I have the answer to this question that I will post shortly. According to my  research, asking and answering a question is encouraged as a source of information

Update: I should clarify that I don't have the answer to the question. I know how I fixed it, but that doesn't preclude another answer being more informative
Fixed gears don't have as much tolerance for mismatched wear patterns as geared bikes, because there is no "give" in the drive train system.

The small amount of wear shown on the used cog is enough to cause this kind of noise in the case of a fixie. I'd be surprised if the same amount of wear caused you any problems at all on a geared machine.

It's also possible that the chain was over tensioned. I know you said that you use a tensioner, and that you didn't change the settings, but you used a new, unstretched, and therefore shorter chain. You may have needed to relax the tension by a small amount.

As you removed the wheel to change the cog, It may be that the tension has changed as well. Hard to say for certain, as you made the noise go away.