Maybe a slightly bent hanger or cable is a touch out of adjustment

 

Maybe a slightly bent hanger or cable is a touch out of adjustment

Yes, that should do the trick. I had this problem as well and that's how I fixed it. The teeth are not all alike.

 

Once per revolution and only on a single cog does not match a hanger misalignment or cable adjustment.

I'd take the 12t cog off and examine it closely. Try putting it face down on a glass plate and see if you can slide feeler gauges under it.

 

Agree with matfam. Try adjusting cable tension first, but just a tad (1/4 turn or so). When you look at a Shimano cog edge-on, you'll see that the teeth are not all alike. Some of the teeth are factory-twisted, others are not. When the cable is tension is just slightly off, the chain can run clean over the cassette, but just catches on the edge of one of the twisted teeth of the adjacent cog.

The feeler gauge check doesn't work because it assumes that the tips of the teeth are all in line, shaped the same and not twisted. It also assumes that the click comes from the cog the chain is running on, which may not be true.

 

I also think it is likely cable tension. I've had similar symptoms when adjusting SRAM cassettes. There are 1 or 2 teeth aligned pretty far out and if there isn't enough slack it can click on those teeth.

 

I agree with the suggestions from wim & others. I'll try tweaking the RD cable tension first. BTW, I went online & looked into buying a new Ultegra 12T cog - it's about $18. But the cog tooth catching the chain might well be the next cog up or down, meaning I could buy the wrong cog! Once you buy 2 - 3 cogs, you could have paid for a new cassette. Thanks for the advice; I'll work on it tonight.
Bob

 

I must have a bent tooth on the 12T cog, right?

Why are you asking us? It's your cog, so why don't you just look at the damn thing?

Other than that, it's pretty much impossible to literally bend the tooth of a cog during normal operation, let alone for the 12t cog. The suggestions above give good places to start.

 

Problem solved:
I slowly passed my chain over the cogs & found the bent tooth.
The sticking was only when the chain was passing over the 12T cog, always in the same spot. But the chain's outer plates were being lightly grabbed whenever the chain passed between the 13T cog & the 1 bent tooth on the 11T cog! When I disassembled the cassette & examined the cogs individually on a flat surface, the inward-bent tooth on the 11T cog was evident. I'll attempt to bend the offending tooth outward. If this fails, I'll buy a new 11T cog.
BTW, switching to an 11-speed chain might work as well, since it's a bit narrower.
Thanks for your help.
Bob

 

I'll attempt to bend the offending tooth outward. If this fails, I'll buy a new 11T cog.

Don't be too surprised if the tooth breaks off. And if you decide to buy a new 11T cog, don't be too surprised if it comes with a bent tooth right out of the factory.

 

good,Try putting it face down on a glass plate and see if you can slide feeler gauges under it. thanks