Well the 19t is ultegra 10 speed, and the 12t is 105 11 speed.
I got the 105 because it was the only 12t cog which had the recess I believed was necessary to fit the circlip. It also matches the appearance of the minimod 12t.
Can't see either of those being the issue then. My money is on the thickness of the circlip material. G'Luck with that.
yeah it sounds like it. I've got some vernier calipers so I can check it against your measurements, that's really helpful, thanks.
@JurekB started
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Well the 19t is ultegra 10 speed, and the 12t is 105 11 speed.
I got the 105 because it was the only 12t cog which had the recess I believed was necessary to fit the circlip. It also matches the appearance of the minimod 12t.