The factory nubbin is just slightly larger than 2x2mm. There are a variety of different cap types for Thinkpads (and other brands like HP and Dell), and many of them have 4x4mm indents which won't fit.
Clit Mouse Nubbin/Trackpoint Cap Replacement
Clit Mouse Nubbin/Trackpoint Cap Replacement
If the mouse nubbin is worn/missing, you can replace it with the super low profile caps for Thinkpads of the T460S-T490S and X1 Carbon Gen 4-8 generations. I've found that these fit the best, both for secure fitment on the isometric stick under the cap and also providing proper clearance for the display to slide over. Aesthetically, the Thinkpad caps flare out creating a larger/more usable touch surface that also fits the concave indent of the keyboard quite nicely. The red accent color is kinda neat, to boot.
The factory nubbin is just slightly larger than 2x2mm. There are a variety of different cap types for Thinkpads (and other brands like HP and Dell), and many of them have 4x4mm indents which won't fit.
The factory nubbin is just slightly larger than 2x2mm. There are a variety of different cap types for Thinkpads (and other brands like HP and Dell), and many of them have 4x4mm indents which won't fit.