I look into the end of a SHCS and reliably select the wrong hex wrench.

My first thought was to train my eye to better improve my success rate. Some hex sizes are very close and I have not been able to see the difference.

My next idea was to build an axially telescopic gage that would fit in the hole. A round cross section works, I don’t need hex. Besides, I’m fresh out of drill bits that cut hex holes. This too became tricky due to very thin walls.

I also thought about an expanding gage but this too was tricky due to the 100 thou min diameter.

Does anyone have a suggestion for a simple tool?

Thanks in advance,

Rick