Very interesting! Although it appears the angle of the bit and blade tips are not precisely perpendicular to the gauge (?)
Ernesto keep in mind you want to measure the body of the sensor, not including fins. They are there for isolation and conform to hole diameter. Had you measured the original 4.0 you would have seen the same dimensions.
I'm not using an insertion tool and only using the caps. However, I take my Relton bits from Wagner and have them sharpened by an industrial blade sharpening shop (even the new ones). I know some will say that you cant keep an sharp edge on a roto-bit but these guys do a great job and my sensors go in smooth and snug. Getting a perfectly round true 3/4 hole is key.
Surface Testing Group
what is the problem again?
I remember a scramble to get a black insertion tool on the market because the white one was destroying probes.
From the pictures I've seen, the black tool pushes on the sleeve so it doesn't ruin the sensor.