When Don checked mine he used his extra sensory finger tip gauge.
Machine shops in NA discuss things in thousandths of an inch. If you talk millimeters they just stare.
Approximately 17,000 mechanoreceptors (receptors that respond to sensations of touch) reside in each human hand, with the majority clustered in your fingertips. Your fingertips' ridges, which make up the loops and whorls seen in your fingerprints, maximize surface area for increased touch sensitivity.
Don just helped guide me through the head thickness measurement exercise last week. As you can see in the image below, my head (on my running engine!) is at or slightly less than minimum thickness.
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