The “63M” refers to Starrett’s 63 Series Long Range Micrometer Heads in the metric version, which have a travel of 0–50 mm.
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Features include: a flat measuring face, micro-lapped spindle, satin chrome finish (to reduce glare / resist rust), and a very stable one-piece hardened spindle.
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There is a ring-type knurled lock nut to hold the spindle where you set it.
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The body is designed to be mounted in a fixture (plain stem), often used in gages, tooling, or special machines.



