READ THE DIMENSION CORRECTLY

Sources, definitions
and scope.

A reference is useful when you can tell what a number describes, where it came from and what it does not establish.

Exact equivalence: 1 inch = 25.4 millimetres exactly. Diameters are stored as integer nanometres. “Exact” means the same reference dimension, not identical manufactured parts. Displayed decimal inches may be rounded.
Thread positions: Threads use their basic major diameter, not pitch or minor diameter. Actual manufactured diameters depend on tolerances. BSP labels are nominal pipe designations; R and Rc positions are basic gauge-plane diameters on a 1:16 taper.
Tapping relationships: Thread labels carry the recommendation for that exact pitch and form. This is attached information, not a second diameter at the thread’s position. Compare drill centres the actual recommended hole diameter. The optional Tap-drill diameters scale is ordered by hole diameter. Recommendations are transcribed from the Alpha / Sterling cutting-tap chart. They vary with material, engagement and tool. They are not intended for forming taps. Rc sizes use its “drill only” column. G and R have no tapping recommendation in this release.
Version 1 coverage: Continuous dimensions: 0–200 mm. Fractions: every 1/64″ in range. Drills: #80–#1, A–Z, 1/64–1″, and metric catalogue sizes 0.3–40.5 mm. Metric threads: selected coarse M1.6–M56 and fine M4–M50. BSW: 1/8–2″; BSF: 3/16–1¾″; UNC: #2–2″; UNF: #3–1½″. BSP: G, R, Rp and Rc, 1/8–2. This is a selected reference catalogue, not every standardised thread.

DATA EDITION 1.0 · 1,265 reference records

Alpha / Sterling drill selection & tapping chart Starrett decimal equivalents · Bulletin 1317 Gühring thread milling reference tables NIST · Engineering Metrology Toolbox

How the tables and chart stay in agreement

The reference pages and interactive chart use the same catalogue. Unit conversions use the exact inch-to-millimetre relationship; named records retain their source identifiers, thread pitches and forms. Tapping relationships are attached to the exact originating thread.

“Basic major diameter” is a reference geometry, not a measurement of a manufactured part. “Nearest” means closest in physical dimension, not equivalent in use. No new drill recommendation is created by proximity.

Data edition 1.0 is a fixed reference snapshot, not a live standards service. Consult the original source and the applicable tooling or thread specification for your work.

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