Engineering Reference
Thread Callout Decoder
Paste the callout as it appears on the drawing. Get the diameter, pitch, tap drill, and what the class actually means.
1/4-20 UNC-2B · Unified inch thread
Major diameter
0.2500 in
6.35 mm
Pitch
20 threads per inch
1.270 mm between crests
Tap drill
0.2000 in
approx. 75% thread
Class 2B
internal, standard fit
Major diameters per ASME B1.1 / ISO 261. Tap drills are workshop reference values; confirm against your tap maker's chart for hard materials.
One callout decoded. Your drawing has forty.
Mavlon reads every thread callout on the drawing, patterns and depths included, and balloons each one onto your AS9102 Form 3.
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Frequently asked questions
What does 1/4-20 UNC-2B mean?
Quarter-inch major diameter, 20 threads per inch, Unified National Coarse, class 2B: an internal thread (tapped hole) at the standard fit. The matching bolt is 1/4-20 UNC-2A.
2A vs 2B vs 3A vs 3B?
The letter is the side: A external, B internal. The number is the fit: 2 standard commercial, 3 tighter with reduced allowance, common on aerospace fasteners. The class changes the pitch diameter limits you inspect against.
What does M8x1.25-6H mean?
Metric thread, 8 mm major diameter, 1.25 mm pitch (coarse for M8), class 6H: the standard internal fit. Written as just M8, the coarse pitch is implied.
How does a threaded hole count on Form 3?
The thread itself is typically one characteristic verified with go/no-go gauges; depth, countersink, and position each get their own row. Patterns (4X) are counted per the customer's convention.