ThreadSizeGuideThreads · Drills · Fasteners

Machining fundamentals

Thread Anatomy: Major Diameter, Minor Diameter, Pitch and Flank

A screw thread is defined by more than outside diameter. The major, pitch and minor diameters, pitch, flank angle, crest, root, lead and tolerance system work together.

Major diameterLargest basic diameter
Minor diameterSmallest basic diameter
PitchAxial spacing
LeadAdvance per revolution

The original diagram on this page uses a simplified 60-degree profile for explanation. It is not a manufacturing drawing or a substitute for a standard profile definition.

Simplified 60-degree thread profile showing pitch, crest, root, major diameter and minor diameter
Original ThreadSizeGuide diagram. Simplified explanatory geometry; not a manufacturing drawing.

Pitch diameter is the functional middle

Pitch diameter is the imaginary diameter where thread and groove widths are equal in the basic profile. It strongly influences fit, but it cannot be reliably established from a simple outside-diameter reading alone.

Pitch and lead

For a single-start thread, lead equals pitch. Multi-start threads advance by multiple pitches per turn, so lead and pitch are not interchangeable in every design.

Important limitation
Simplified diagrams omit truncation, allowance, tolerance and inspection details. Use the governing standard for production geometry.

Technical references

Standards references identify scope and terminology; protected dimensional tables are not reproduced.