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    Threads & Fasteners

    Thread Pitch Basics

    Understand TPI, pitch, and common Unified thread classes.

    thread
    pitch
    TPI
    class-of-fit
    beginner

    Formula

    Pitch = 1 / TPI

    Units: inch

    Machine-readable reference

    Structured metadata for formulas, tags, canonical status, and future diagram support.

    Slug

    thread-pitch-basics

    Category

    Threads & Fasteners

    Canonical

    Yes

    Updated

    4/23/2026

    Units

    inch

    Tags

    thread, pitch, TPI, class-of-fit

    Source

    Machinery's Handbook, 31st ed., Threads

    Thread Pitch Basics

    Thread pitch is the distance from one thread crest to the next. In Unified inch threads, the usual shop-floor value is TPI, or threads per inch.

    Quick conversions

    • Pitch = 1 / TPI for inch threads
    • Fine threads give more threads engaged per inch
    • Coarse threads are usually better for fast assembly and softer materials

    Common classes

    • 2A and 2B: standard general-purpose fit
    • 3A and 3B: tighter precision fit

    Source: Machinery's Handbook, 31st ed., Threads