Fastener thread designations tell engineers how a screw, bolt, nut, or tapped hole is sized and how its threads are shaped. This cheat sheet helps students read common metric and Unified thread callouts used in drawings, specifications, and shop work. It also connects pitch, threads per inch, major diameter, and thread series so students can choose matching fasteners and avoid assembly errors.
The most important ideas are that metric threads usually give pitch in millimeters, while Unified inch threads usually give threads per inch. A larger pitch means farther spacing between neighboring threads, while a larger threads per inch value means closer spacing. Standard callouts also include diameter, thread form, fit class, handedness when needed, and sometimes thread depth or length.
Key Facts
- A common metric thread callout has the form M10 x 1.5, where M means ISO metric thread, 10 is the nominal major diameter in millimeters, and 1.5 is the pitch in millimeters.
- Metric pitch is the axial distance from one thread crest to the next, so pitch = distance between matching points on adjacent threads.
- A common Unified inch thread callout has the form 1/4-20 UNC, where 1/4 is the nominal major diameter in inches, 20 is the threads per inch, and UNC is the coarse thread series.
- For inch threads, pitch in inches equals 1 divided by threads per inch, so pitch = 1/TPI.
- Coarse threads have larger pitch and fewer threads per length, while fine threads have smaller pitch and more threads per length.
- Right-hand threads tighten clockwise when viewed from the head or end, while left-hand threads are marked LH and tighten counterclockwise.
- A typical internal thread tap drill estimate for metric threads is tap drill diameter = major diameter - pitch.
- Thread fit classes describe looseness or tightness of assembly, such as 2A for common external Unified threads and 2B for common internal Unified threads.
Vocabulary
- Major diameter
- The largest diameter of an external thread or the diameter measured across the crests of a screw thread.
- Pitch
- The distance measured parallel to the fastener axis from one thread crest to the next matching crest.
- Threads per inch
- The number of thread peaks counted along one inch of an inch-based threaded fastener.
- UNC
- Unified National Coarse is an inch thread series with relatively large pitch for general-purpose fastening.
- UNF
- Unified National Fine is an inch thread series with smaller pitch and more threads per inch than UNC of the same diameter.
- Thread class
- A designation that describes the allowed tolerance and fit between mating internal and external threads.
Common Mistakes to Avoid
- Confusing pitch with threads per inch is wrong because pitch is a distance between threads, while TPI is a count per inch.
- Reading M8 x 1.25 as 1.25 threads per millimeter is wrong because the 1.25 value is the pitch in millimeters per thread.
- Mixing UNC and UNF parts of the same diameter is wrong because the thread spacing is different, so the parts will not assemble correctly.
- Leaving off LH for a left-hand thread is wrong because a standard unmarked thread is assumed to be right-hand.
- Using major diameter as the tap drill size is wrong because the drilled hole must be smaller than the major diameter so the tap can form internal threads.
Practice Questions
- 1 A bolt is labeled M12 x 1.75. What is the nominal major diameter and what is the pitch?
- 2 A 3/8-16 UNC bolt has 16 threads per inch. What is its pitch in inches?
- 3 Estimate the metric tap drill diameter for an M10 x 1.5 internal thread using tap drill diameter = major diameter - pitch.
- 4 Two bolts both have a 1/2 inch nominal diameter, but one is 1/2-13 UNC and the other is 1/2-20 UNF. Explain why their nuts are not interchangeable.