Thread Callouts on Engineering Drawings: M, UN, Rc/Rp and More

Thread callouts on engineering drawings encode pitch, tolerance class, fit, and form in a compact notation — misreading a thread designation leads to incorrect tooling, failed assembly, and potential safety incidents in pressure-containing or load-bearing joints.

Threaded fasteners and threaded features are among the most common elements in mechanical assemblies, yet thread callouts are frequently misread or incompletely specified. A fully defined thread callout communicates the thread form, nominal diameter, pitch (or threads per inch), tolerance class, fit, length of engagement, and special requirements (left-hand, coated, etc.). This article covers the major thread standards used in mechanical engineering drawings: metric ISO threads (M), Unified National threads (UN), and pipe threads (Rc/Rp and NPT/NPTF).

Metric ISO Threads (M): The Global Standard

Metric threads are defined in ISO 261 (general purpose) and ISO 965 (tolerances). The callout format is:

M[nominal diameter] × [pitch] — [tolerance class] — [LH if left-hand]

Examples:

• M20×2.5 — 6H: 20mm nominal diameter, 2.5mm pitch, 6H tolerance class (internal thread / hole)
• M20×2.5 — 6g: same diameter and pitch, 6g tolerance class (external thread / bolt or stud)
• M10×1.0 — 6H: M10 fine pitch (1.0mm), internal thread (fine pitch coarse for M10 is 1.5mm)
• M16×2 — 6H LH: left-hand thread (unusual, specified only when functional requirement exists)

When the pitch is the standard coarse pitch for that diameter (per ISO 261), it may be omitted in some company standards. Best practice is to always specify the pitch explicitly to avoid ambiguity — M20×2.5 is clearer than M20 even when 2.5mm is the coarse pitch for M20.

Metric Thread Tolerance Classes Explained

The tolerance class consists of a number (tolerance grade) and a letter (fundamental deviation):

• For internal threads (holes): uppercase letters — H is the most common (zero allowance, fundamental deviation at nominal size), G gives a small positive allowance (slightly larger hole, used for coated parts)
• For external threads (fasteners): lowercase letters — h gives zero allowance (tight upper limit), g gives a small negative allowance (loose fit), e gives a larger negative allowance (free running)
• Tolerance grade numbers — 4 is tight, 6 is general-purpose medium, 8 is loose. Both pitch diameter and major diameter have individual tolerance designations, but in shorthand notation (e.g., 6H, 6g), the same grade applies to both.

Common thread class combinations and their applications:

External Class	Internal Class	Fit Type	Application
6g	6H	Normal/medium	Standard bolts and nuts, general fasteners
6h	6H	Tight	Close-fit fasteners, precision assemblies
6e	6H	Free running	Fasteners subject to contamination, coating compensation
4h	5H	Fine	Precision adjustment screws, instrumentation
6g	7H	Loose	Fasteners used with thread inserts, coated fasteners

Length of Engagement

Thread callouts on drawings for tapped holes typically include the thread depth (length of engagement). Format on the drawing:

M10×1.5 — 6H ↧ 20 THRU (or with a depth symbol)

• THRU: Thread runs all the way through the part thickness
• ↧ 20: Thread depth is 20mm from the surface (does not include the drill tip runout zone)
• DRILL ↧ 25: Sometimes the drill depth (to tip of drill) is separately specified, especially for blind holes with CNC programming requirements

A general rule: length of engagement should be 1.0–1.5 × nominal diameter for steel-to-steel threading. For aluminum or other soft materials receiving steel fasteners, 2.0–2.5 × nominal diameter or thread inserts (Helicoil type) are recommended.

Unified National Threads (UN): The American Standard

Unified threads are defined in ASME B1.1. The callout format is:

[Nominal diameter]-[TPI] [thread series]-[class][A or B] [LH if left-hand]

Examples:

• 1/2-13 UNC-2A: 1/2 inch nominal diameter, 13 TPI (threads per inch), Unified National Coarse series, Class 2 external thread (A = external)
• 1/2-13 UNC-2B: same but Class 2 internal thread (B = internal / hole)
• 3/8-24 UNF-3A: 3/8 inch, 24 TPI, Unified National Fine series, Class 3 external
• 1/4-28 UNF-2B LH: left-hand thread, internal

Thread series:

• UNC: Unified National Coarse — standard for most commercial fasteners, better resistance to stripping, easier assembly with some contamination
• UNF: Unified National Fine — finer pitch, higher thread engagement per inch, used in automotive and aerospace for better vibration resistance
• UNEF: Unified National Extra Fine — very fine pitch, thin-walled sections and precision adjustment mechanisms
• UN: Unified National — constant-pitch series (8UN, 12UN, 16UN) for large diameters

Tolerance classes for Unified threads:

• Class 1: Loose fit — for ease of assembly and disassembly in field conditions
• Class 2: General purpose — standard commercial fasteners; allows manufacturing tolerances and some contamination
• Class 3: Tight fit — precision assemblies, critical fasteners; minimal clearance between threads

Pipe Threads: Tapered and Parallel

Pipe threads are used for fluid and gas connections. They come in two main categories: tapered (sealing by thread interference) and parallel (sealing by a separate seal or sealant).

ISO Pipe Threads (ISO 7-1, ISO 228-1):

• Rc: Tapered internal pipe thread (female, used in fittings and ports). Seals by thread engagement alone when assembled with Rp or R.
• Rp: Parallel internal pipe thread (female, parallel). Seals with a gasket at the port face.
• R: Tapered external pipe thread (male). Standard on pipes, fittings, and valves.

ISO pipe thread designations include the nominal pipe size: R 1/2 (tapered external, 1/2 inch pipe), Rc 1/2 (tapered internal), Rp 3/4 (parallel internal, 3/4 inch pipe).

NPT and NPTF (ASME B1.20.1 and B1.20.3):

• NPT: National Pipe Taper — tapered thread, seals by thread interference, but typically requires thread sealant (PTFE tape or pipe dope) for reliable sealing under pressure
• NPTF: National Pipe Taper Fuel — “Dryseal” thread with tighter tolerances. Designed to seal by controlled thread interference without sealant. Required for hydraulic and fuel systems.

NPT/NPTF callout: 1/2 NPT or 1/2 NPTF (nominal pipe size, which is neither the OD nor the thread pitch diameter).

Thread Designation on Engineering Drawings: Placement Rules

On engineering drawings, thread callouts are placed using a leader line pointing to the thread or its center line, with the full callout in the note:

• For tapped holes, the callout is placed in the view that shows the hole as a circle (top view or section view), pointing to the thread circle
• The callout reads: thread designation, depth (with depth symbol or THRU), and any special requirement (e.g., INSERT HELICOIL, CHAMFER 1×45° AT ENTRY)
• For external threads (bolts, studs), the callout points to the threaded cylinder in the most descriptive view, typically the side view
• Thread length is specified separately from the overall part length for studs and bolts

Special Thread Callouts

Left-hand threads: “LH” appended to the callout (M12×1.75 — 6H LH or 1/2-13 UNC-2B LH). Left-hand threads are used in applications where right-hand threads would self-loosen (left pedal on bicycles, left-hand gas cylinder valves for hazardous gases).

Thread inserts: When a tapped hole will receive a thread insert (Helicoil, Keensert, etc.), the tapped hole size is the insert’s OD thread designation, not the bolt thread. Example: STI M10×1.5 (Screw Thread Insert for M10×1.5 bolt). The note “INSERT PER [specification]” should accompany the callout.

Partial thread symbols: On section views, threads are shown as alternating solid and dashed circles (parallel to the thread helix). Solid lines = crest of thread; dashed lines = root. Thread depth in a blind hole is shown by the distance the solid thread line extends from the surface.

Metric vs Unified Quick Reference

Property	Metric ISO (M)	Unified National (UN)
Governing standard	ISO 261 / ISO 965	ASME B1.1
Diameter unit	mm	inch (fraction or decimal)
Pitch specification	mm per thread (e.g., 1.5)	TPI — threads per inch (e.g., 20)
Thread angle	60°	60°
External tolerance class	6g (general), 6h (tight)	2A (general), 3A (tight)
Internal tolerance class	6H (general), 5H (fine)	2B (general), 3B (tight)
Left-hand designation	LH suffix	LH suffix

Conclusion

Thread callouts on engineering drawings are compact but information-dense. Each element — the thread form designation, nominal size, pitch, tolerance class, and handedness — has a specific functional meaning that directly affects tooling selection, assembly procedure, and sealing performance. Mastering the callout formats for metric ISO, Unified National, and pipe threads, and understanding what the tolerance class numbers and letters mean in practice, ensures that threaded features are manufactured and assembled correctly every time. When in doubt about an unusual thread designation, always refer back to the governing standard (ISO 965, ASME B1.1, or ISO 7-1) before committing to tooling or setup.