Thread Standards: UNC, UNF, Metric & Specialty

Threads convert rotational motion to linear motion, create secure mechanical joints, and enable precision adjustment. Despite their ubiquity, thread specification causes more confusion than almost any other drawing callout—especially when multiple standards intersect.

This guide covers the thread systems you’ll encounter in manufacturing, when to use each, and how to specify them correctly.

Thread System Overview

Unified Threads (Inch)

The Unified thread system dominates in the United States and much of North America. Governed by ASME B1.1 (Unified Inch Screw Threads), this family includes several series to match different application needs. UNC (Unified National Coarse) serves as the standard general-purpose thread, while UNF (Unified National Fine) provides finer pitch for adjustment and vibration resistance. For thin-wall applications, UNEF (Unified National Extra Fine) offers the finest pitch, and UNS (Unified National Special) covers non-standard combinations when standard series don’t meet specific requirements.

Metric Threads (ISO)

Metric threads follow the international standard used throughout most of the world, governed by ISO 68-1, ISO 261, and ISO 262. The system is straightforward: coarse pitch is the default for each diameter (M10 implies 1.5mm pitch), while fine pitch is explicitly specified after the diameter (M10×1.0 for 1.0mm pitch).

Pipe Threads

Pipe threads are specialized for fluid connections. The primary standards are ASME B1.20.1 covering NPT and NPS threads, and ASME B1.20.3 covering NPTF dryseal threads.

Unified Threads (UNC/UNF)

Thread Geometry

Parameter	Description
Major diameter	Nominal thread size
Minor diameter	Root diameter (critical for strength)
Pitch diameter	Theoretical contact point
Pitch	Distance between threads (1/TPI)
Thread angle	60° included
Thread form	Truncated roots and crests

Common UNC Sizes

Size	TPI	Major Dia	Tap Drill
#6-32	32	0.138″	#36 (0.107″)
#8-32	32	0.164″	#29 (0.136″)
#10-24	24	0.190″	#25 (0.150″)
1/4-20	20	0.250″	#7 (0.201″)
5/16-18	18	0.313″	F (0.257″)
3/8-16	16	0.375″	5/16″ (0.313″)
1/2-13	13	0.500″	27/64″ (0.422″)
3/4-10	10	0.750″	21/32″ (0.656″)
1-8	8	1.000″	7/8″ (0.875″)

Common UNF Sizes

Size	TPI	Major Dia	Tap Drill
#6-40	40	0.138″	#33 (0.113″)
#8-36	36	0.164″	#29 (0.136″)
#10-32	32	0.190″	#21 (0.159″)
1/4-28	28	0.250″	#3 (0.213″)
5/16-24	24	0.313″	I (0.272″)
3/8-24	24	0.375″	Q (0.332″)
1/2-20	20	0.500″	29/64″ (0.453″)
3/4-16	16	0.750″	11/16″ (0.688″)

Classes of Fit

Thread fit classes define the allowable tolerance and clearance between mating threads. For external threads on screws and bolts, Class 1A provides a loose fit for easy assembly, Class 2A is the standard commercial fit used in most applications, and Class 3A delivers a tight fit for precision work. Internal threads in nuts and tapped holes follow the same pattern: Class 1B for loose fit, Class 2B for standard commercial use, and Class 3B for precision applications.

In practice, the 2A/2B combination handles the vast majority of applications. You should only specify a different class when your design specifically demands tighter or looser tolerance than standard.

UNC vs UNF Selection

Factor	UNC	UNF
Tensile strength	Higher (larger minor dia)	Lower
Fatigue strength	Lower	Higher
Vibration resistance	Less	More
Ease of assembly	Easier	More critical
Cross-threading risk	Lower	Higher
Fine adjustment	Limited	Better
Stripping resistance	Higher	Lower

The choice between UNC and UNF depends on your application priorities. UNC should be your default for most applications, particularly when quick assembly matters or thread damage is possible during service. The coarser pitch tolerates more abuse, strips less easily, and assembles faster. Choose UNF when vibration is present, when you need fine adjustment capability, or when thread engagement length is limited—the finer pitch provides better vibration resistance and more precise positioning.

Metric Threads

Thread Designation

Metric threads follow a straightforward designation format: M[diameter]×[pitch]. For example, M10×1.5 indicates a 10mm diameter with 1.5mm coarse pitch, while M10×1.0 specifies the same diameter with 1.0mm fine pitch. When you see just M10 without a pitch value, it implies the coarse pitch (1.5mm for this diameter).

Common Metric Sizes

Size	Pitch (Coarse)	Tap Drill
M3	0.5	2.5mm
M4	0.7	3.3mm
M5	0.8	4.2mm
M6	1.0	5.0mm
M8	1.25	6.8mm
M10	1.5	8.5mm
M12	1.75	10.2mm
M16	2.0	14.0mm
M20	2.5	17.5mm

Tolerance Classes

External (bolt)	Internal (nut)	Application
6g	6H	Standard commercial (most common)
4g6g	4H5H	Precision
8g	7H	Loose fit

Pipe Threads

NPT (National Pipe Thread Tapered)

NPT threads are the workhorse of fluid connections, using a tapered profile for general-purpose sealing when combined with thread sealant (tape or compound).

Size	TPI	Taper	Tap Drill
1/8	27	3/4″ per foot	R
1/4	18	3/4″ per foot	7/16″
3/8	18	3/4″ per foot	37/64″
1/2	14	3/4″ per foot	45/64″
3/4	14	3/4″ per foot	59/64″
1	11.5	3/4″ per foot	1-5/32″

When assembling NPT connections, always use sealant—either PTFE tape or pipe compound. Be careful not to over-tighten, as the tapered threads can crack thin-walled housings. Standard engagement calls for 3-4 turns hand-tight followed by 2-3 turns with a wrench.

NPTF (Dryseal)

NPTF threads feature a modified form that creates metal-to-metal sealing without sealant. The thread roots and crests interfere to form the seal, making NPTF ideal for applications where sealant contamination is unacceptable. This comes at a cost: NPTF is more expensive to manufacture than standard NPT.

NPS (National Pipe Straight)

NPS threads are parallel rather than tapered, so they don’t seal by themselves. Instead, they rely on gaskets or O-rings to prevent leakage. Straight threads are easier to manufacture than tapered threads and are commonly used in hydraulic fittings with O-ring boss (ORB) designs.

SAE Straight Thread (O-Ring Boss)

The SAE straight thread system combines parallel threads with O-ring sealing at a machined face. This approach provides more reliable sealing than tapered threads alone, allows multiple assembly and disassembly cycles without degradation, and has become standard in modern hydraulic systems.

Specialty Threads

Acme Threads

Acme threads use a trapezoidal profile with a 29° included angle, optimized for power transmission rather than fastening. You’ll find them on lead screws, vises, jacks, and machine tool components. They offer higher load capacity than standard V-threads, generate less friction during operation, and are easier to machine than true square threads.

Buttress Threads

Buttress threads feature an asymmetric profile with 7° and 45° flank angles, designed to handle high axial loads in one direction. They’re used in artillery breeches, press screws, and high-pressure vessel closures where the load direction is predictable.

Square Threads

Square threads provide maximum efficiency for power transmission applications, but their perpendicular flanks make them difficult to manufacture. Modern practice has largely replaced them with Acme threads, which offer nearly equivalent performance with simpler machining.

Whitworth (BSW/BSF)

The British Whitworth system uses a 55° thread angle rather than the 60° of Unified threads. While no longer common in new designs, you’ll encounter Whitworth threads on vintage equipment, some British Commonwealth applications, and imported machinery and tooling.

Machining Considerations

Tapping

Tap drill selection determines thread engagement percentage. The standard 75% thread engagement balances strength and tapping difficulty, while 65% thread makes tapping easier with only slightly reduced strength. The theoretical formula is: Tap drill = Major dia - (Pitch × % thread × 1.08). In practice, standard tap drill charts are more practical and account for real-world factors.

When tapping blind holes, remember to allow depth for the tap chamfer (typically 2-3 threads for a taper tap) plus space for chips. For thread depth, full engagement of 1.5× the nominal diameter provides full joint strength—going deeper doesn’t add significantly. See our DFM guide for more detail.

Thread Milling

Thread milling offers several advantages over conventional tapping. A single thread mill can produce multiple thread sizes sharing the same pitch, making it economical for shops running varied parts. Thread milling handles interrupted cuts better than tapping, can thread blind holes nearly to the bottom, provides superior chip evacuation in deep holes, and allows adjustment for thread fit by adjusting the toolpath.

Single-Point Threading (Lathe)

Single-point threading on a lathe is the standard approach for external threads and large internal threads. Achieving good results requires attention to process parameters: plan on 29-30 passes for coarse threads in steel, use constant chip load with decreasing depth of cut per pass, and set the compound at 29° for Unified threads to improve chip flow and surface finish.

Thread Rolling

Thread rolling cold-forms threads rather than cutting them, producing superior results for certain applications. Rolled threads are stronger due to work-hardening of the surface, exhibit better fatigue resistance, have smoother surface finish, and generate no chips. The process requires a specific blank diameter between the minor and pitch diameters of the finished thread.

Specification on Drawings

Basic Format

External threads (screws) follow this format: [Diameter]-[Pitch] [Series]-[Class] × [Length]

1/4-20 UNC-2A × 0.750
M10×1.5-6g × 20

Internal threads (tapped holes) use similar notation: [Diameter]-[Pitch] [Series]-[Class] × [Depth]

1/4-20 UNC-2B × 0.500 MIN FULL THD
M10×1.5-6H THRU

Notes to Include

Complete thread specifications require more than just the basic callout. For blind holes, specify full thread depth and drill depth as separate dimensions—the drill must go deeper than the threads. Always include the thread class unless the standard 2A/2B (inch) or 6g/6H (metric) is acceptable. Call out any required chamfers or thread relief. For critical applications, note whether thread gauge inspection is required.

Working With NextGen Components

We machine threads across all common standards, including UNC/UNF from #2 through 2″, metric from M2 through M48, pipe threads (NPT, NPTF, NPS), and specialty threads like Acme and buttress on request.

Thread gauging and inspection documentation available. Contact us with your thread requirements.