The 8UN (8-draad serie) is a constant-pitch thread standard within the Unified Thread Standard (UTS) family, governed by ASME B1.1. Unlike UNC and UNF — where TPI changes with diameter — every 8UN thread from 1-1/16″ to 6″ uses exactly 8 draadgangen per inch. This reference page provides the complete dimensional data you need for design, machining, and Inspectie.
At BravoFabs, we machine 8UN threads daily on CNC lathes for oil & gas flanges, pressure vessel studs, and heavy machinery components. This data is compiled from ASME B1.1 and verified against our Productie experience.
Kernpunten
- 8UN = constant 8 TPI across all diameters — one threading insert can machine multiple sizes
- Certificatiedocumenten & inspectierapporten range: 1-1/16″ through 6″ per ASME B1.1 Table 2 (selected sizes also defined above 6″)
- Thread form: 60° V-thread with 0.125″ pitch (1/8″ per thread), flat root per UN profile
- Pitch diameter tolerance available in classes 2A/2B (general) and 3A/3B (precision)
- Primary industries: oil & gas (API flanges), Drukvaten (ASME B16.5), heavy equipment, marine propulsion
8UN Complete Certificatiedocumenten & inspectierapporten and Pitch Data (ASME B1.1)
| Nominal Certificatiedocumenten & inspectierapporten | TPI | Spoed (in) | Basic Minor Ø Internal (in) | Basic Minor Ø External (in) | Tapboor (in) |
|---|---|---|---|---|---|
| 1-1/16″ | 8 | 0.125 | 0.9375 | 0.9102 | 15/16 (0.9375) |
| 1-3/16″ | 8 | 0.125 | 1.0625 | 1.0352 | 1-1/16 (1.0625) |
| 1-5/16″ | 8 | 0.125 | 1.1875 | 1.1602 | 1-3/16 (1.1875) |
| 1-7/16″ | 8 | 0.125 | 1.3125 | 1.2852 | 1-5/16 (1.3125) |
| 1-9/16″ | 8 | 0.125 | 1.4375 | 1.4102 | 1-7/16 (1.4375) |
| 1-11/16″ | 8 | 0.125 | 1.5625 | 1.5352 | 1-9/16 (1.5625) |
| 1-13/16″ | 8 | 0.125 | 1.6875 | 1.6602 | 1-11/16 (1.6875) |
| 2″ | 8 | 0.125 | 1.8750 | 1.8477 | 1-7/8 (1.8750) |
| 2-1/4″ | 8 | 0.125 | 2.1250 | 2.0977 | 2-1/8 (2.1250) |
| 2-1/2″ | 8 | 0.125 | 2.3750 | 2.3477 | 2-3/8 (2.3750) |
| 2-3/4″ | 8 | 0.125 | 2.6250 | 2.5977 | 2-5/8 (2.6250) |
| 3″ | 8 | 0.125 | 2.8750 | 2.8477 | 2-7/8 (2.8750) |
| 3-1/4″ | 8 | 0.125 | 3.1250 | 3.0977 | 3-1/8 (3.1250) |
| 3-1/2″ | 8 | 0.125 | 3.3750 | 3.3477 | 3-3/8 (3.3750) |
| 3-3/4″ | 8 | 0.125 | 3.6250 | 3.5977 | 3-5/8 (3.6250) |
| 4″ | 8 | 0.125 | 3.8750 | 3.8477 | 3-7/8 (3.8750) |
| 4-1/4″ | 8 | 0.125 | 4.1250 | 4.0977 | 4-1/8 (4.1250) |
| 4-1/2″ | 8 | 0.125 | 4.3750 | 4.3477 | 4-3/8 (4.3750) |
| 5″ | 8 | 0.125 | 4.8750 | 4.8477 | 4-7/8 (4.8750) |
| 5-1/2″ | 8 | 0.125 | 5.3750 | 5.3477 | 5-3/8 (5.3750) |
| 6″ | 8 | 0.125 | 5.8750 | 5.8477 | 5-7/8 (5.8750) |
Note: Basic minor diameter for internal threads = nominal – 1.125 × pitch. For external threads, the basic minor diameter includes the thread root truncation per ASME B1.1 Figure 2. Tap drill sizes are for approximately 75% thread engagement — adjust for material and Toepassing.
8UN Pitch Certificatiedocumenten & inspectierapporten Toleranties (Classes 2A/2B and 3A/3B)
Pitch diameter is the most critical dimension for thread fit. For 8UN threads at 8 TPI, the pitch diameter tolerance per ASME B1.1 is:
| Tolerance Class | Toepassing | PD Tolerance (8 TPI) | Typisch gebruik |
|---|---|---|---|
| 2A (External) | General industrial | 0.0089″ | Standard studs, bolts |
| 2B (Internal) | General industrial | 0.0106″ | Standard nuts, tapped holes |
| 3A (External) | Precisie / aerospace | 0.0057″ | High-precision studs |
| 3B (Internal) | Precisie / aerospace | 0.0069″ | Aerospace nuts,精密螺纹 |
For oil & gas Toepassings, most specifications call for Class 2A on studs and Class 2B on nuts. Class 3A/3B is reserved for Toepassings where thread fit must be controlled to prevent fatigue crack initiation — such as rotating equipment shafts and aerospace fasteners.
8UN Thread Engagement: Minimum Lengte Requirements
For full-strength thread engagement (bolt breaking before thread stripping), the minimum engagement length depends on the material combination:
| Nut Materiaal | Bolt Materiaal | Min Engagement (× diameter) | Min Engagement at 1-5/16″-8UN |
|---|---|---|---|
| Staal | Staal | 0.8D | 1.05″ |
| Staal | Aluminium | 1.5D | 1.97″ |
| Aluminium | Staal | 2.0D | 2.63″ |
| Cast Iron | Staal | 1.0D | 1.31″ |
| Stainless (304) | Stainless (304) | 1.2D | 1.58″ |
At 8 TPI, each full turn advances the bolt 0.125″. For a 1-5/16″-8UN steel nut on a steel stud, you need approximately 9 full turns for full-strength engagement. Less engagement = risk of thread stripping before bolt fracture.
Common Industries and Toepassings for 8UN Threads
| Industry | Toepassing | Governing Standard |
|---|---|---|
| Oil & Gas | API 6A flanges, wellhead studs, BOP bonnet bolts | API 6A, ASME B16.5 |
| Drukvaten | Heat exchanger studs, manway bolts, vessel flange bolting | ASME BPVC Section VIII, ASME B16.5 |
| Heavy Apparatuur | Hydraulic cylinder end caps, track pin retainers | Manufacturer spec (CAT, Komatsu) |
| Marine | Propeller shaft nuts, rudder stock bolts, hatch dog bolts | Classification society (ABS, DNV, LR) |
| Power Generation | Turbine casing studs, steam valve bonnet bolts | ASTM A193, ASTM A453 |
| Aerospace | Landing gear bolts, engine mount studs | ASME B1.1, AMS specs |
8UN Thread Machining: CNC Setup Parameters
For CNC lathe single-point threading of 8UN threads, we use the following starting parameters (adjust for material and insert grade):
| Parameter | Aluminium (6061) | Koolstofstaal (1018/1045) | Stainless (304/316) | Alloy Staal (4140) |
|---|---|---|---|---|
| Cutting Speed (SFM) | 300-500 | 200-350 | 150-250 | 150-250 |
| Number of Passes | 8-12 | 12-16 | 14-18 | 14-20 |
| Infeed per Pass | 0.015″ → 0.002″ | 0.012″ → 0.001″ | 0.010″ → 0.001″ | 0.010″ → 0.0005″ |
| Infeed Method | Modified flank | Modified flank | Modified flank | Radial (for hardness) |
| Insert Grade | Uncoated carbide | PVD TiAlN | PVD TiAlN | CVD Al₂O₃ |
| Coolant | Soluble oil (5-8%) | Soluble oil (8-10%) | Heavy-duty (10-12%) | Heavy-duty (10-12%) |
Note on 8UN vs UNC tooling: Because 8UN is constant 8 TPI across all diameters, a single 8 TPI full-profile threading insert can machine any 8UN diameter — no insert changes between sizes. This is a significant Productie advantage for shops that machine multiple 8UN diameters in the same setup.
8UN Thread Inspectie Checklist
Every 8UN threaded part we produce at BravoFabs is verified against this checklist:
- Go/No-Go thread plug gauge (internal) or thread ring gauge (external) — primary acceptance test
- Optische comparator at 20× — verify 60° thread angle, root radius, crest flatness
- Three-wire pitch diameter measurement — for precision Class 3A/3B threads, measured to ±0.0002″
- Visuele inspectie under magnification — check for torn threads, burrs, chatter marks
- Materiaal certification review — verify heat number and material grade match the PO requirements
- Thread protector installation — for finished studs shipping to site, prevent handling damage
For more on thread series selection and interchangeability, see our guides on UNC vs UNF vs 8UN comparison, UNC and 8UN interchangeability rules, and CNC thread manufacturing methods.
8UN Thread Ontwerp Tips for Engineers
Specify the Full Thread Callout on Drawings
A complete 8UN thread callout on an engineering drawing should include: nominal diameter, threads per inch, series designation, tolerance class, and any special requirements. Example: “1-5/16-8UN-2A” tells the machinist everything needed — diameter, TPI, series, and fit class. Ambiguous callouts like “8UN thread” or “8 TPI” without a diameter and tolerance class will result in questions from the shop, adding 1-2 days to your quote turnaround.
Choose the Right Tolerance Class: 2A/2B vs 3A/3B
For 95% of industrial Toepassings, Class 2A (external) and 2B (internal) are sufficient. These provide a running fit with adequate clearance for assembly and are the default for oil & gas studs and nuts. Reserve Class 3A/3B for Toepassings where thread fit directly affects fatigue life — rotating shafts, aerospace fasteners, and精密定位螺纹. Class 3 adds roughly 15-25% to machining cost due to tighter Inspectie requirements. If you are unsure which class to specify, see our thread selection decision guide for a step-by-step framework.
Materiaal Selection for 8UN Threaded Parts
8UN threads are most commonly machined in 4140/4142 alloy steel (ASTM A193 B7 for studs, ASTM A194 2H for nuts), 304/316 Roestvrij Staal (ASTM A193 B8/B8M for corrosive service), and 17-4 PH stainless (ASTM A564 for high-strength corrosive Toepassings). For non-pressure Toepassings, 1018/1045 Koolstofstaal with verzinken is the economical choice. Aluminium 8UN threads are rare — at large diameters, aluminum lacks the thread strength needed for high-torque assembly. If weight reduction is critical, consider a steel threaded insert in an aluminum body.
Best voor 8UN: 4140 heat-treated to 22-32 HRC — balances machinability with thread strength. Avoid: Unalloyed aluminum above 1″ diameter — thread stripping risk exceeds acceptable margins at typical 8UN torque values.
Need 8UN Thread Machining? Offerte Aanvragen with Full Spec Review
BravoFabs produces custom 8UN threaded components from 1-1/16″ to 6″ diameter in Koolstofstaal, stainless, alloy steel, and aluminum. We provide full thread Inspectie reports with Elke batch — Go/No-Go gauge results, Optische comparator images, and material certifications.
Upload your drawing with the complete thread callout (e.g., “2-1/2-8UN-2A, ASTM A193 B7, with 3.1 material cert”). We’ll confirm the machining plan and Inspectie criteria in your quote. Snel antwoord met offerte binnen 24 uur →
Need 8UN Threaded Parts? Free CNC Quote with Drawing Review
Verstuur us your drawing or STEP file and we’ll provide a detailed machining quote — including material recommendations, thread Inspectie plan, and delivery timeline. ISO-certified, serving industrial clients in DE/US/CA/EU.
🔗 Related Thread & Manufacturing Hulpmiddelen
- 8UN vs UNC Thread Serie: Complete Comparison Guide — pillar reference
- Can UNC Replace 8UN? Thread Interchangeability Guide — bolt substitution rules
- UNC vs UNF vs 8UN: Thread Selection Guide — decision framework for engineers
- Hoe maak je draad in metaalbewerking — CNC threading methods
- CNC-draaidienst — single-point threading for shafts and cylindrical parts
