The 8UN (Série à 8 filets) 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 filets par pouce. This reference page provides the complete dimensional data you need for design, machining, and inspection.
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 production experience.
Points Clés
- 8UN = constant 8 TPI across all diameters — one threading insert can machine multiple sizes
- Diamètre 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), Récipients sous pression (ASME B16.5), heavy equipment, marine propulsion
8UN Complete Diamètre and Pitch Data (ASME B1.1)
| Nominal Diamètre | TPI | Pas (po) | Basic Minor Ø Internal (in) | Basic Minor Ø External (in) | Foret de taraudage (po) |
|---|---|---|---|---|---|
| 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 application.
8UN Pitch Diamètre Tolérances (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 | Application | PD Tolerance (8 TPI) | Utilisation typique |
|---|---|---|---|
| 2A (External) | General industrial | 0.0089″ | Standard studs, bolts |
| 2B (Internal) | General industrial | 0.0106″ | Standard nuts, tapped holes |
| 3A (External) | Précision / aerospace | 0.0057″ | High-precision studs |
| 3B (Internal) | Précision / aerospace | 0.0069″ | Aerospace nuts,精密螺纹 |
For oil & gas applications, most specifications call for Class 2A on studs and Class 2B on nuts. Class 3A/3B is reserved for applications where thread fit must be controlled to prevent fatigue crack initiation — such as rotating equipment shafts and aerospace fasteners.
8UN Thread Engagement: Minimum Longueur Requirements
For full-strength thread engagement (bolt breaking before thread stripping), the minimum engagement length depends on the material combination:
| Nut Matériau | Bolt Matériau | Min Engagement (× diameter) | Min Engagement at 1-5/16″-8UN |
|---|---|---|---|
| Acier | Acier | 0.8D | 1.05″ |
| Acier | Aluminium | 1.5D | 1.97″ |
| Aluminium | Acier | 2.0D | 2.63″ |
| Cast Iron | Acier | 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 Applications for 8UN Threads
| Industry | Application | Governing Standard |
|---|---|---|
| Oil & Gas | API 6A flanges, wellhead studs, BOP bonnet bolts | API 6A, ASME B16.5 |
| Récipients sous pression | Heat exchanger studs, manway bolts, vessel flange bolting | ASME BPVC Section VIII, ASME B16.5 |
| Heavy Équipements de qualité utilisés quotidiennement | 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) | Acier au carbone (1018/1045) | Stainless (304/316) | Alloy Acier (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 production advantage for shops that machine multiple 8UN diameters in the same setup.
8UN Thread Inspection 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
- Comparateur optique 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″
- . L’inspection couvre : under magnification — check for torn threads, burrs, chatter marks
- Matériau 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 Conception 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 applications, 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 applications 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 inspection requirements. If you are unsure which class to specify, see our thread selection decision guide for a step-by-step framework.
Matériau 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 Acier inoxydable (ASTM A193 B8/B8M for corrosive service), and 17-4 PH stainless (ASTM A564 for high-strength corrosive applications). For non-pressure applications, 1018/1045 Acier au carbone with placage au zinc 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.
Meilleur pour 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? Obtenir un Devis with Full Spec Review
BravoFabs produces custom 8UN threaded components from 1-1/16″ to 6″ diameter in Acier au carbone, stainless, alloy steel, and aluminum. We provide full thread inspection reports with Chaque lot — Go/No-Go gauge results, Comparateur optique 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 inspection criteria in your quote. Réponse rapide avec devis sous 24 heures →
Need 8UN Threaded Parts? Free CNC Quote with Drawing Review
Envoyer us your drawing or STEP file and we’ll provide a detailed machining quote — including material recommendations, thread inspection plan, and delivery timeline. ISO-certified, serving industrial clients in DE/US/CA/EU.
🔗 Related Thread & Manufacturing Ressources
- 8UN vs UNC Thread Série: 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
- Les champs obligatoires sont indiqués avec Fabriquer un Filetage dans la Fabrication Métallique — CNC threading methods
- Service de tournage CNC — single-point threading for shafts and cylindrical parts
