The 8UN (8-Gang-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 Gewindegänge pro Zoll. This reference page provides the complete dimensional data you need for design, machining, and Inspektion.
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 Produktion experience.
Kernaussagen
- 8UN = constant 8 TPI across all diameters — one threading insert can machine multiple sizes
- Durchmesser 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), Druckbehälter (ASME B16.5), heavy equipment, marine propulsion
8UN Complete Durchmesser and Pitch Data (ASME B1.1)
| Nominal Durchmesser | TPI | Steigung (in) | Basic Minor Ø Internal (in) | Basic Minor Ø External (in) | Gewindebohrer (Zoll) |
|---|---|---|---|---|---|
| 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 Anwendung.
8UN Pitch Durchmesser Toleranzen (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 | Anwendung | PD Tolerance (8 TPI) | Typische Verwendung |
|---|---|---|---|
| 2A (External) | General industrial | 0.0089″ | Standard studs, bolts |
| 2B (Internal) | General industrial | 0.0106″ | Standard nuts, tapped holes |
| 3A (External) | Präzision / aerospace | 0.0057″ | High-precision studs |
| 3B (Internal) | Präzision / aerospace | 0.0069″ | Aerospace nuts,精密螺纹 |
For oil & gas Anwendungs, most specifications call for Class 2A on studs and Class 2B on nuts. Class 3A/3B is reserved for Anwendungs where thread fit must be controlled to prevent fatigue crack initiation — such as rotating equipment shafts and aerospace fasteners.
8UN Thread Engagement: Minimum Länge Requirements
For full-strength thread engagement (bolt breaking before thread stripping), the minimum engagement length depends on the material combination:
| Nut Material | Bolt Material | Min Engagement (× diameter) | Min Engagement at 1-5/16″-8UN |
|---|---|---|---|
| Steel | Steel | 0.8D | 1.05″ |
| Steel | Aluminium | 1.5D | 1.97″ |
| Aluminium | Steel | 2.0D | 2.63″ |
| Cast Iron | Steel | 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 Anwendungs for 8UN Threads
| Industry | Anwendung | Governing Standard |
|---|---|---|
| Oil & Gas | API 6A flanges, wellhead studs, BOP bonnet bolts | API 6A, ASME B16.5 |
| Druckbehälter | Heat exchanger studs, manway bolts, vessel flange bolting | ASME BPVC Section VIII, ASME B16.5 |
| Heavy Ausrüstung | Hydraulic cylinder end caps, track pin retainers | Hersteller 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) | Kohlenstoffstahl (1018/1045) | Stainless (304/316) | Alloy Steel (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 Produktion advantage for shops that machine multiple 8UN diameters in the same setup.
8UN Thread Inspektion 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
- Profilprojektor 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″
- Visuelle Prüfung under magnification — check for torn threads, burrs, chatter marks
- Material 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 Design 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 Anwendungs, 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 Anwendungs 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 Inspektion requirements. If you are unsure which class to specify, see our thread selection decision guide for a step-by-step framework.
Material 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 Edelstahl (ASTM A193 B8/B8M for corrosive service), and 17-4 PH stainless (ASTM A564 for high-strength corrosive Anwendungs). For non-pressure Anwendungs, 1018/1045 Kohlenstoffstahl with Verzinkung 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.
Am besten geeignet für 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? Angebot einholen with Full Spec Review
BravoFabs produces custom 8UN threaded components from 1-1/16″ to 6″ diameter in Kohlenstoffstahl, stainless, alloy steel, and aluminum. We provide full thread Inspektion reports with Jede Charge — Go/No-Go gauge results, Profilprojektor 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 Inspektion criteria in your quote. Angebot anfordern →
Need 8UN Threaded Parts? Free CNC Quote with Drawing Review
Senden us your drawing or STEP file and we’ll provide a detailed machining quote — including material recommendations, thread Inspektion plan, and delivery timeline. ISO-Zertifiziert, serving industrial clients in DE/US/CA/EU.
🔗 Related Thread & Manufacturing Ressourcen
- 8UN vs UNC Thread Reihe: 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
- Wie man Gewinde in der Metallverarbeitung herstellt — CNC threading methods
- CNC-Drehservice — single-point threading for shafts and cylindrical parts
