8UN Thread Specifications: Complete Durchmesser, TPI & Tolerance Chart

The 8UN (8-Gang-Serie) is a constant-pitch thread standard within the Unified Thread Standard (UTS) family, governed von 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 Seite 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.

Wichtige Erkenntnisse

• 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 Vollständige Durchmesser- und Steigungsdaten (ASME B1.1)

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 Flankendurchmessertoleranzen (Klassen 2A/2B und 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:

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 Gewindeeingriff: Mindestlängenanforderungen

For full-strength thread engagement (bolt breaking before thread stripping), the minimum engagement length depends on the material combination:

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.

Häufige Branchen und Anwendungen für 8UN-Gewinde

8UN Gewindebearbeitung: CNC-Einrichtungsparameter

For CNC lathe single-point threading of 8UN threads, we use the following starting parameters (adjust for material and insert grade):

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 Gewindeprüfliste

Every 8UN threaded part we produce at BravoFabs is verified against this checklist:

1. Go/No-Go thread plug gauge (internal) or thread ring gauge (external) — primary acceptance test
2. Profilprojektor at 20× — verify 60° thread angle, root radius, crest flatness
3. Three-wire pitch diameter measurement — for precision Class 3A/3B threads, measured to ±0.0002″
4. Visuelle Prüfung under magnification — check for torn threads, burrs, chatter marks
5. Material certification review — verify heat number and material grade match the PO requirements
6. 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 Gewindekonstruktionstipps für Ingenieure

Vollständige Gewindeangabe auf Zeichnungen angeben

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.

Wählen Sie die richtige Toleranzklasse: 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-von-step framework.

Materialauswahl für 8UN-Gewindeteile

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.

Benötigen Sie 8UN-Gewindebearbeitung? Angebot mit vollständiger Spezifikationsprüfung anfordern

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 →