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

The 8UN (8-Thread Series) 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 threads per inch. 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.

Punti Chiave

• 8UN = constant 8 TPI across all diameters — one threading insert can machine multiple sizes
• Diametro 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), pressure vessels (ASME B16.5), heavy equipment, marine propulsion

8UN Complete Diametro and Pitch Data (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 Applicazione.

8UN Pitch Diametro Tolleranze (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:

For oil & gas Applicaziones, most specifications call for Class 2A on studs and Class 2B on nuts. Class 3A/3B is reserved for Applicaziones where thread fit must be controlled to prevent fatigue crack initiation — such as rotating equipment shafts and aerospace fasteners.

8UN Thread Engagement: Minimum Length Requirements

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.

Common Industries and Applicaziones for 8UN Threads

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):

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:

1. Go/No-Go thread plug gauge (internal) or thread ring gauge (external) — primary acceptance test
2. Optical comparator 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. documentate. L'ispezione include: 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 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 Applicaziones, 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 Applicaziones 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.

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 Acciaio inossidabile (ASTM A193 B8/B8M for corrosive service), and 17-4 PH stainless (ASTM A564 for high-strength corrosive Applicaziones). For non-pressure Applicaziones, 1018/1045 Acciaio al carbonio with zincatura is the economical choice. Aluminum 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 for 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? Richiedi un preventivo with Full Spec Review

BravoFabs produces custom 8UN threaded components from 1-1/16″ to 6″ diameter in Acciaio al carbonio, stainless, alloy steel, and aluminum. We provide full thread inspection reports with every batch — Go/No-Go gauge results, optical 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 inspection criteria in your quote. Richiedi un Preventivo →