UNCボルトは8UNの代わりになるか？ねじ互換性ガイド

One of the most common questions from maintenance engineers and procurement managers: “I have a UNC bolt — can I use it in this 8UN threaded hole?” The short answer is usually no, but the complete answer depends on the specific 直径, 用途, and governing code. This guide explains exactly when interchangeability is possible and when it will fail.

At BravoFabs, we regularly machine replacement parts for industrial equipment where the original thread specification must be matched exactly. Understanding interchangeability rules prevents costly field failures.

重要ポイント

• UNC and 8UN bolts are NOT interchangeable at most 直径s — different thread geometry means different load distribution
• At exactly 1″ 直径 only, UNC and 8UN share 8 TPI — the thread geometry is identical, but the engineering 設計ation matters for code compliance
• Field substitution without engineering review is the #1 cause of flange leaks and stud failures in oil & gas
• Always check the governing code (ASME B16.5, API 6A, ASTM A193) before substituting thread series
• When in doubt, machine a new part to the correct specification — the cost of a replacement stud is negligible compared to a flange leak

Why UNC and 8UN Are Different — Thread Geometry Explained

Both UNC and 8UN belong to the Unified Thread 標準 (UTS) family and share the same 60° thread form angle. But the pitch progression is fundamentally different:

• UNC pitch varies with 直径 — #4-40, 1/4-20, 3/8-16, 1/2-13, 3/4-10, 1-8. As 直径 increases, TPI 減少する.
• 8UN pitch is constant — every 直径 from 1-1/16″ to 6″ uses exactly 8 TPI.

This means at any 直径 other than 1″, a UNC bolt and an 8UN bolt of the same nominal size will have different thread pitches. Attempting to thread a 3/4-10 UNC bolt into a 3/4-8UN nut will result in cross-threading, thread damage, or — if forced — a jammed assembly with zero holding strength.

The 1-Inch Exception: Where UNC Meets 8UN

At exactly 1″ nominal 直径, both UNC and 8UN specify 1インチあたり8山. The thread form, pitch 直径, and tolerance classes are identical. A 1-8 UNC bolt will physically thread into a 1-8UN nut without resistance (see UNC vs UNF vs 8UN full comparison for dimensional data).

However, interchangeability at 1″ is a dimensional coincidence, not a 設計 intent. Here’s why you should still be careful:

Bottom line: A 1-8 UNC bolt and a 1-8UN stud are geometrically compatible but not code-compatible. If your flange is stamped ASME B16.5 Class 600, the bolting must meet ASTM A193 B7 with full 材料 traceability — a hardware-store Grade 8 bolt will not pass 検査, even if it threads in perfectly.

UNC vs 8UN Interchangeability Chart by 直径

When Field Substitution Is Acceptable (And When It’s Not)

Acceptable Substitution Scenarios

• Non-pressure, non-structural 用途s at exactly 1″ 直径 — a 1-8 UNC bolt can replace a 1-8UN bolt for a machine guard, bracket, or access cover where thread fit is the only requirement
• Temporary/emergency repair with engineering approval and a defined replacement timeline — “run it for one shift while the correct stud is being machined”
• Prototype/test 組立品 where the final production part will use the correct specification — this is 一般的な分野 R&D and fit-check stages

When Substitution Is Unacceptable

• Pressure-containing parts — flanges, valve bonnets, wellhead connectors. A bolt failure here can release pressurized fluid or gas. Never substitute.
• Safety-critical structures — crane bolts, lifting eyes, suspension components. The margin of safety assumes the specified 材料 grade.
• Any assembly governed by a code or 標準 — ASME BPVC, API, ASTM, ISO. The inspector will check head markings; a mismatch means rejection.
• High-temperature service above 400°C (750°F) — UNC and 8UN fasteners may use different 材料 grades with different thermal expansion coefficients. A mismatch can cause bolt relaxation and loss of preload.
• Cyclic fatigue 用途s — the thread root radius and surface finish requirements differ between general-purpose UNC and pressure-rated 8UN studs.

How to Identify UNC vs 8UN Bolts in the Field

When you have a box of mixed bolts and need to sort them quickly:

1. Check the head marking — UNC bolts typically have grade markings (three radial lines = Grade 5, six = Grade 8). 8UN stud bolts for pressure service are marked B7, B8, L7, or B8M per ASTM A193.
2. Measure the TPI with a thread pitch gauge — if it’s not 8 TPI, it’s not 8UN. If it’s above 1″ 直径 and reads 8 TPI, it could be either.
3. Check the bolt length and configuration — 8UN stud bolts for flanges are often fully threaded with two nuts. UNC bolts typically have a partial thread and a single nut.
4. Look for traceability markings — pressure-rated studs often have heat numbers stamped on one end. General-purpose UNC bolts do not.

Cost Comparison: UNC vs 8UN Replacement Parts

From a procurement perspective, UNC bolts are significantly cheaper than 8UN stud bolts — often 40-60% less for the same 直径. This price gap tempts buyers to substitute, but the savings evaporate the moment a substituted bolt fails:

For custom-machined threaded components (non-標準 直径s, special 材料s, tight 公差), the cost difference narrows because both require CNC setup. At BravoFabs, we machine both UNC and 8UN threads and the cost is driven by 材料, 直径, and tolerance — see our thread 製造ガイド for CNC methods — not by which UTS series you choose. Specify the correct series; the machining cost will be similar.

Explore more: UN vs UNC thread pillar guide · UNC vs UNF vs 8UN comparison · 8UN thread specifications · Thread interchangeability rules · Thread selection decision tree

カスタム UNC or 8UN Bolts Machined? 見積もりを依頼

BravoFabs machines replacement studs, bolts, and custom threaded components in any UTS series — including non-標準 直径s and exotic alloys. We provide 材料 certifications, thread 検査 reports, and full traceability for pressure-rated parts.

送信 us your drawing with the thread specification (e.g., “1-5/16-8UN-2A, ASTM A193 B7”) and we’ll quote within 24 hours. Upload your drawing →