Bolt and Nut Grade Pairing Guide for Metric and Imperial Systems
Bolts and nuts are made in graded strength classes so that their combined joint can hold a designed load without failure. Common imperial (SAE) grades are 2, 5, and 8, while metric property classes include 4.6, 5.8, 8.8, 10.9, and 12.9[1][2]. Broadly, SAE Grade 2 (no head markings) corresponds to low-strength use, Grade 5 (3 radial lines) is medium-strength, and Grade 8 (6 lines) is high-strength[1]. In metric, Class 8.8 (≈800 MPa tensile) is mid-strength (similar to Grade 5), Class 10.9 (≈1040 MPa) is high-strength (similar to Grade 8), and Class 12.9 (≈1220 MPa) is ultra-high-strength[3][4]. These grades determine the tensile load a bolt (and its matching nut) can safely carry. Below we explain how to match nuts to bolts by grade, the dangers of mismatching, considerations of corrosion protection, and recommended uses in various industries.
Strength-Based Pairing: Matching Nut and Bolt Grades
Generally, a nut should be of equal or slightly lower strength than its mating bolt. Nuts and bolts are manufactured to complementary standards so that their threads share strength. For example, ISO 898-2 nut property classes (e.g. 8, 10, 12) are designed to pair with corresponding bolt classes (8.8, 10.9, 12.9) to prevent thread stripping under load[5][6]. In practice, engineers often choose a nut that is just below the bolt’s grade, ensuring that if the joint is overloaded the nut will strip before the bolt breaks[7][8]. This provides a controlled failure mode (loose threads) that is safer and easier to inspect or replace than a snapped bolt.
For example, pairing an ASTM A193 Grade B7 bolt (high-strength alloy) with an ASTM A194 Grade 2H nut (quenched/tempered medium carbon) ensures the nut yields first, preventing catastrophic bolt fracture[7][9]. If a lower-grade nut is used under a high-grade bolt (e.g. Class 8.8 bolt with a Class 4 nut), the threads can shear and fail at much below the bolt’s capacity[8]. Conversely, using a stronger-than-necessary nut (e.g. Grade 8 nut on a Grade 5 bolt) does not make the joint any stronger; the weaker bolt will still be the failure point[9]. In summary: the nut should match or modestly under-grade the bolt. Table 1 and Table 2 below list common bolt grades with their recommended nut grades and typical applications.
- Match vs. under-strength: Industry guidance advises matching nut grade to bolt (same-class nuts), noting that “a stronger nut doesn’t increase a weaker bolt’s capacity”[9]. However, using a nut one grade lower (e.g. Class 8 nut on a Class 10.9 bolt) deliberately sacrifices the nut first for a safer failure[7][6].
- Mismatching hazards: If the nut is much weaker, it will strip (threads shear) under load[8]. If the nut is stronger, the weaker bolt may yield or break first, which can be worse (broken bolts are hard to remove). Thus nuts are typically specified so that the bolt is the stronger element, allowing the nut to fail first under extreme load[7].
- Imperial nut grades: SAE nuts are marked by clock-face notches (e.g. a line at the 8 o’clock position for Grade 8). Standard practice is to pair SAE Grade 2 bolt with Grade 2 nut, Grade 5 bolt with Grade 5 nut, and Grade 8 bolt with Grade 8 nut[9]. Mixing metric and SAE threads is not allowed (thread pitch and form differ)[9].
- Metric nut classes: ISO metric nuts use property classes (e.g. 8, 10, 12). A bolt of class 8.8 should use a Class 8 nut; a 10.9 bolt uses Class 10 nut; a 12.9 bolt uses Class 12 nut[6][8]. (Class 5.8 bolts can use Class 5 nuts for consistency.) These pairings ensure at least the minimum thread shear strength of the nut meets the bolt’s design strength.
Corrosion Resistance and Fastener Material
Corrosion is a key consideration in fastener pairing. In corrosive environments (outdoors, chemicals, saltwater), material choice and coating become as important as strength. General guidelines from corrosion charts include[10]:
- Galvanized/coated steel: Zinc plating offers moderate protection (salt-spray resistance ~50–100 hours) for light-duty indoor/outdoor use. Hot-dip galvanizing (HDG) provides much longer life (~500–1,000+ hours) and is common for outdoor and structural steel fasteners[11]. Advanced coatings (zinc-flake like Geomet or proprietary coatings like Ruspert) can reach >1,000 hours salt-spray resistance for demanding applications (automotive, high-corrosion environments)[11].
- Stainless steel: A2 (304) and A4 (316) stainless steel fasteners are inherently corrosion-resistant. A2-70 (≈700 MPa proof) is often used for general outdoors or food-grade; A4-80 (≈800 MPa proof) is marine-grade with superior resistance[12][13]. In coastal or chemical exposures, 316 stainless (A4 series) is preferred.
- Avoid dissimilar materials: Mixing carbon-steel bolts with stainless nuts (or vice versa) can create galvanic corrosion, especially in moist or salted environments. One source notes stainless/carbons contact can increase corrosion rates 3–5×[14]. Therefore, fasteners and mating hardware should be of the same material or coated compatibly, or both stainless/galvanized to minimize galvanic effects.
- Locking and finishing: For all pairs, adding a matching corrosion-resistant washer or using locking nuts (nylon-insert, prevailing torque nuts) may improve joint longevity. Always ensure the nut coating is compatible with the bolt (e.g. zinc-plated nut with zinc-plated bolt) to avoid differential corrosion.
Applications: Industrial, Structural, and Automotive
Choosing bolt/nut grades depends on the application:
- Industrial machinery: Medium and high-strength fasteners (metric 8.8 or 10.9; SAE Grade 5 or 8) are common. Use Grade 5/8 bolts with matching nuts for assemblies in general machinery. For heavy-equipment or safety-critical parts, higher grades (10.9 or 12.9 / Grade 8) are used. All nuts should meet or slightly exceed bolt strength. In corrosive or wet factory settings, consider HDG or stainless fasteners[10]. Hardened washers (ASTM F436) are recommended with high-strength bolts in machinery.
- Structural steel: Steel construction uses specialized high-strength bolts. ASTM A325/A490 (heavy-hex, structural) bolts are roughly equivalent to metric 8.8 and 10.9. These require heavy hex nuts (ASTM A563) or heavy hex A194 nuts. For example, A325 Type 1 (plain carbon or alloy, ~120 ksi tensile) is paired with A563 Grade C/D/DH heavy hex nuts (or A194 2H as an alternative)[15]. A325 Type 3 (weathering steel) uses A563 Grade C3/DH3 nuts[15]. A490 bolts (alloy ~150 ksi) require A563 Grade D/DH or A194 2H nuts. All these use hardened washers per ASTM F436[15]. Coatings matter: galvanized A325/A490 bolts (Type 3) need matching galvanized nuts (A563 C3/DH3) to maintain weather resistance.
- Automotive: Passenger vehicles use a range of grades. Interior trim and body panels typically use low grades (SAE Grade 2, metric 5.8) with plain nuts[2]. Chassis, suspension, and engine brackets typically use Grade 5 (8.8) bolts with Grade 5 nuts[16]. Critical drivetrain and motor-mount connections often use Grade 8 (10.9) bolts with Grade 8 nuts[17]. For example, a steering knuckle or suspension subframe often uses 8.8/Grade 5 hardware, while a crankshaft pulley or main driveline flange uses 10.9/Grade 8. Higher-grade 12.9 bolts appear in racing or high-performance parts. Automotive fasteners are usually zinc-plated or phosphate-finished; in high-corrosion vehicles (marine or specialty), stainless A2-70/A4-80 may be used. Always use the same corrosion protection on both bolt and nut (e.g. both zinc-plated).
- Miscellaneous: Aircraft and aerospace often use even higher grades (e.g. 12.9 or special alloys) with matching nuts. Wood or masonry anchors use lower grades (e.g. ASTM A307). In general practice, match fastener grades to their structural role: heavy loads and dynamic stresses demand higher-class bolts and nuts, whereas light-duty assemblies can use lower-grade parts. When in doubt, select the nut to match the bolt’s grade and use washers or locking devices per engineering standards.
Pairing Tables
The tables below summarize common pairings of bolt grades and nut grades for metric and SAE systems, along with typical uses (based on engineering standards and manufacturer data[3][2][15]).
Table 1: Metric Bolt Classes and Matching Nut Classes
| Bolt Class | Tensile Strength (MPa) | Nut Property Class | Typical Uses |
|---|---|---|---|
| 4.6 | ~400 | 4 | Low-strength, general use (e.g. light brackets) |
| 5.8 | ~500 (proof: 380) | 5 | Low-medium (furniture, cabinets, auto trim) |
| 8.8 | ~800 | 8 | General machinery, medium automotive (Grade 5 eq.) |
| 10.9 | ~1,040 | 10 | Heavy machinery, engine components (Grade 8 eq.) |
| 12.9 | ~1,220 | 12 | Critical/high-load (racing engines, safety parts) |
| A2-70 (SS) | ~500 (proof: ~700) | A2 (≈8) | Stainless 304 fastener; outdoor/general use |
| A4-80 (SS) | ~700 (proof: ~830) | A4 (≈10) | Stainless 316 fastener; marine/chemical exposure |
Table 2: SAE Bolt Grades and Matching Nut Grades
| Bolt Grade | Tensile Strength (psi) | Nut Grade | Typical Uses |
|---|---|---|---|
| Grade 2 | 60–74 ksi | Grade 2 | Low-strength (furniture, light automotive body brackets) |
| Grade 5 | 105–120 ksi | Grade 5 | Medium (automotive chassis, steering, suspension) |
| Grade 8 | 150 ksi | Grade 8 | High (heavy machinery, engine mounts, driveline) |
| A307 Gr A/B | ~60–85 ksi | A307 Nuts | Structural bolts (bridges, flanges) – use heavy hex nuts per ASTM |
| A449/C1040 | ~150 ksi | Grade 8* | Anchor and hardware bolts; use heavy nuts per spec |
*Note: Structural-grade bolts (A325/A490, A307 Grade B, etc.) use ASTM A563/A194 heavy hex nuts (e.g., ASTM A325 Type 1 with A563 Grade C/D/DH or A194 Grade 2H nuts[15]). Always consult the relevant fastener specification for the correct nut.
Sources: Bolt and nut strength data and pairings are adapted from SAE J429, ISO 898, and ASTM standards and industry guides[18][2][15]. Corrosion recommendations from fastener finish guides[10].
[1] [3] [9] [18] Bolt and Nut Grades Explained: Head Markings, Strength Charts & Applications
[2] [4] [16] [17] Automotive Fastener Strengths/Grade Classes – Auto Fasteners
https://auto-fasteners.com/automotive-fastener-strengths-grade-classes/
[5] [7] Understanding Nut and Bolt Grades: Why the Nut Should Be Weaker Than the Bolt – Components For Industry
[6] [12] Bolt Grade Markings Explained for Better Fastener Choices
[8] [14] Is It Safe to Mix and Match Fastener Nuts Occasionally? | XINCHI
https://www.xinchi-industry.com/news/is-it-safe-to-mix-and-match-fastener-nuts-occasionally/
[10] [11] [13] Guide to Fastener Finishes and Corrosion Resistance – BCP Fasteners
[15] ASTM A325 – Portland Bolt
https://www.portlandbolt.com/technical/specifications/astm-a325/
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