The difference between Grade 5 fasteners vs. Grade 8 fasteners and when to use each type
When rebuilding or restoring a car or truck, and you want to upgrade the bolts to a stronger grade than what your vehicle originally was built with, there are several options when choosing the ideal type of fastener to use. But it all depends on the application; is the bolt for a brake caliper, suspension arm, or the hold the rear axle in place?
The chart shown here, courtesy of Fastenermart.com, outlines the different types of fastener grades and what their strength properties are:
Many fasteners (screws, etc.), especially smaller sizes, are typically not graded—their strength isn’t specified. Larger sizes and those for specialized purposes, though, are made to meet certain strength requirements. Inch fasteners—those commonly used in North America—will have a grade or ASTM rating. Property class (often just “class”) specifies metric fasteners. Special markings on screw heads and nuts identify the fastener’s grade. Replace a graded fastener with same or higher grade (don’t replace a Grade 8 fastener with a Grade 5 or 2). If in doubt, ask a professional for assistance. The chart below summarizes some of the more common grades and classes available here at Fastener Mart.
| Common fastener grades and property classes. | |
| Grade/Class | Strength |
| Grade A Nuts | Strength exceeds Grade 2. |
| ASTM A325 | Bolts meet ASTM A325 Type 1 standards for structural steel joints. |
| Grade B Nuts | Strength is similar to Grade 5. |
| Grade B7 Threaded Stud & Rod | Same as Grade 5. Use with Grade 2H and Grade C nuts. |
| Grade C Nuts | Strength exceeds Grade 5. Use with heat-treated medium-strength steel fasteners like ASTM A325 structural bolts. |
| Grade G Nuts | Strength is similar to Grade 8. Use with ASTM A325 structural bolts. |
| Grade 2 | Low strength. |
| Grade 2H Nuts | Strength is similar to Grade 5. Use with ASTM A325 structural bolts. |
| Grade 5 | Medium strength. |
| Grade 8 | High strength. |
| Metric Class 4 | Similar to Grade 2. |
| Metric Class 8.8 | Similar to Grade 5. |
| Metric Class 10.9 | Similar to Grade 8. |
| Metric Class 12.9 | The highest metric class for strength, it exceeds Grade 8. |
Because there are so many different grades and fasteners all look about the same, medium carbon and alloy steel strength grades are marked for identification. Bolts and screws also bear a mark to identify the manufacturer. If strength is important, ensure that both the grade and manufacturer identification markings are present.
SAE Hex Cap Screw Grades
Screws are marked on the top of their heads, the exceptions being small (under about 1/4″), slotted and recessed head fasteners where there is insufficient room for head markings. A head with no markings is Grade 2, one with 3 lines spaced 120 degrees is Grade 5, and 6 lines spaced at 60-degree intervals is Grade 8.
A more detailed explanation, courtesy of toweltrades.net, states: Fasteners are critical components in various industries, and bolts are among the most commonly used types. However, not all bolts are created equal. Bolts come in different grades, each with unique properties and applications. When it comes to choosing between a grade 8 and grade 5 bolt, the decision is not always straightforward. However, understanding the differences between the two grades can help make an informed decision.
Bolts are graded based on their material properties, including tensile strength, yield strength, and hardness. Grade 5 bolts are made of medium carbon steel with a tensile strength of 120,000 PSI and a yield strength of 92,000 PSI. On the other hand, grade 8 bolts are made of alloy steel with a tensile strength of 150,000 PSI and a yield strength of 130,000 PSI. The higher strength of grade 8 bolts makes them more suitable for demanding applications that require more durability and reliability.
One of the primary applications for grade 8 bolts is in the automotive industry, where high-stress and high-vibration conditions are common. In this industry, grade 8 bolts are preferred for critical components such as engine mounts, suspension systems, and steering components. These bolts are also used in heavy equipment, construction, and aerospace industries, where safety and reliability are paramount.
In contrast, grade 5 bolts are typically used in low to medium-stress applications, such as general construction, automotive, and machinery. They are also commonly used in non-critical applications such as holding non-structural components in place.
Another significant factor to consider when choosing between grade 5 and grade 8 bolts is the cost. Grade 8 bolts are generally more expensive than grade 5 bolts due to their higher strength and more stringent manufacturing requirements. However, the higher cost may be worth it if the application requires the additional strength and reliability.
It is also essential to note that using the wrong grade of bolt for an application can result in catastrophic failures, leading to property damage, injuries, and even fatalities. Therefore, it is crucial to follow the manufacturer’s recommendations and industry standards when selecting bolts for an application.
In conclusion, choosing between a grade 8 and grade 5 bolt depends on the application’s requirements. If the application involves high-stress and high-vibration conditions or critical components, grade 8 bolts are the preferred choice due to their higher strength and reliability. However, if the application involves low to medium-stress conditions or non-critical components, grade 5 bolts can be a cost-effective solution. Ultimately, the choice of bolt grade should always prioritize safety and reliability.