Bolt Grades Explained: Grade 5 vs Grade 8 vs Stainless (Which Do You Need?)

Bolt grade determines how much load a bolt can handle before it yields or breaks. Using an undergrade bolt in a structural connection is a safety failure waiting to happen. Here is exactly what each grade means and when to use it.

SAE Bolt Grade Chart

Metric Bolt Class Chart

Grade 5 vs Grade 8 — When to Use Each

Use Grade 5 for: General construction connections, deck hardware, fence post hardware, most residential structural applications, and any application where mild steel is appropriate. Grade 5 is the standard for most construction hardware.

Use Grade 8 for: Heavy equipment, automotive suspension components, applications with high vibration or impact loading, and any connection where failure would be catastrophic. Grade 8 is harder but more brittle — it can snap suddenly under shock load where Grade 5 would deform first as a warning.

Important: Grade 8 is NOT always better. In applications with shock or impact loading, Grade 5 is often the correct choice because it deforms before breaking, giving warning of overload.

How to Read Bolt Head Markings

SAE bolts have radial lines on the head. Count the lines and add 2 to get the grade — 3 lines = Grade 5, 6 lines = Grade 8. Grade 2 has no marks. Metric bolts show the class number directly (8.8, 10.9, 12.9). Stainless bolts are marked SS, A2, or A4.

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Frequently Asked Questions

What is the difference between Grade 5 and Grade 8 bolts?

Grade 5 bolts have 120,000 psi tensile strength and 3 radial lines on the head. Grade 8 bolts have 150,000 psi tensile strength and 6 radial lines. Grade 8 is stronger but more brittle - it can snap suddenly under shock load where Grade 5 deforms first as a warning. For most construction use Grade 5. Use Grade 8 for high-stress non-impact applications.

How do I identify bolt grades by head markings?

SAE bolts have radial lines on the head - count the lines and add 2 to get the grade. 3 lines equals Grade 5, 6 lines equals Grade 8. Grade 2 has no marks. Metric bolts show the class number directly on the head - 8.8 is roughly equivalent to Grade 5, 10.9 to Grade 8. Stainless bolts are marked SS, A2 for 304, or A4 for 316.

What bolt grade should I use for deck hardware?

Use Grade 5 hot-dipped galvanized or stainless steel bolts for deck hardware and outdoor structural connections. Grade 8 is not typically required for residential deck construction. More important than grade is the coating - standard zinc-plated bolts corrode rapidly in ACQ pressure-treated lumber. Use HDG or stainless for any outdoor application.

Can I mix Grade 5 and Grade 8 bolts?

Avoid mixing grades in the same joint. The stronger Grade 8 bolt will attract more load than the Grade 5 in the same connection leading to uneven load distribution. If one bolt in a group needs to be Grade 8 use Grade 8 for all bolts in that connection.

What is the metric equivalent of Grade 8 bolts?

Class 10.9 metric bolts are approximately equivalent to SAE Grade 8 with 1040 MPa tensile strength. Class 8.8 is approximately equivalent to Grade 5. Class 12.9 exceeds Grade 8 and is used in critical machinery and automotive applications. The class number is stamped directly on the bolt head.

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