How to Choose the Right Hex Head Cap Screw Grade
By Mutual Screw & Supply | Published: 2026-10-20
Category: Buying Guides
Learn how to select the correct hex head cap screw grade for your project. Compare strength, material, and thread options to make an informed choice.
Quick answer
Choose hex head cap screw grades based on required tensile strength, corrosion resistance, and thread engagement. For high-strength general use, Grade BD alloy steel works well; for corrosive environments, 316 stainless steel is better; for lighter-duty metric applications, Class 8.8 steel is a cost-effective choice.
Strength vs. corrosion resistance: the main trade-off
Higher strength grades like Grade BD alloy steel offer excellent load capacity for heavy machinery but lack corrosion protection. In contrast, 316 stainless steel provides superior corrosion resistance in marine or chemical environments, though its tensile strength is lower than high-strength alloy grades.
Class 8.8 steel bolts sit in the middle—they offer good strength for most industrial assemblies at a lower cost, but the zinc plating may wear over time in harsh conditions. Your choice depends on whether strength or longevity in a corrosive setting matters more.
Thread style affects joint integrity
Partially threaded screws provide a longer unthreaded shank, which improves shear strength and alignment in applications where the bolt passes through multiple components. Fully threaded screws offer maximum thread engagement for clamping force but reduce shear capacity slightly.
For example, the 2"-4-1/2 x 9" Grade BD cap screw is partially threaded, ideal for structural connections where shear loads are high. The M10-1.25 x 25mm Class 8.8 screw is fully threaded, better for applications needing precise clamping.
Decision rules for choosing a hex head cap screw grade
- Rule 1: Match grade to load requirements: Use Grade BD alloy steel for high-stress applications like heavy equipment or structural frames. Use Class 8.8 steel for moderate loads in machinery or automotive assembly. For corrosive environments, choose 316 stainless steel even though its strength is lower than Grade BD. Always verify the tensile strength rating from the manufacturer if not listed on the product page.
- Rule 2: Consider thread style for your joint design: Choose partially threaded screws when the bolt passes through thick materials and shear strength is critical. Choose fully threaded screws when clamping force and thread engagement are more important than shear capacity.
- Rule 3: Match finish to environment: Plain finishes (like Grade BD) need additional coating or lubrication for outdoor or corrosive use. Zinc-plated finishes (like Class 8.8) resist light corrosion indoors. For saltwater or chemical exposure, 316 stainless steel is the reliable choice. If you need corrosion resistance but cannot use stainless, consider adding a protective coating after installation.
Product facts at a glance
- 2"-4-1/2 X 9" Hex Head Cap Screws | Grade BD Alloy Steel Plain | Partially Threaded (1 / Pkg.): 2"-4-1/2 x 9" hex head cap screw, Grade BD Alloy Steel, plain finish, partially threaded, sold individually. This grade delivers high tensile strength for heavy machinery and structural joints where shear loads are significant, but the plain finish requires protection in wet environments.
2"-4-1/2 X 9" Hex Head Cap Screws | Grade BD Alloy Steel Plain | Partially Threaded (1 / Pkg.) - M10-1.25 X 25mm Hex Head Cap Screws | Class 8.8 Steel Zinc Plated | Fully Threaded (100 / Pkg.): M10-1.25 x 25mm hex head cap screw, Class 8.8 Steel, zinc plated, fully threaded, sold in packs of 100. A cost-effective metric option for general assembly with good strength and moderate corrosion resistance from the zinc plating, ideal for indoor or dry applications.
M10-1.25 X 25mm Hex Head Cap Screws | Class 8.8 Steel Zinc Plated | Fully Threaded (100 / Pkg.)
Who should choose which grade?
Best for
- Heavy machinery and structural joints requiring high shear strength: Grade BD alloy steel partially threaded
- General metric assembly with moderate loads and indoor use: Class 8.8 steel fully threaded
Not for
- Corrosive or marine environments: Grade BD and Class 8.8 plain or zinc finishes will corrode; choose 316 stainless steel instead
- Applications needing maximum thread engagement in thin materials: Partially threaded screws leave an unthreaded shank that may not provide enough grip
