ASTM A193 B7 vs Class 10.9 Bolts: High-Strength Guide

Quick answer

Choose ASTM A193 B7 when your project uses inch-series threads, needs a high-strength alloy steel cap screw or stud bolt for machinery or equipment assembly, and the joint will be protected from moisture. Choose Class 10.9 metric bolts when your design is metric, you want a fully threaded high-strength fastener, and you need a predictable metric thread pitch. Neither grade is a corrosion-resistant stainless; both are plain steel and depend on the surrounding environment for protection.

What each grade actually is

ASTM A193 B7 is an inch-series specification for alloy steel bolting used in high-temperature and high-pressure service, commonly found as hex head cap screws and stud bolts. Class 10.9 is a metric property class defined by ISO 898-1, meaning the bolt has a specified minimum tensile strength and a yield strength roughly 90% of that tensile value. In practical terms, both are heat treated fasteners: the steel is quenched and tempered to reach high strength rather than being used in a soft, as-rolled condition.

The difference you feel at the bench is the thread system. B7 fasteners follow inch threads such as 5/8-11, while Class 10.9 fasteners follow metric threads such as M16-2. Mixing the two on the same joint is not a strength problem; it is a fit problem, because the nuts, washers, and tapped holes will not interchange.

Strength and thread configuration

Both grades are high strength bolt grades, but they are not interchangeable by diameter alone. A 5/8-11 B7 cap screw and an M16-2 Class 10.9 cap screw are close in size but differ in thread pitch, so the nut and hole must match the bolt you choose. Thread configuration also changes how the bolt behaves in the joint: partially threaded cap screws put the unthreaded shank in shear and allow the threads to stay out of the bearing area, while fully threaded cap screws give you more adjustment range and are common in metric assemblies.

When you tighten either grade, the goal is clamp load, not torque for its own sake. Use the torque value specified for your joint and lubricant condition, and use hardened washers under the turning face if the joint design calls for them. Because these are heat treated fasteners, do not cut, weld, or heat them after installation without engineering approval, as that can alter the properties the grade was chosen for.

Corrosion and environment

Both B7 and Class 10.9 are supplied in plain, uncoated condition in the catalog examples here. Plain alloy steel will rust in outdoor, humid, or wash-down environments unless it is protected by paint, oil, plating, or a controlled indoor atmosphere. If your application is a marine deck, a pressure-treated wood structure, or a constantly wet location, a stainless fastener is usually the better starting point rather than a high-strength alloy steel bolt.

Do not assume that higher strength means better corrosion resistance. Strength comes from alloy content and heat treatment; corrosion resistance comes from the alloy family and finish. If you need both, specify a coated or stainless product and confirm it meets the strength requirement for your joint.

How to choose on the job

Start with the hole and the nut, not the bolt. If the tapped hole or nut is inch-series, B7 is the natural match. If it is metric, Class 10.9 is the natural match. Next, check the drawing for thread length: a partially threaded cap screw is often specified where the shank passes through a shear plane, and a fully threaded cap screw is often specified where the bolt must accommodate a range of grip lengths.

Finally, check the quantity and packaging against your build. Buying a pack that matches your assembly count reduces leftover mixed hardware and makes it easier to keep one grade in one bin. If your design mixes inch and metric fasteners, label the bins clearly so a metric nut never gets started on an inch bolt.

Trade-offs to weigh before you buy

• Inch vs metric system: B7 follows inch threads like 5/8-11; Class 10.9 follows metric threads like M16-2. The bolt you can actually install is the one that matches your nut and tapped hole. Match the thread system first, then compare strength.
• Thread configuration: A partially threaded cap screw keeps threads out of the bearing area, while a fully threaded cap screw offers more adjustment range. The catalog B7 example is partially threaded; the catalog Class 10.9 example is fully threaded. Pick the thread style that matches the joint drawing, not the one that is in stock.
• Corrosion protection: Both grades are plain steel in the examples here, so they need paint, oil, or a dry environment to resist rust. A yellow zinc finish or a 316 stainless lag screw is a different answer to the corrosion question. If the joint stays wet, choose a corrosion-resistant product instead of a plain high-strength bolt.

Catalog examples of each grade

• 5/8-11 X 2-1/4" Heavy Hex Cap Screws | ASTM A193 B7 Alloy Steel Plain | Partially Threaded (20 / Pkg.): ASTM A193 B7 Alloy Steel Plain hex head cap screws, partially threaded, size 5/8-11 x 2-1/4", sold 20 per package. This is an inch-series high-strength cap screw with a partially threaded shank, which suits machinery and equipment assembly where the unthreaded portion can sit in the bearing or shear area.
• M16-2 X 40mm Hex Head Cap Screws | Class 10.9 Steel Plain | Fully Threaded (50 / Pkg.): M16-2 x 40mm hex head cap screws, Class 10.9 Steel Plain, fully threaded, sold 50 per package. This is a metric high-strength cap screw with full threads, which gives more adjustment range and matches metric nuts and tapped holes.
• 1/4-20 X 2" Hex Head Cap Screws | USA Made | PFC9 Alloy Steel Yellow Zinc (100 / Pkg.): 1/4-20 x 2" hex head cap screws, USA Made, PFC9 Alloy Steel Yellow Zinc, sold 100 per package. A smaller inch-series alloy steel cap screw with a yellow zinc finish, useful when you want a plated finish and a domestic source for lighter-duty assemblies.
• 3/4" X 12" Hex Head Lag Screws | 316 Stainless Steel (15 / Pkg.): 3/4" x 12" hex head lag screws, 316 Stainless Steel, sold 15 per package, intended for heavy-duty wood and structural timber connections. A stainless option for wood-to-wood and timber connections where corrosion resistance matters more than the high-strength alloy steel grades discussed above.

Which grade fits your job

Best for

• Inch-series machinery, equipment assembly, and general industrial fastening where a high-strength alloy steel cap screw or stud bolt is specified
• Applications where a partially threaded shank is needed for shear or bearing, such as the 5/8-11 x 2-1/4" B7 cap screw
• Metric designs that call for a fully threaded high-strength cap screw, such as the M16-2 x 40mm Class 10.9 bolt

Not for

• Wet, outdoor, or wash-down environments where plain steel will rust; choose a stainless or coated fastener instead
• Joints where the thread system does not match the nut or tapped hole; a B7 bolt will not fit a metric nut and a Class 10.9 bolt will not fit an inch nut
• Applications that require welding or post-installation heating of the fastener without engineering approval