Property Class 8.8 Steel Square-Head Bolts are supplied to ISO 898-1 property class 8.8. Corrosion behaviour is requires plating (zinc) for corrosion protection, the material is magnetic, and the usable service range is commonly limited to approx. +300 °C; confirm against the coating and the load. In practice this grade is the workhorse medium-carbon alloy class for structural joints.
- Grade
- ISO 898-1 property class 8.8
- Corrosion resistance
- Requires plating (zinc) for corrosion protection
- Magnetic response
- Magnetic
- Service temperature
- commonly limited to approx. +300 °C; confirm against the coating and the load
Property values are typical for the grade. Certified mechanical test data is available on request for production orders.
Property Class 8.8 Steel Square-Head Bolts are supplied to ASME B18.2.1 / SAE J429 Grade 5. The grade provides requires plating (zinc) for corrosion protection and is magnetic; the practical service range is commonly limited to approx. +300 °C; confirm against the coating and the load. Property data, grade comparison and the full size table follow.
Why Choose Property Class 8.8 Steel for Square-Head Bolts
The workhorse medium-carbon alloy class for structural joints. For square-head bolts, this matters when requires plating (zinc) for corrosion protection is the primary requirement.
Mechanical & Physical Properties
| Grade designation | ISO 898-1 property class 8.8 |
|---|
| Magnetic response | Magnetic |
|---|
| Service temperature | commonly limited to approx. +300 °C; confirm against the coating and the load |
|---|
| Corrosion characteristic | Requires plating (zinc) for corrosion protection |
|---|
Corrosion Behavior in Service
Carbon and alloy steel square-head bolts require a coating for corrosion protection. Zinc plating (with or without passivation) is the most common; hot-dip galvanizing is used for outdoor structural service; black-oxide is mainly a tarnish-prevention finish for tool-room use.
Galvanic & Galling Considerations
For square-head bolts in steel, the dominant compatibility issue is hydrogen embrittlement at high property classes (10.9 and above) when electroplated. Plating must be followed by an appropriate baking treatment, or a non-electroplated finish (mechanical zinc, zinc flake) should be substituted.
Sizes of Square-Head Bolts Available in Property Class 8.8 Steel
| Size | Thread form | Head width (in) | Head height (in) | Tensile strength (psi) | Material / grade | Reference standard |
|---|
| 1/4-20 | fully or partially threaded | 3/8 | 11/64 | 120000 | yellow-zinc-chromate steel, Grade 5 | ASME B18.2.1 / SAE J429 |
| M6 / M8 / M10 / M12 / M14 / M16 / M20 / M24 / M30 | fully or partially threaded | | | | yellow-zinc-chromate steel, Grade 5 | ASME B18.2.1 / SAE J429 |
| 5/16-18 through 1-1/4-7 | fully or partially threaded | | | | yellow-zinc-chromate steel, Grade 5 | ASME B18.2.1 / SAE J429 |
Dimensions are independent of the material: every size above is available in Property Class 8.8 Steel.
Custom manufacturing · Made to drawing
Need this bolt in a grade that is not listed?
Grade 8 8 Steel is one of the grades the catalog describes. Head form and drive can be specified to the assembly rather than to a standard: a larger bearing face, a security drive, or a socket sized for the tool already on the bench. Made by cold heading or hot forging.
Minimum order from 2,000 pieces; samples within 7 days of drawing approval.
Custom bolt manufacturingSend a drawing
Applications & Industries
What Square-Head Bolts are specified for, industry by industry. Stocked from 1/4-20 to 5/16-18 through 1-1/4-7 (screw size).
Industries that specify Square-Head Bolts
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Steel frames
Partial or full thread options fit different grip lengths.
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Maintenance work
Metric and inch sizes cover mixed legacy equipment.
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Fabricated machinery
Square heads provide wrench faces when a hex head is not desired.
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Agricultural structures
Grade 5 steel covers general structural and equipment joints.
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Machine building & OEM
The default fastener on production equipment: covers, brackets, motor mounts and sub-assemblies tightened to a specified torque.
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Automotive & transport
Chassis, driveline and body fixings that see vibration and are assembled to a controlled preload.
Typical applications
- matching the Grade 5 material to the joint requirement
- using the stated ASME and SAE designations when ordering
- tightening a square head with an open wrench
- choosing full or partial thread from the grip
- assemblies that are dismantled for service and re-torqued
- joints where the property class is stated on the drawing
Service conditions to specify against
- Marine and coastal
- Available in A4 (316) stainless for chloride exposure — coastal structures, pool halls, water treatment and deck hardware, where A2 will eventually pit.
- High load
- Supplied in ISO 898-1 property classes, so the joint can be designed to a stated proof load and tightened to a calculated torque rather than by feel.
- Torque and preload
- The joint is designed around bolt tension, so lubrication, coating and the friction coefficient all change the torque figure — state them together.
Where to use something else
- Do not assume the square head provides the same wrench clearance as a standard hex head; check the actual envelope.
- Do not use a machine thread into unthreaded sheet or timber; use a tapping screw, rivet nut or insert made for that parent material.
Full application guide: where Square-Head Bolts are used, how to specify them, and the cases they are wrong for
