Property Class 12.9 Steel Machine Screws, Inch (UNC & UNF) are supplied to ISO 898-1 property class 12.9. Corrosion behaviour is requires plating or black-oxide 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 highest common strength class, for high-load machine assembly.
- Grade
- ISO 898-1 property class 12.9
- Corrosion resistance
- Requires plating or black-oxide 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.
For machine screws, inch (unc & unf) in Property Class 12.9 Steel, the controlling selection factors are corrosion exposure, strength requirement and service temperature. The grade data for Property Class 12.9 Steel (ISO 898-1 property class 12.9) is summarized below together with every size supplied.
Why Choose Property Class 12.9 Steel for Machine Screws, Inch (UNC & UNF)
The highest common strength class, for high-load machine assembly. For machine screws, inch (unc & unf), this matters when requires plating or black-oxide protection is the primary requirement.
Mechanical & Physical Properties
| Grade designation | ISO 898-1 property class 12.9 |
|---|
| Magnetic response | Magnetic |
|---|
| Service temperature | commonly limited to approx. +300 °C; confirm against the coating and the load |
|---|
| Corrosion characteristic | Requires plating or black-oxide protection |
|---|
Corrosion Behavior in Service
Carbon and alloy steel machine screws, inch (unc & unf) 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 machine screws, inch (unc & unf) 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 Machine Screws, Inch (UNC & UNF) Available in Property Class 12.9 Steel
| Size | UNC threads per inch | UNF threads per inch | Major diameter (in) | Pan head diameter (in) | Pan head height (in) | Drive | Reference standard |
|---|
| #2 | 56 | 64 | 0.086 | 0.167 | 0.053 | PH1 | ASME B18.6.3 |
| #4 | 40 | 48 | 0.112 | 0.219 | 0.068 | PH1 | ASME B18.6.3 |
| #6 | 32 | 40 | 0.138 | 0.270 | 0.082 | PH2 | ASME B18.6.3 |
| #8 | 32 | 36 | 0.164 | 0.322 | 0.096 | PH2 | ASME B18.6.3 |
| #10 | 24 | 32 | 0.190 | 0.373 | 0.110 | PH2 | ASME B18.6.3 |
| 1/4" | 20 | 28 | 0.250 | 0.492 | 0.144 | PH2 / PH3 | ASME B18.6.3 |
Dimensions are independent of the material: every size above is available in Property Class 12.9 Steel.
Custom manufacturing · Made to drawing
Need this screw in a grade that is not listed?
Grade 12 9 Steel is one of the grades the catalog describes. Lengths and thread lengths outside the standard table are the commonest request, followed by a drive the standard does not offer and a head marked with your own part number. Made by cold heading or warm forging.
Minimum order from 2,000 pieces; samples within 7 days of drawing approval.
Custom screw manufacturingSend a drawing
Applications & Industries
What Machine Screws are specified for, industry by industry. Stocked from #2 to 1/4" (screw size).
Industries that specify Machine Screws
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Electronics & rack hardware
Gauge-series screws in panels and chassis.
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Maintenance of imported plant
Matching an existing UNC or UNF thread in service.
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US-specified equipment
Assemblies drawn in inches, where a metric screw will not thread.
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Maintenance & MRO
Stocked replacements matched to an existing property class, thread and finish.
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Food, pharma & cleanroom
Stainless assemblies that are washed down, where the finish matters as much as the strength.
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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.
Typical applications
- assemblies dimensioned in the inch gauge series
- UNC and UNF are not interchangeable at the same nominal size
- assemblies that are dismantled for service and re-torqued
- joints where the property class is stated on the drawing
- clamping two machined faces to a calculated preload
- bolting brackets, motors and gearboxes to a frame
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 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 Machine Screws are used, how to specify them, and the cases they are wrong for
