Fastener Grades and Head Markings
Read the head marking and the equipment specification before choosing a replacement. Three radial lines commonly identify SAE Grade 5 hex cap screws; six identify Grade 8. Metric numbers such as 8.8 are a different strength-class system. Match the specified fastener, thread, finish, and companion hardware together.
In this guide
Start with the marks you can see
Photograph the whole head. Keep the grade mark and maker mark separate.
| Visible clue | What it suggests | What it does not prove |
|---|---|---|
| Three radial lines on an inch hex cap screw | SAE J429 Grade 5 | Diameter, thread pitch, finish, or approval for this machine |
| Six radial lines on an inch hex cap screw | SAE J429 Grade 8 | That it can replace every lower-strength fastener |
| 8.8, 10.9, or 12.9 | Metric steel property class | Thread diameter or pitch |
| A2-70 or A4-80 | Stainless steel group and property class | Universal chemical resistance or equivalence to Grade 8 |
| No readable grade mark | Identification remains incomplete | “Definitely Grade 2” |
The sketches show common recognition patterns, not a counterfeit inspection method. A worn head, small fastener, special product, or different standard may use another marking system. Letters or a logo often identify the maker. Do not count them as radial grade lines.
Separate the size from the strength
M10 × 1.5 describes the thread. A separate 8.8 marking describes a mechanical property class.
A buyer can receive two bolts with the same diameter, pitch, and length but different strength, head geometry, coating, or intended service. “It fits the hole” answers only part of the replacement question.
For metric carbon/alloy steel classes, the first number gives nominal tensile strength in hundreds of MPa; the second indicates the nominal yield-strength ratio in tenths. Class 8.8 illustrates the naming rule: 8 × 100 = 800 MPa nominal tensile strength, and 0.8 × 800 = 640 MPa nominal yield strength. Actual acceptance requirements depend on the standard and size. This arithmetic is not a working-load rating.
Tensile strength concerns the maximum tensile stress the material sustains. Yield strength concerns permanent stretching. Proof stress is a specified test level below unacceptable permanent extension. None tells a buyer how much an installed bracket, lift point, or assembly may safely carry.
Write one complete replacement line
Keep form, size, strength, finish, and quantity in the same request.
Example request: “Hex cap screw, 3/8-16 UNC × 1-1/2 in under-head length, SAE J429 Grade 5, specified zinc finish, quantity 50; confirm matching nut and washer requirements against the equipment drawing.” This is a naming example, not a recommendation for an unknown joint.
| Record | Useful evidence |
|---|---|
| Product form | Hex head, socket head, flange, shoulder, stud, or special; photo of side profile |
| Dimensions | Thread diameter and pitch, length, threaded length, shoulder dimensions if present |
| Strength requirement | Drawing/OEM specification and all visible head marks |
| Material and coating | Exact specified designation, not “silver” or “gold” |
| Complete joint | Nut, washers, mating material, locking feature, and installation instruction |
| Procurement | Quantity, machine/asset, required documents, due date |
If the finish is unknown, write “finish unknown” and attach the photo. A gold-colored surface is not proof of Grade 8. A black finish does not identify a socket screw's strength standard.
A higher number does not approve a substitution
Use the drawing or OEM requirement. Refer a proposed change to the person responsible for the joint.
Changing the fastener can change clamp load, nut compatibility, bearing behavior, fatigue performance, and installation requirements. A socket head cap screw may follow a different standard from an SAE hex cap screw. An ASTM structural bolt set carries requirements beyond a familiar strength number. A stainless fastener may solve corrosion while failing the specified mechanical requirements.
Do not transfer a torque value to a different coating or lubricant. Friction changes how tightening torque produces tension. High-strength steel also needs an approved coating process because susceptible material, hydrogen, and sustained stress can combine to cause delayed cracking.
Keep structural, lifting, pressure, vehicle, guarding, and other engineered hardware tied to its controlled specification. A buyer's useful next action is to collect the marks and drawing, then ask for confirmation of the proposed replacement. A strength comparison chart cannot supply that approval.
Keep the identity after the box is opened
Retain the supplier item number, lot information when required, and the full size/grade/finish label.
Put look-alike grades or finishes in separate locations. A shared bin destroys the distinction the head marks were meant to preserve. Keep matched or traceable sets together when the specification requires them.
At receiving, compare the order line, package label, markings, and dimensions. Quarantine a mismatch instead of relabeling it from appearance. Photograph a failed part before cleaning or discarding it; fracture and contact marks may be needed by the technical owner.
Frequently Asked Questions
Is Grade 8 the same as metric 8.8?
No. They belong to different systems. The similar-looking numbers do not establish equivalent dimensions, mechanical requirements, or interchangeability.
Can I identify strength from coating color?
No. Read the mark and package specification. Several grades can share the same color, and one grade can be supplied with several coatings.
What if the head is too damaged to read?
Preserve the part and ask for the drawing, OEM number, original package, or purchase history. Keep the strength unknown until the evidence resolves it.
Sources and Review Boundaries
Published by GO4FORGE. Revised 2026-10-29. Editorial method. These notes help narrow sourcing evidence and selection risk; they do not replace the responsible site authority, OEM procedure, code requirement, or qualified engineering/safety review.
- GO4FORGE product knowledge files Internal product-knowledge references distilled into public decision guides.
- GO4FORGE RFQ evidence model Shop-floor evidence patterns for quotes, substitutions, and sourcing review.
- Portland Bolt: fastener marking requirements SAE grade marks and manufacturer identification.
- Fastenal: Grade 5 hex cap screw product standard Product form, thread class, finish and traceability are separate specification fields.
- Fastenal: strength categories and substitution limits Similar strength does not make different product standards interchangeable.
- Bossard: stainless steel groups and property classes Stainless grade-group, property-class and maker markings.
- SAE J429: mechanical and material requirements for inch fasteners
- ASTM A563: carbon and alloy steel nuts
- ASTM F3125: high-strength structural bolts
- ASTM F1941/F1941M: electrodeposited coatings on mechanical fasteners
- IFI Fastener Standards reference family
Related Guides
Plan the first stock point or improve an existing one
You can begin with the available space and the work being done; a finished parts list is optional. Ask FORGE about this topic.