SAE J429 — inch fastener grades 2, 5 and 8, and how they sit next to a metric class

Scope — and what it does not cover

SAE J429 sets the mechanical and material requirements for inch-series externally threaded fasteners — bolts, screws and studs — by grade: 2, 5, 5.2, 8 and 8.2, with the requirement changing by diameter range. It fixes proof load, yield strength, tensile strength, core hardness and the head marking. It does NOT cover nuts (SAE J995), washers (ASTM F436), structural bolting (ASTM F3125, formerly A325 and A490), coatings of any kind, or metric fasteners.

Standards are copyrighted documents. We explain scope, intent and practical application in our own words; obtain the current edition from ISO, ASTM, DIN or your national standards body.

What actually bites in production

• 1The requirement depends on diameter, not on grade alone. Grade 5 drops from 120 ksi to 105 ksi minimum tensile once the diameter passes 1 in.; a certificate quoting one number for every size has skipped a step.
• 2Head marking is part of the specification: no radial lines is grade 2, three lines is grade 5, six lines is grade 8. Marking is how the grade survives on the shop floor after the paperwork has been filed.
• 3Proof load, not tensile strength, is the number a joint is designed around. It is the load the fastener must carry with no measurable permanent set, and it is the target a heat treater has to hit.
• 4Grade 8 sits at roughly 33–39 HRC. That is above the threshold where electroplating brings internal hydrogen embrittlement into the process, so the baking requirement has to be written down rather than assumed.
• 5Grades 5.2 and 8.2 reach the same strength through low-carbon boron martensite. They are not automatically acceptable where 5 and 8 are specified, and the difference matters at elevated temperature.
• 6Inch and metric are different systems on paper. Grade 8 and class 10.9 land at similar strength, but the documents, the tests, the hardness bands and the marking rules are all different.

What to write on the drawing or PO

• Grade and the full inch thread designation, including UNC or UNF and the class of fit — for example 1/2-13 UNC-2A.
• The diameter range the grade is required over, so the correct row of the table is on record for the size actually ordered.
• Which lot tests are required — proof load, wedge tensile, hardness — and whether testing follows ASTM F606 methods.
• The nut and washer documents separately: SAE J995 grade for the nut, ASTM F436 for hardened washers.
• The coating as its own line (ASTM B633, or ASTM F1941 for fasteners) and whether de-embrittlement baking is required, with temperature, hold and maximum delay after plating.

What goes wrong when it is cited loosely

• Ordering "grade 8" without the size. The table changes with diameter, and the matching nut is a different document again.
• Substituting class 10.9 metric for grade 8 inch because the strengths are close. The thread is different and the specification is different; the substitution has to be an engineering decision on paper.
• Specifying grade 8 with a thick zinc coating and no baking clause. That hardness band is exactly where the risk lives.
• Treating the head mark as the certificate. The marking states an intent; the proof load and hardness records are the evidence.