JIS B 1051 — the Japanese property-class standard for carbon and alloy steel fasteners

Scope — and what it does not cover

JIS B 1051 is the Japanese Industrial Standard for the mechanical properties of bolts, screws and studs in carbon and alloy steel — the document a Japanese drawing means when it writes 強度区分 8.8. It is technically aligned with ISO 898-1, so the class arithmetic, the hardness window with its upper limit, and the minimum tempering temperature follow the same logic. It does NOT cover nuts (JIS B 1052-2), stainless fasteners (JIS B 1054), surface treatment (JIS B 1044 for electroplated coatings on fasteners, JIS H 0404 for the plating symbol written on the drawing), thread tolerances (JIS B 0209), or the dimensional standard of the part.

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

• 1JIS B 1051 and ISO 898-1 are aligned documents, not two different requirements. A part made to class 10.9 under one is made to class 10.9 under the other — but the drawing still has to name which one governs, because editions and acceptance clauses are managed separately.
• 2Japanese drawings write the class as 強度区分, and Japanese buyers search the JIS number rather than the ISO one. A certificate that quotes only the ISO number can make a conforming lot look unreferenced during an incoming inspection.
• 3Legacy Japanese drawings still carry the T-series — 4T, 6T, 8T, 10T. The digit is the nominal tensile strength in units of 10 kgf/mm², so 8T is roughly 800 N/mm². The T grade says nothing about the yield ratio, so it does not translate one-for-one into a modern class.
• 4The hardness requirement has a ceiling exactly as in ISO 898-1. A lot deliberately run hard "for safety" is out of class and closer to the hydrogen-embrittlement region, not further from it.
• 5Surface treatment lives in different documents. JIS B 1044 covers electroplated coatings on fasteners; the symbol actually written on a Japanese drawing, such as Ep-Fe/Zn 8/CM2, comes from JIS H 0404 and already fixes the thickness.
• 6The salt-spray test a Japanese customer cites is JIS Z 2371. Like ISO 9227 it defines the chamber and the solution, not a pass duration for your coating.

What to write on the drawing or PO

• The governing document and the class together — "JIS B 1051 強度区分 10.9" — with thread designation and pitch.
• Which acceptance tests apply per lot, and whether the test is on the full-size fastener or a machined test piece.
• The surface treatment on its own line, using the JIS H 0404 symbol in full, and whether de-embrittlement baking is required with its window.
• Whether the certificate must be issued in Japanese, English or both, and whether it must quote the JIS number rather than the ISO number.
• If the drawing is old and carries a T grade, the modern class that replaces it, agreed in writing before the order is placed.

What goes wrong when it is cited loosely

• Reading 8T as "8.8". The nominal tensile strengths are similar; the statement about yield behind the two numbers is not the same statement.
• Treating 強度区分 as a hardness specification. It is a strength class that carries a hardness window, and the window has a ceiling as well as a floor.
• Writing a JIS plating symbol on the drawing and a different coating thickness in the notes. The symbol already fixes the thickness, so the two lines contradict each other.
• Assuming a JIS-referenced part automatically satisfies an ISO-referenced customer audit. The technical content lines up; the document reference on the paperwork does not update itself.