ISO 965 — metric thread tolerances 6H, 6g, 6e and 6G, and where the room for plating comes from

Scope — and what it does not cover

ISO 965 is the tolerance system for ISO general purpose metric screw threads: Part 1 sets the principles and the fundamental deviations, Part 2 the limits of size for general purpose external and internal threads, Part 3 the deviations for constructional threads. A tolerance class is a grade number plus a position letter — 6H for a nut, 6g for a bolt — where the letter fixes the allowance and the number fixes the width of the tolerance band. It does NOT specify the thread profile (ISO 68-1), the diameter and pitch series (ISO 261 and ISO 262), coating thickness (ISO 4042), or any mechanical property.

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

• 1Capital letters are internal threads: H has zero allowance, G a positive one. Lower case letters are external threads: h has zero allowance, and g, f and e move the whole thread progressively smaller.
• 2The allowance is the fundamental deviation and it follows PITCH, not diameter. At 0.8 mm pitch position g gives about 24 µm; at 1.5 mm pitch it gives about 32 µm. Same letter, different room.
• 3Thread tolerances describe the thread as produced, before coating. Unless the drawing says otherwise, the coated part is still expected to respect the maximum-material limit of the class — which is precisely where "the GO gauge no longer goes" comes from.
• 4Electroplating grows the effective pitch diameter by roughly four times the coating thickness, because the layer lands on both flanks of every thread. The practical thickness ceiling is therefore about the allowance divided by four.
• 5When the coating will not fit there are three honest fixes: produce the external thread to 6e or 6f before plating, tap the nut to 6G, or reduce the specified thickness. Plating thin and hoping the gauge passes is not one of them.
• 6A tolerance class is a pair. Writing 6g on the bolt and saying nothing about the nut leaves the fit undefined; 6H/6g is the normal general purpose combination and should be stated as such.

What to write on the drawing or PO

• The full thread designation with the class on both halves — M10 × 1.5 - 6g for the bolt, M10 × 1.5 - 6H for the nut.
• Whether the tolerance class applies before or after coating, in words. This one line prevents most gauge disputes.
• If a coating is specified, the pre-plate class chosen to accommodate it (6e or 6f) and the matching gauge set.
• Which gauges are the referee — GO and NO-GO ring and plug gauges to the stated class — and who holds the master.
• For a nut tapped after coating, say so and state the tapping class, because the flanks then carry no coating at all.

What goes wrong when it is cited loosely

• Specifying 6g together with a coating thickness that cannot physically fit inside the allowance for that pitch, then treating the gauge failure as a plating defect.
• Assuming a bigger diameter gives more room. The allowance follows pitch, so an M20 × 1.5 fine thread has exactly the same g allowance as an M10 × 1.5.
• Gauging a plated part with a pre-plate gauge set, or the reverse, and reporting the mismatch as a process fault.
• Quietly changing the external thread to 6e to make the coating fit, then letting the assembly plant discover a looser fit and call it undersize.