6g vs 6h thread tolerance: when to use which.
Both 6g and 6h are external thread tolerance classes per ISO 965-1. They have identical tolerance band width (grade 6). They differ only in position — 6g sits below nominal with an allowance for plating, 6h sits at nominal with zero allowance. Confusing them sends parts back for re-work. This page covers the difference, when to use each, and the common Indian-shop mistakes.
The two parts of a thread class designation
An ISO 965 thread class designation like 6g has two pieces:
- The number (6) is the tolerance grade. It determines how wide the tolerance band is. Smaller number = tighter band. Grades 3, 4, 5, 6, 7, 8, 9 are defined; 6 is the production standard.
- The letter (g, h, e, f for external; G, H, E, F for internal) is the tolerance position. It determines where the band sits relative to nominal. Lowercase letters are external threads, uppercase are internal.
So 6g and 6h have the same band width but different positions. The band for 6g sits below nominal; the band for 6h sits at nominal.
Side-by-side comparison
External thread with allowance
Position: below nominal (negative fundamental deviation).
Upper limit: below nominal by the fundamental deviation 'es'.
Why it exists: allowance built in for plating, coating, or surface treatment.
Used for: Production bolts, screws, studs (almost all commercial fasteners).
External thread with zero allowance
Position: at nominal (zero fundamental deviation).
Upper limit: at nominal.
Why it exists: theoretical class for uncoated, gauge-grade threads.
Used for: Thread gauges, ground-thread shafts, master calibration parts.
Fundamental deviations 'es' for 6g by size
The "es" value is the upper deviation — how far below nominal the upper limit of the thread sits. Values from ISO 965-1:2013.
| Thread | Pitch (mm) | es for 6g (μm) | IT6 (μm) | ei for 6g (μm) |
|---|---|---|---|---|
| M3 × 0.5 | 0.50 | −17 | 67 | −84 |
| M4 × 0.7 | 0.70 | −19 | 80 | −99 |
| M5 × 0.8 | 0.80 | −19 | 90 | −109 |
| M6 × 1.0 | 1.00 | −26 | 106 | −132 |
| M8 × 1.25 | 1.25 | −28 | 118 | −146 |
| M10 × 1.5 | 1.50 | −32 | 132 | −164 |
| M12 × 1.75 | 1.75 | −34 | 150 | −184 |
| M14 × 2.0 | 2.00 | −38 | 170 | −208 |
| M16 × 2.0 | 2.00 | −38 | 170 | −208 |
| M20 × 2.5 | 2.50 | −42 | 190 | −232 |
| M24 × 3.0 | 3.00 | −48 | 212 | −260 |
| M30 × 3.5 | 3.50 | −53 | 236 | −289 |
For 6h, the 'es' value is 0 for all sizes; 'ei' = -IT6. Values apply to pitch diameter; major and minor diameters use the same fundamental deviation but their own IT grade.
The most-used thread class combinations
| Combination | Fit type | Typical use |
|---|---|---|
| 6H/6g | Medium (standard) | Default for production fasteners with thin plating (zinc 5-12 μm) |
| 6H/6h | Close (zero allowance) | Ground-thread shafts, thread gauges, calibration masters |
| 5H/4h | Fine (tight) | Precision instruments, optical mounts, aerospace critical fasteners |
| 7H/8g | Loose | After heavy coating (hot-dip galvanize), high-temperature service, fast assembly |
| 6G/6e | Loose (less common) | Older drawings; replaced by 7H/8g in modern practice |
When to use 6g
- Production bolts and screws that will be plated. Zinc plating adds 5-12 μm per surface; 6g leaves room for this without requiring the nut to be re-tapped after plating.
- Tapped holes in production parts (paired with 6H internal). The standard combo for everything from automotive brackets to general-purpose machinery.
- Default when the drawing doesn't specify a thread class. ISO 965-1 says 6H/6g is the assumed default when only thread size is given.
When to use 6h
- Thread gauges and calibration masters. Gauges must be at exactly nominal; any allowance would invalidate the gauge.
- Ground-thread shafts. Where the thread is the final form (no plating, no coating), 6h gives the tightest production-feasible fit.
- Optical instrument shafts. Where precision is more important than ease of assembly, 6h removes the play that 6g introduces.
- Mating with a 6H internal in a high-precision application. 6H/6h is essentially line-to-line at the upper limit — minimal backlash but requires both threads to be made well within tolerance.
5 common mistakes
- Using 6h on production bolts that will be zinc-plated. The plating adds material; the thread becomes too large to fit a 6H nut. The fix: switch to 6g, or tap the nut after plating.
- Using 6g on a thread gauge. The gauge sits below nominal, so it accepts threads that are out of spec on the small side. The fix: use 6h or 4h for gauges.
- Confusing 6g (external) with 6G (internal). Lowercase = external (bolt), uppercase = internal (nut). The G/g position has clearance; the H/h position is at nominal.
- Not specifying a class on the drawing. Leaves room for shop interpretation. ISO 965 default is 6H/6g but auditors prefer explicit callouts on drawings that go to multiple suppliers.
- Mixing tolerance grades and positions. A 4g external thread is valid (tight tolerance, with allowance) but unusual. Stick to standard combinations unless there's a specific functional reason for an unusual class.
References
- ISO 965-1:2013 — ISO general purpose metric screw threads — Tolerances — Part 1: Principles and basic data
- ISO 965-2:2013 — Part 2: Limits of sizes for general purpose external and internal screw threads — Medium quality
- ISO 965-3:2013 — Part 3: Deviations for constructional screw threads
- ISO 262:1998 — ISO general purpose metric screw threads — Selected sizes