The Tolerance-Band Code: What Number and Letter Mean

Per GB/T 197 "General purpose metric screw threads—Tolerances", the metric thread tolerance-band code has two parts: tolerance grade (number) + fundamental deviation (letter). In "6g" for example, "6" is the tolerance grade and "g" is the fundamental-deviation code. You read them together, e.g. 6g as "grade-6 g deviation".

Component	Meaning	External Thread (lowercase)	Internal Thread (uppercase)
Tolerance grade (number)	Tolerance-band width; larger number = wider tolerance, lower precision	Common 4, 6, 8	Common 5, 6, 7
Fundamental deviation (letter)	Position of the band relative to basic profile (upper/lower deviation)	e, f, g, h	G, H
Deviation direction	h/H = zero deviation; g/G/e/f = directional deviation	h lower deviation = 0; g/e/f negative	H lower deviation = 0; G positive

Remember the pattern: external threads use lowercase letters (g/h/e/f), internal threads use uppercase (G/H). h and H have zero fundamental deviation—external h has upper deviation es = 0, internal H has lower deviation EI = 0; g, e, f are negative deviations for external threads (smaller than basic size), and G is a positive deviation for internal threads (larger than basic size).

6g and 6H: The Baseline Fit for External and Internal Threads

6g is the most common tolerance band for external threads (bolts, screws), and 6H is the most common band for internal threads (nuts, tapped holes). The pair forms the medium-fit "6H/6g", the default choice for ordinary fastener joints and the national standard's recommended preferred combination.

Band	Object	Deviation Trait	Typical Use
6g	External thread (bolt)	Negative fundamental deviation; leaves slight coating clearance	General fasteners, post-zinc-plate threads, routine assembly
6h	External thread	Zero deviation; tighter fit	Precision fits, pre-plate threads, special tight fits
6e / 6f	External thread	Larger negative deviation; more coating allowance	Threads needing thicker electroplate
6H	Internal thread (nut)	Zero deviation (lower deviation EI=0)	General nuts, tapped holes
6G	Internal thread	Positive fundamental deviation; larger clearance	Rough/high-temperature/lubricated internal threads

Note: the core of a thread fit is the pitch diameter. Major and minor diameter tolerances are relatively loose; what really decides screwing tightness is the fit of the pitch-diameter tolerance band. Inspection also centers on pitch diameter.

Where Basic Sizes Come From: Pitch and Minor Diameter per GB/T 196

The tolerance band is a "deviation", and the deviation lands on a basic size. The basic profile dimensions of metric threads are given by GB/T 196 "General purpose metric screw threads—Basic dimensions". Its core is the original triangle height H, and the derived pitch and minor diameters:

H = (√3/2)·P = 0.866025 P ; pitch diameter d₂ = d − 0.6495 P ; minor diameter d₁ = d − 1.0825 P

where P is the pitch (mm). GB/T 196 Table 1 gives pitch and minor diameter values for each size (mm); commonly used sizes are excerpted below:

Size	Pitch P	Pitch d₂	Minor d₁
M3	0.5	2.675	2.459
M4	0.7	3.545	3.242
M5	0.8	4.480	4.134
M6	1.0	5.350	4.917
M8	1.25	7.188	6.647
M10	1.5	9.026	8.376
M12	1.75	10.863	10.106

These pitch and minor diameters are the baseline for machining and inspection: the external 6g band is stacked on basic sizes d₂ and d₁, and the internal 6H likewise.

Choosing Common Fits: General Joints and Post-Plating Threads

For actual selection, judge quickly by the table below:

• General steel joint, no or thin coating: default 6g/6H medium fit; best versatility, fullest stock.
• Needs zinc or other electroplate: machine the external thread to 6g (coating allowance); for thick coatings use 6e/6f, then pair with a 6H nut.
• Precision / tight fit: external thread 6h (zero deviation); tighter fit, better coaxiality.
• High temperature, rough, needs large clearance for lubrication: internal thread 6G, avoiding thermal galling.
• Moving joint with fit requirements: choose grade by engagement length (medium/short/long); long engagement may relax the grade.

Inspection: How to Use Go/No-Go Gauges

Whether a thread is acceptable is judged with thread gauges (go gauge, no-go gauge; GB/T 3934), on the principle "go goes, no-go stops":

1. Go gauge (T): checks the functional pitch diameter and basic size; it should screw in by hand smoothly over the full thread length—if it will not go in, the thread was made too small and pitch diameter is out of tolerance.
2. No-go gauge (Z): checks the single pitch diameter; it should not screw in more than 2 threads (usually no more than 2 pitches)—if the no-go also screws in many turns, the pitch diameter was made too large and is out of tolerance.

For external threads such as bolts use ring gauges (go ring, no-go ring); for internal threads such as nuts and tapped holes use plug gauges. Go/no-go gauges only judge "accept/reject" and do not measure specific dimensions; use a thread micrometer or profile projector to read the actual pitch diameter. Incoming-inspection best practice is to gauge every batch sample while it is still on the line, because a pitch-diameter drift that is invisible to the eye is exactly what the go/no-go pair is designed to catch, and catching it at the supplier's end is far cheaper than discovering it at assembly.