⚡ Key Takeaways
- The default pairing: internal thread 6H with external thread 6g covers about 90% of general fastening.
- Reading a tolerance class: the number is the accuracy grade (3 tightest, 8 loosest) and the letter is the position (H internal, g external).
- Clearance fits: H/g and H/h; transition: H/k; interference: H/N.
- Standard: GB/T 197-2003, aligned with ISO 965-1.
- Important: bolts default to 6g and nuts to 6H — do not upgrade the class casually.
📑 Contents
1. How to read a tolerance class
2. Common fits and where they are used
3. Choosing the accuracy grade
4. Selection FAQ
1. How to read a tolerance class
2. Common fits and where they are used
3. Choosing the accuracy grade
4. Selection FAQ
1. How to read a tolerance class
Example: M8×1.25-6H/6g
- 6: tolerance grade — the smaller the number, the tighter the tolerance;
- H: fundamental deviation position of the internal thread (H sits on the basic line);
- g: fundamental deviation position of the external thread (g sits below the basic line, giving clearance).
Internal threads use capitals (G/H), external threads use lower case (e/f/g/h).
| Position | Deviation | Character |
|---|---|---|
| External g | Negative (below the basic line) | Clearance, accommodates plating |
| External h | Zero | Tightest, precision fit |
| External e/f | Large negative | Large clearance, hot or dirty conditions |
| Internal H | Zero | The baseline |
| Internal G | Positive | Still assembles after plating |
2. Common fits and where they are used
| Fit | Nature | Application |
|---|---|---|
| 6H/6g | Clearance | General bolts and nuts, by far the most common |
| 6H/6h | Minimum clearance | Precision location |
| 6G/6e | Large clearance | Plated parts, high temperature |
| 6H/5g6g | Precision | Machine tools, instruments |
| 7H/6n | Transition / interference | Threaded inserts, permanent joints |
3. Choosing the accuracy grade
- Grades 4/5: aerospace, precision instruments;
- Grade 6: general machinery, automotive, standard fasteners;
- Grades 7/8: construction, steel structures, rough work.
Important: bolts ship at 6g and nuts at 6H by default. Parts to be plated should be made with a thicker allowance (6f/6e) so that after plating they come back to 6g — otherwise they will not screw in.
4. Selection FAQ
Q: Does the 8.8 marking mean accuracy grade 8? No — 8.8 is the property class, not a thread tolerance grade.
Q: Why won't a zinc-plated bolt screw in? Plating shrinks the thread; specify an undersize-before-plating 6g or an internal 6G thread.
Q: Is grade 3 better than grade 6? It is tighter but costs two to three times as much, and is rarely necessary.
🛠️ Engineering support: Toprecision supplies the full tolerance range to GB/T 197 — 4H5H, 6H, 6g and 6e.
👉 Engineering services ｜ 👉 Request a technical quote
👉 Engineering services ｜ 👉 Request a technical quote