Types of centres
The tap manufacturer, according to the diameter and the application, defines the types of centres. Types of centre and chamfer are generally combined together as in the following table, but for specific applications exceptions are possible.
Male centre
| Thread type | Form A | Form B | Form C | Form D | Form E |
|---|---|---|---|---|---|
| M | M2 ≤ Ø ≤ M8 | M2 ≤ Ø ≤ M8 | M2 ≤ Ø ≤ M8 | M2 ≤ Ø ≤ M8 | – |
| MF | M2 ≤ Ø ≤ M6 | M4 ≤ Ø ≤ M6 | M2 ≤ Ø ≤ M6 | M5 ≤ Ø ≤ M6 | – |
| UNC | Nr.2-56 ≤ Ø ≤ 1/4″-18 | Nr.2-56 ≤ Ø ≤ 1/4″-18 | Nr.2-56 ≤ Ø ≤ 1/4″-18 | Nr.2-56 ≤ Ø ≤ 1/4″-18 | – |
| UNF | Nr.2-64 ≤ Ø ≤ 1/4″-28 | Nr.2-64 ≤ Ø ≤ 1/4″-28 | Nr.2-64 ≤ Ø ≤ 1/4″-28 | – | – |
| G | – | – | – | – | – |
Half centre
| Thread type | Form A | Form B | Form C | Form D | Form E |
|---|---|---|---|---|---|
| M | M8 < Ø ≤ M10 | M8 < Ø ≤ M10 | M8 < Ø ≤ M10 | M8 < Ø ≤ M10 | – |
| MF | M6 < Ø ≤ M10 | M6 < Ø ≤ M10 | M6 < Ø ≤ M10 | M6 < Ø ≤ M10 | – |
| UNC | 5/16″-18 ≤ Ø ≤ 3/8″-16 | 5/16″-18 ≤ Ø ≤ 3/8″-16 | 5/16″-18 ≤ Ø ≤ 3/8″-16 | – | – |
| UNF | 5/16″-24 ≤ Ø ≤ 3/8″-24 | 5/16″-24 ≤ Ø ≤ 3/8″-24 | 5/16″-24 ≤ Ø ≤ 3/8″-24 | – | – |
| G | Ø = 1/8″-28 | Ø = 1/8″-28 | Ø = 1/8″-28 | – | – |
Female centre
| Thread type | Form A | Form B | Form C | Form D | Form E |
|---|---|---|---|---|---|
| M | Ø > M10 | Ø > M10 | Ø > M10 | Ø > M10 | All sizes |
| MF | Ø > M10 | Ø > M10 | Ø > M10 | Ø > M10 | All sizes |
| UNC | Ø ≥ 7/16″-14 | Ø ≥ 7/16″-14 | Ø ≥ 7/16″-14 | Ø ≥ 5/16″-18 | – |
| UNF | Ø ≥ 7/16″-20 | Ø ≥ 7/16″-20 | Ø ≥ 7/16″-20 | – | – |
| G | Ø ≥ 1/4″-19 | Ø ≥ 1/4″-19 | Ø ≥ 1/4″-19 | – | – |
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