NPT: the American tapered pipe thread
NPT does not seal on a face, it seals on the thread itself: because it is tapered, the flanks wedge together as you tighten. That makes it the first choice for pipe connections in hydraulics, pneumatics and gas engineering built to American standards. VÖLKEL makes 48 versions from NPT 1/16 x 27 to NPT 2.1/2 x 8, to DIN 374 form C in HSSE, as finishing taps to ISO 529 form C in HSS-G, and to JIS B-4430 in form B and form C.
Tapered cutting means: to depth, not to a stop
Torque rises with every turn, because the taper engages more material as it goes. So the tap is not driven to a stop but to the standard engagement depth. The hole is countersunk to a taper beforehand, or drilled to the tap drill size specified for NPT.
Sealant belongs to the job
NPT is self-sealing, but in practice a sealant is almost always used, usually PTFE tape or hemp. The NPTF version (Dryseal) is toleranced to seal without a sealant, and NPS is the parallel variant for mechanical joints without pressure.
Do not confuse it with G or Rc
NPT has a flank angle of 60°. British Rc (BSPT) is tapered as well but uses 55°, and G (BSP) is parallel with 55°. No combination of these will screw together. The differences are set out in the article pipe thread types explained, the direct comparison in NPT vs BSPT.
Left-hand NPT tools are listed under NPT left hand, short versions among the short machine taps.
All sizes at a glance
48 articles in 3 series. The article number links straight to the tool.
Machine Tap JIS B-4430
| Threadnominal size | Tapping drillØ in mm | Form B HSSE | Form C HSS-G | Form C 35° Spiral Flute HSSE | Form C 35° Spiral Flute HSS-G |
|---|---|---|---|---|---|
| NPT 1/16 x 27 | 6.3 | 98502 | 98402 | – | – |
| NPT 1/8 x 27 | 8.5 | 98506 | 98406 | – | 98706 |
| NPT 1/4 x 18 | 11.2 | 98514 | 98414 | – | 98714 |
| NPT 3/8 x 18 | 14.5 | 98518 | 98418 | – | 98718 |
| NPT 1/2 x 14 | 18 | 98522 | 98422 | – | 98722 |
| NPT 3/4 x 14 | 23 | 98530 | 98430 | – | 98730 |
| NPT 1" x 11.5 | 29 | 98538 | 98438 | – | 98738 |
| NPT 1.1/4 x 11.5 | 38 | 98546 | 98446 | – | – |
| NPT 1.1/2 x 11.5 | 44 | 98554 | 98454 | – | – |
| NPT 2" x 11.5 | 56 | 98570 | 98470 | – | – |
| NPTF 1/16 x 27 | – | – | – | 98702 | – |
| NPTF 1.1/4 x 11.5 | – | – | – | 98746 | – |
| NPTF 1.1/2 x 11.5 | – | – | – | 98754 | – |
| NPTF 2" x 11.5 | – | – | – | 98770 | – |
Machine Tap Bottoming ISO 529 Form C HSS-G
| Threadnominal size | Tapping drillØ in mm | Article no. |
|---|---|---|
| NPT 1/16 x 27 | 6.3 | 88410 |
| NPT 1/8 x 27 | 8.5 | 88412 |
| NPT 1/4 x 18 | 11.2 | 88414 |
| NPT 3/8 x 18 | 14.5 | 88416 |
| NPT 1/2 x 14 | 18 | 88418 |
| NPT 3/4 x 14 | 23 | 88422 |
| NPT 1" x 11.5 | 29 | 88426 |
| NPT 1.1/4 x 11.5 | 38 | 88434 |
| NPT 1.1/2 x 11.5 | 44 | 88442 |
| NPT 2" x 11.5 | 56 | 88454 |
| NPT 2.1/2 x 8 | 67 | 88464 |
Machine Tap DIN 374 Form C HSSE
| Threadnominal size | Tapping drillØ in mm | Article no. |
|---|---|---|
| NPT 1/16 x 27 | 6.3 | 73432 |
| NPT 1/8 x 27 | 8.5 | 73434 |
| NPT 1/4 x 18 | 11.2 | 73436 |
| NPT 3/8 x 18 | 14.5 | 73438 |
| NPT 1/2 x 14 | 18 | 73440 |
| NPT 3/4 x 14 | 23 | 73442 |
| NPT 1" x 11.5 | 29 | 73444 |
Tapping drill: drill diameter recommended by VÖLKEL (Technical Information “Tapping drill sizes”).
