■ Recommended Bored Hole Size Table for American Taper Pipe Threads (NPT) (Refer to ANSI/ASME B1.20.1-1983)

unit: mm

Minor DiameterBored Hole Size (reference)reference Tap
SizeL1L3L1+L3Pipe End (Basic Diameter Position)Position away from Pip (L1+L3)e End byMaximum Size ofBasic Diameter
Maximum ValueMinimum ValueToleranceMaximum ValueMinimum ValueToleranceStraight Bored HolePosition ℓg
NPT1/16- 274.0642.8226.8866.5106.3880.1226.0805.9580.1226.0512.00
NPT1/8- 274.1022.8226.9248.8578.7360.1228.4258.3030.1228.3912.05
NPT1/4- 185.7864.23410.02011.51411.3570.15710.88810.7300.15710.8517.45
NPT3/8- 186.0964.23410.33014.95314.7960.15714.30814.1500.15714.2717.65
NPT1/2- 148.1285.44313.57118.48518.3230.16317.63717.4750.16317.6022.85
NPT3/4- 148.6115.44314.05423.83123.6680.16322.95222.7900.16322.9122.95
NPT1 - 11.510.1606.62716.78729.86829.6960.17328.81928.6470.17328.7827.40
NPT11/4- 11.510.6686.62717.29538.62538.4520.17337.54437.3720.17337.5028.10
NPT11/2- 11.510.6686.62717.29544.69544.5220.17343.61443.4410.17343.5728.40
NPT2 - 11.511.0746.62717.70156.73256.5600.17355.62655.4540.17355.5828.00

Remarks 1. Pipe End is Basic Diameter Position (E1). NPT INTERNAL THREAD Remarks 2. Effective Thread Length is the length away from Pipe End by (L1+L3). L3 L1

Remarks 3. Concerning bored hole shape, considering load

on taps, taper bored hole is recommended.

Remarks 4. When applying taper bored hole, by referring to

NPT EXTERNAL THREAD values in shown columns ⑤, ⑥ ⑧, ⑨,

and prepare the taper hole by using pipe reamer (1/16 taper). By referring to values shown in column ⑪, select the drill diameter before

reaming by taking reamer's margin into account.

Remarks 5. When preparing straight bored hole, by referring

to values shown in column ⑪, select drill

diameter.