■ Recommended Bored Hole Size Table for American Dryseal Taper Pipe Threads (NPTF) (Refer to ANSI B1.20.3-1976, re-confirmed in 1982) unit: mm Bored Hole reference Minor Diameter Size (reference) Tap L3

Size L1 L1+L3+1P Pipe End Position of

(3P)MaximumBasic
(Basic Diameter Position)( L1+L3+1P )Size ofDiameter
MaximumMinimumMaximumMinimumStraightPosition
ToleranceToleranceBored Holeℓg
ValueValueValueValue
NPTF1/16- 274.0642.8227.8276.5056.4140.0916.0155.9230.0915.9912.00
NPTF1/8 - 274.1022.8227.8658.8528.7610.0918.3628.2700.0918.3412.05
NPTF1/4 - 185.7864.23411.43111.48411.3970.08610.77010.6840.08610.7517.45
NPTF3/8 - 186.0964.23411.74114.92314.8360.08614.18914.1030.08614.1717.65
NPTF1/ 2- 148.1285.44315.38618.41918.3330.08617.45917.3730.08617.4422.85
NPTF3/4 - 148.6115.44315.86823.76423.6780.08622.77322.6870.08622.7522.95
NPTF1 - 11.510.1606.62718.99629.81229.7260.08628.62528.5380.08628.6027.40
NPTF11/4 - 11.510.6686.62719.50438.56938.4830.08637.35037.2630.08637.3328.10
NPTF11/ 2- 11.510.6686.62719.50444.63944.5520.08643.42043.3340.08643.4028.40
NPTF2 - 11.511.0746.62719.91056.67756.5900.08655.43255.3450.08655.4128.00

HAND ENGAGEMENT(L1)+L3 + ONE TURN FULL THREAD Remarks 1. Pipe End is Basic Diameter Position (E1).

L 3 3p L1 Remarks 2. Effective Thread Length is the length away from Pipe End

by (L1+L3+1P). NPTF INTERNAL THREAD Remarks 3. Concerning bored hole shape, considering load on taps, taper bored hole is recommended. NPTF EXTERNAL THREAD Remarks 4. When applying taper bored hole, by referring to values

shown in columns ⑤, ⑥ and ⑧, ⑨, prepare the taper

hole by using pipe reamer (1/16 taper). By referring to

values in shown 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.

■Percentage of Thread Engagement & Relation between Percentage of Thread Height and Area Removed at A Thread Height

Percentage of Thread Engagement

internal Basic Major Dia. − Hole Size Before Tapping threads

100

2× (Basic Thread Overlap)

Basic Thread Overlap

Metric & Unified Thread 0.5413P

Whitworth Thread 0.5664P

Pipe Thread (Rc, Rp, G, PT, PS, PF) 0.6403P

P=Pitch

As shown above, when the thread height increases, the amount of material to be removed increases rapidly, so it is an advantage to tap users to keep the hole size (thread minor diameter) as large as possible.