■ 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

SizeL1L3 (3P)L1+L3+1P(BasicPipe End Diameter PoMinor Di sition)ameter (Position of L1+L3+1P)Bored Hole Size (reference) Maximum Size ofreference Tap Basic Diameter
Maximum ValueMinimum ValueToleranceMaximum ValueMinimum ValueToleranceStraight Bored HolePosition ℓg
①②③④⑤⑥⑦⑧⑨⑩⑪⑫
1/16- 27NPTF4.0642.8227.8276.5056.4140.0916.0155.9230.0915.9912.00
1/8- 27NPTF4.1022.8227.8658.8528.7610.0918.3628.2700.0918.3412.05
1/4- 18NPTF5.7864.23411.43111.48411.3970.08610.77010.6840.08610.7517.45
3/8- 18NPTF6.0964.23411.74114.92314.8360.08614.18914.1030.08614.1717.65
1/2- 14NPTF8.1285.44315.38618.41918.3330.08617.45917.3730.08617.4422.85
3/4- 14NPTF8.6115.44315.86823.76423.6780.08622.77322.6870.08622.7522.95
1 - 111/2NPTF10.1606.62718.99629.81229.7260.08628.62528.5380.08628.6027.40
11/4- 111/2NPTF10.6686.62719.50438.56938.4830.08637.35037.2630.08637.3328.10
11/2- 111/2NPTF10.6686.62719.50444.63944.5520.08643.42043.3340.08643.4028.40
2 - 111/2NPTF11.0746.62719.91056.67756.5900.08655.43255.3450.08655.4128.00

Remarks 1. Pipe End is Basic Diameter Position (E1).

HAND ENGAGEMENT(L1)+L3Remarks 2.Effective Thread Length is the length
+ ONE TURN FULL THREADaway from Pipe End by (L1+L3+1P).
Remarks 3.Concerning bored hole shape, considering load
L L 3= 3p
on taps, taper bored hole is recommended.
Remarks 4.When applying taper bored hole, by referring
NPTF INTERNAL THREADto values shown in columns ⑤ , ⑥ and ⑧ ,
⑨ , prepare the taper hole by using pipe
NPTF EXTERNAL THREADreamer (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

Technical Inf 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.