A good tap drill is 90% (± 2 pp) of major diameter for fine threads. For metric V threads, the concept of major minus pitch (i.e., the major or widest diameter of the intended screw in millimeters minus the pitch of the threads of that screw in millimeters per thread) yields a good tap drill diameter.
What size drill bit is needed for a 3/8 NPT tap?
Tap Size Drill Size Tap and Drill Set
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3/8" NPT Tap 37/64" Drill Bit 3/8" NPT Tap and Drill Set
1/2" NPT Tap 23/32" Drill Bit 1/2" NPT Tap and Drill Set
3/4" NPT Tap 59/64" Drill Bit 3/4" NPT Tap and Drill Set
1" NPT Tap 1-5/32" Drill Bit 1" NPT Tap and Drill Set
What is the diameter of 3/8 NPT?
What size hole do I need to drill for a 3/8 tap?
Tap Fractional Drill Bit Letter Drill Bit
-------- -------------------- ----------------
5/16"-18 17/64" F
5/16"-24 - I
3/8"-16 5/16" -
3/8"-24 21/64" Q
What size drill bit to use with a 3/8-16 tap?
To Tap This Size Screw Or Bolt: Use This Drill Bit: Decimal Inches
------------------------------- ------------------- --------------
8mm-1.00mm 7.1mm .2795
5/16-32 NEF 9/32" .2812
9mm-1.25mm 7.9mm .3110
3/8-16 NC 5/16" .3125
How do you drill a hole in a NPT?
https://www.youtube.com/watch?v=Kq7Vhn5EuqI
What size hole do you drill for 1/2 NPT?
Tap Size Drill Size Tap and Drill Set
------------ ---------------- --------------------------
1/4" NPT Tap 7/16" Drill Bit 1/4" NPT Tap and Drill Set
3/8" NPT Tap 37/64" Drill Bit 3/8" NPT Tap and Drill Set
1/2" NPT Tap 23/32" Drill Bit 1/2" NPT Tap and Drill Set
3/4" NPT Tap 59/64" Drill Bit 3/4" NPT Tap and Drill Set
What size hole do you drill for a 1/4 NPT tap?
If you intend to use the drill-only method to prepare the starter hole for a 1/4-18 NPT tap, use a 7/16 inch drill bit. If you choose to use the drill-and-reamer methods, use a 27/64 inch bit, followed by a 1/4-18 tapered pipe reamer. In either case, you should resist the temptation to use old drill bits.