Threads

Standard Screw Threads and Tap Drill Sizes

American Threads

U.S. Machine Screw Standard

No.	Thread	Tap Drill	Clear Drill
0	80	56	52
1	72	53	48
2	56	49	43
3	48	47	39
4	40	44	32
5	40	39	30
6	32	36	28
8	36	29	19
10	32	21	11
12	24	16	2

U.S. National Fine Standard

Size	Thread	Tap Drill
1/4	28	3
5/16	24	I
3/8	24	Q
7/16	20	25/64
1/2	20	29/64

U.S. National Coarse Standard

Size	Thread	Tap Drill
1/4	20	7
5/16	18	F
3/8	16	5/16
7/16	14	U
1/2	13	27/64
9/16	12	31/64
5/8	11	17/32

British Threads

British Standard

Size	Thread	Tap Drill
5/32	40	#30
3/16	40	5/32
7/32	40	3/16
1/4	40	7/32
5/16	32	J
3/8	32	Q
3/8	40	11/32

British Standard

Size	Diameter mm	Diameter inch	Tap Drill	Clear Drill
0	6.0	0.2362	10	C
1	5.3	0.2087	17	2
2	4.7	0.1851	24	12
3	4.1	0.1614	29	19
4	3.6	0.1417	32	27
5	3.2	0.1260	37	30
6	2.8	0.1102	43	34
7	2.5	0.0984	46	38
8	2.2	0.0866	50	43
9	1.9	0.0748	53	47
10	1.7	0.0670	55	50
11	1.5	0.0591	56	52
12	1.3	0.0512	60	54
14	1.0	0.0394	70	59

Machine Screw Diameters

By H. J. Coventry

The Miniature Locomotive, July-August 1954

Machine screws are numbered as distinct from Cap Screws, Bolts, etc, and range usually from #0 to #12. A convenient method of finding the exact diameter if a table is not handy is as follows:

Multiply the number of 13 and add sixty thousandths. The answer will be thousandths.

Examples:

• #0 -- 0 multiplied by 13 equals 0, plus 0.060 equals 0.060
• #3 -- 3 multiplied by 13 equals 39, plus 0.060 equals 0.099
• #5 -- 5 multiplied by 13 equals 65, plus 0.060 equals 0.125
• #10 -- 10 multiplied by 13 equals 0.130, plus 0.060 equals 0.190

Pipe Threads

Ciszewski posted on PracticalMachinist.com:

You can tap NPT without taper reaming. Morse Cutting Tools recommends:

• 1/4 NPT - drill with a 7/16 bit to a minimum depth of 0.78125 & tap to a depth of 0.72525
• 3/8 NPT - drill with a 37/64 bit to a minimum depth of 0.8125 & tap to a depth of 0.7565
• 1/2 NPT - drill with a 45/64 bit to a minimum depth of 1.03125 & tap to a depth of 0.96025

Following these recommendations will only give you a full thread for the first 2 to 3 threads.

External Links

• "Making Scale Hex Head Bolts", Home Metal Shop Club
• "National Pipe Thread Taper", The Engineering Toolbox
• "Pipe Threads (NPT/API)", UnifiedAlloys.com