Wich drill diameter when thread tapping

When tapping threads, it's important to pre-drill with the correct diameter.

The tables on this page list these bore sizes for use with both cutting taps and forming taps. What the difference is between roll tap and cutting tap, you read below.

Roll tap

A roll tap or forming tap usually has a longer life and works faster. It produces no chips, a stronger thread and is also suitable for blind holes. A disadvantage is the higher purchase price. In addition, much more machine power is required to let the forming tap do its job (approx. 50% higher compared to cutting tapping). Special attention is needed for the right lubricant because of friction and with a lot of heat development as a result.

Cutting tap

With a cutting tap - usually called a thread tap - the thread is cut by machining. A tap is usually made of HSS (High Speed Steel) and has sharply ground cutting teeth. A lubricant is also used for tapping. The supply of cutting oil and the removal of the chips takes place through the recessed grooves of the tap. A set of taps usually consists of: 3 taps for metric threads, 2 taps for gas threads, 1 tap for machine taps. There are taps for the generally used right-hand thread, but also for left-hand thread.

In other words
As the name suggests, cutting taps produce threads by progressively cutting away the space between threads into which the fastener´s external threads will fit. Forming taps, by contrast, remove no material. Instead, they move or displace material inside the hole to build up the threads and dig into the grooves.

Lubricants

When tapping, a lubricant suitable for the material is used, which serves to lubricate and cool. Today there are excellent universal oils available, for example drilling oil, cutting oil, tap oil, even 3 in 1 in aerosol.

Thread pitch refers to the distance between threads and is expressed in millimeters. This measurement is taken along the length of the fastener. For example, if the distance between two threads is 1.75mm, this means the thread pitch is 1.75

Metric

Metric threads are the most commonly used threads today. Metric threads are used in many common applications such as fasteners in construction, machinery and construction.

The "M" designation for metric screws indicates the nominal outer diameter of the screw thread, in millimetres.

Metric	Drill diameters for tapping threads

M1-M64	Thread pitch (P)	cutting tap	roll tap
ø mm	mm	ø mm	ø mm
1	0.25	0.75	0.90
1.6	0.35	1.25	1.40
1.8	0.35	1.45	1.65
2	0.40	1.60	1.80
2.2	0.45	1.75	2.00
2.5	0.45	2.05	2.30
3	0.50	2.50	2.80
3.5	0.60	2.90	3.20
4	0.70	3.30	3.70
4.5	0.75	3.80	4.15
5	0.80	4.20	4.60
6	1.00	5.00	5.50
7	1.00	6.00	6.50
8	1.25	6.80	7.40
9	1.25	7.80	8.40
10	1.50	8.50	9.30
11	1.50	9.50	10.30
12	1.75	10.30	11.20
14	2.00	12.00	13.00
16	2.00	14.00	15.00
18	2.50	15.50	16.80
20	2.50	17.50	18.90
22	2.50	19.50	20.80
24	3.00	21.00	22.70
27	3.00	24.00
30	3.50	26.50
33	3.50	29.50
36	4.00	32.00
39	4.00	35.00
42	4.50	37.50
45	4.50	40.50
48	5.00	43.00
52	5.00	47.00
56	5.50	50.50
64	6.00	58.00

Metric Fine

With Metric Fine threads, the courses are closer together. This provides a larger contact surface, thus greater friction. A fine thread with a small pitch therefore has a smaller chance of coming loose than a coarse thread with a large pitch. In fine-mechanical technology, fixings with Metric Fine threads are regularly used.

Metric Fine	Drill diameters for tapping threads

MF3-MF40	Thread pitch (P)	cutting tap	roll tap
ø mm	mm	ø mm	ø mm
3	0.35	2.65	2.85
3.5	0.35	3.20	3.35
4	0.50	3.50	3.80
5	0.50	4.50	4.80
5.5	0.50	5.00	5.25
6	0.75	5.30	5.70
7	0.75	6.30	6.65
8	0.75	7.30	7.70
8	1.00	7.00	7.50
9	1.00	8.00	8.50
10	1.00	9.00	9.50
10	1.25	8.80	9.40
11	1.00	10.00	10.50
12	1.00	11.00	11.50
12	1.25	10.80	11.40
12	1.50	10.50	11.30
14	1.00	13.00	13.50
14	1.25	12.80	13.40
14	1.50	12.50	13.30
15	1.00	14.00	14.50
15	1.50	13.50	14.30
16	1.00	15.00	15.50
16	1.50	14.50	15.30
18	1.00	17.00	17.50
18	1.50	16.50	17.30
18	2.00	16.00	17.00
20	1.00	19.00	19.50
20	1.50	18.50	19.30
20	2.00	18.00	19.00
22	1.00	21.00	21.50
22	1.50	20.50	21.30
22	2.00	20.00	21.00
24	1.00	23.00	23.50
24	1.50	22.50	23.30
24	2.00	22.00	23.00
25	1.00	24.00
25	1.50	23.50
25	2.00	23.00
27	1.50	25.50
27	2.00	25.00
28	1.50	26.50
28	2.00	26.00
30	1.50	28.50
30	2.00	28.00
32	1.50	30.50
32	2.00	30.00
33	2.00	31.00
35	1.50	33.50
36	1.50	34.50
36	2.00	34.00
36	3.00	33.00
38	1.50	36.50
39	3.00	36.00
40	1.50	38.50
40	2.00	38.00
40	3.00	37.00

British Standard Whitworth

BSW threads are standardized from 1/8”-40BSW up to and including 6”-2.5BSW. Because some BSW diameter-pitch combinations are the same as those with UNC threads, they can in principle be interchanged. However, due to the apex angle configuration and some other minor differences, this is not recommended.

Whitworth	Drill diameters for tapping threads

BSW 3/32 - 2	Thread pitch (P)	cutting tap	roll tap
ø ”	TPI	ø mm	ø mm
3/32	48	1.85	2.10
1/8	40	2.55	2.90
5/32	32	3.20	3.60
3/16	24	3.70	4.30
7/32	24	4.50
1/4	20	5.10	5.80
5/16	18	6.50	7.30
3/8	16	7.90	8.80
7/16	14	9.20	10.20
1/2	12	10.50	11.60
9/16	12	12.00	13.20
5/8	11	13.50	14.70
3/4	10	16.50	17.70
7/8	9	19.25	20.75
1	8	22.00	23.75
1.1/8	7	24.75
1.1/4	7	28.00
1.3/8	6	30.50
1.1/2	6	33.50
1.5/8	5	35.50
1.3/4	5	39.00
1.7/8	4.1/2	41.50
2	4.1/2	41.50

British Standard Pipe

British Standard Pipe also called gas thread (G) and pipe thread exists in two different versions.

The straight or parallel thread (BSP), which is mainly used constructively.
The conical or tapered thread (BSPT), where one or both parts have a conical thread. This thread is suitable for sealing connections (usually with the addition of a sealant) for gas or liquids.

BSP	Drill diameters for tapping threads

BSP 1/8 - 3	Thread pitch (P)	cutting tap	roll tap
ø ”	TPI	ø mm	ø mm
1/8	28	8.80	9.30
1/4	19	11.80	12.50
3/8	19	15.25	16.00
1/2	14	19.00	20.00
5/8	14	21.00	22.00
3/4	14	24.50	25.50
7/8	14	28.25	29.30
1	11	30.75	32.00
1.1/4	11	39.50
1.1/2	11	45.00
1.3/4	11	51.00
2	11	57.00
2.1/4	11	63.00
2.1/2	11	72.50
2.3/4	11	79.00
3	11	85.50

Unified National thread

In the American system, each size comes in two different thread sizes. Depending on the application, you can choose from a fine or coarse thread. This way you always have the choice of a bolt that is optimally geared to its application. UNC (Unified National Coarse) is the designation for the coarse thread, where UNF (Unified National Fine) is the designation for the fine thread.

	Drill diameters for tapping threads

UNC No.2 - No.12	Thread pitch (P)	cutting tap	roll tap
ø mm	mm	ø mm	ø mm
No.2	56	1.85	2.00
No.3	48	2.10	2.30
No.4	40	2.35	2.60
No.5	40	2.65	2.90
No.6	32	2.85	3.20
No.8	32	3.50	3.80
No.10	24	3.90	4.40
No.12	24	4.50	5.00
UNC 1/4 - 2	Thread pitch (P)	cutting tap	roll tap
ø ”	TPI	ø mm	ø mm
1/4	20	5.10	5.80
5/16	18	6.60	7.30
3/8	16	8.00	8.80
7/16	14	9.40	10.30
1/2	13	10.80	11.90
9/16	12	12.20
5/8	11	13.50	14.80
3/4	10	16.50	17.90
7/8	9	19.50
1	8	22.25	24.00
1.1/8	7	25.00
1.1/4	7	28.00
1.3/8	6	30.75
1.1/2	6	34.00
1.3/4	5	39.50
2	4.1/2	45.00

	Drill diameters for tapping threads

UNF No.2 - No.12	Thread pitch (P)	cutting tap	roll tap
ø mm	mm	ø mm	ø mm
No.2	64	1.90	2.00
No.3	56	2.15	2.30
No.4	48	2.40	2.60
No.5	44	2.70	2.90
No.6	40	2.95	3.20
No.8	36	3.50	3.90
No.10	32	4.10	4.50
No.12	28	4.70	5.10
UNF 1/4 - 1.1/2	Thread pitch (P)	cutting tap	roll tap
ø ”	TPI	ø mm	ø mm
1/4	28	5.50	6.00
5/16	24	6.90	7.50
3/8	24	8.50	9.10
7/16	20	9.90	10.60
1/2	20	11.50	12.10
9/16	18	12.90
5/8	18	14.50	15.30
3/4	16	17.50	18.30
7/8	14	20.40
1	12	23.25
1.1/8	12	26.50
1.1/4	12	29.50
1.3/8	12	32.75
1.1/2	12	36.00

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