How long is a 1/4 inch chucking reamer?

Flute 1-1/2″, overall 6″ (ASME B94.2, straight shank). A 1/2″ is 2″ flute, 8″ overall.

What size is a No. 5 taper pin reamer?

0.2409″ at the small end to 0.2994″ at the large end; taper pin reamers taper 1/4″ per foot.

What countersink angle do I use for flat head screws?

82° for inch flat head screws, 90° for metric flat heads.

What size is a #3 center drill?

A #3 combined drill and countersink has a 7/64″ drill and a 1/4″ body (plain type, 60°).

What is a 100 degree countersink for?

100° flat head screws and rivets, used in aircraft and sheet metal work.

Home / Resources / Reamer & Countersink Sizes

Reamer & Countersink Sizes

Chucking reamer lengths, taper pin reamer sizes, center drill (combined drill and countersink) sizes, and which screws each countersink angle fits.

A reamer finishes a drilled hole to an exact size with a smooth wall: drill a little under size, then ream. A chucking reamer is the machine reamer with a straight shank; a taper pin reamer cuts the tapered hole a taper pin drives into. A countersink cuts the cone a flat head screw sits flush in — its included angle must match the screw head. A center drill (combined drill and countersink) drills a small pilot and its 60° cone in one pass, the seat a lathe center runs in.

The letters match the table columns. Not to scale.

Chucking reamer lengths — ASME B94.2 (inches)
Reamer	Decimal	Flute F	Overall L
3/64″	0.0469	1/2	2-1/2
1/16″	0.0625	1/2	2-1/2
5/64″	0.0781	3/4	3
3/32″	0.0938	3/4	3
7/64″	0.1094	7/8	3-1/2
1/8″	0.1250	7/8	3-1/2
9/64″	0.1406	1	4
5/32″	0.1562	1	4
11/64″	0.1719	1-1/8	4-1/2
3/16″	0.1875	1-1/8	4-1/2
13/64″	0.2031	1-1/4	5
7/32″	0.2188	1-1/4	5
15/64″	0.2344	1-1/2	6
1/4″	0.2500	1-1/2	6
17/64″	0.2656	1-1/2	6
9/32″	0.2812	1-1/2	6
19/64″	0.2969	1-1/2	6
5/16″	0.3125	1-1/2	6
21/64″	0.3281	1-1/2	6
11/32″	0.3438	1-1/2	6
23/64″	0.3594	1-3/4	7
3/8″	0.3750	1-3/4	7
25/64″	0.3906	1-3/4	7
13/32″	0.4062	1-3/4	7
27/64″	0.4219	1-3/4	7
7/16″	0.4375	1-3/4	7
29/64″	0.4531	1-3/4	7
15/32″	0.4688	1-3/4	7
31/64″	0.4844	2	8
1/2″	0.5000	2	8
17/32″	0.5312	2	8
9/16″	0.5625	2	8
19/32″	0.5938	2	8
5/8″	0.6250	2-1/4	9
21/32″	0.6562	2-1/4	9
11/16″	0.6875	2-1/4	9
23/32″	0.7188	2-1/4	9
3/4″	0.7500	2-1/2	9-1/2
25/32″	0.7812	2-1/2	9-1/2
13/16″	0.8125	2-1/2	9-1/2
27/32″	0.8438	2-1/2	9-1/2
7/8″	0.8750	2-5/8	10
29/32″	0.9062	2-5/8	10
15/16″	0.9375	2-5/8	10
31/32″	0.9688	2-5/8	10
1″	1.0000	2-3/4	10-1/2
1-1/16″	1.0625	2-3/4	10-1/2
1-1/8″	1.1250	2-7/8	11
1-3/16″	1.1875	2-7/8	11
1-1/4″	1.2500	3	11-1/2
1-3/8″	1.3750	3-1/4	12
1-1/2″	1.5000	3-1/2	12-1/2

Straight shank, high-speed steel, nominal. Decimal from our conversion chart. Sizes our two sources both list; for any other size, give us a call.

Taper pin reamers — ASME B94.2 (inches)
Reamer	Small end	Large end	Flute F	Overall L
No. 8/0	—	—	—	—
No. 7/0	0.0497	0.0666	13/16	1-13/16
No. 6/0	0.0611	0.0806	15/16	1-15/16
No. 5/0	0.0719	0.0966	1-3/16	2-3/16
No. 4/0	0.0869	0.1142	1-5/16	2-5/16
No. 3/0	0.1029	0.1302	1-5/16	2-5/16
No. 2/0	0.1137	0.1462	1-9/16	2-9/16
No. 0	0.1287	0.1638	1-11/16	2-15/16
No. 1	0.1447	0.1798	1-11/16	2-15/16
No. 2	0.1605	0.2008	1-15/16	3-3/16
No. 3	0.1813	0.2294	2-5/16	3-11/16
No. 4	0.2071	0.2604	2-9/16	4-1/16
No. 5	0.2409	0.2994	2-13/16	4-5/16
No. 6	0.2773	0.3540	3-11/16	5-7/16
No. 7	0.3297	0.4220	4-7/16	6-5/16
No. 8	0.3971	0.5050	5-3/16	7-3/16
No. 9	0.4805	0.6066	6-1/16	8-5/16
No. 10	0.5799	0.7216	6-13/16	9-5/16

Reference diameters, inches. Taper pin reamers taper 1/4″ per foot, the same as the pins. A dash: only one of our sources lists it.

Center drills (combined drill and countersink), plain 60° (inches)
Size	Drill dia.	Drill length	Body dia.	Overall
#0	1/32″	—	1/8″	1-1/4″
#1	3/64″	3/64″	1/8″	1-1/4″
#2	5/64″	5/64″	3/16″	1-7/8″
#3	7/64″	7/64″	1/4″	2″
#4	1/8″	1/8″	5/16″	2-1/8″
#5	3/16″	3/16″	7/16″	2-3/4″
#6	7/32″	7/32″	1/2″	3″
#7	1/4″	1/4″	5/8″	3-1/4″
#8	5/16″	5/16″	3/4″	3-1/2″

Nominal, inches. A dash: our sources differ, or only one lists it — give us a call.

Countersink angles — what each one fits
Included angle	Fits
60°	Center holes — the 60° seat a lathe or grinder center runs in (center drills)
82°	Inch flat head screws: machine, socket, wood and tapping screws
90°	Metric flat head screws
100°	100° flat head screws and rivets — aircraft and sheet metal work
120°	—

The angle is the full angle across the cone. A dash: our sources don’t tie that angle to a screw head. See our machine screw and flat head socket charts for the heads.

Good to know

Match the countersink to the screw, not the other way round. An 82° inch flat head in a 90° metric countersink (or the reverse) touches on one edge and won’t sit flush or bear evenly.

A reamer only takes a skim. Drill a little under the reamer size, and run the reamer slower than you would the drill.

Quick answers

How long is a 1/4 inch chucking reamer?: Flute 1-1/2″, overall 6″ (ASME B94.2, straight shank). A 1/2″ is 2″ flute, 8″ overall.
What size is a No. 5 taper pin reamer?: 0.2409″ at the small end to 0.2994″ at the large end; taper pin reamers taper 1/4″ per foot.
What countersink angle do I use for flat head screws?: 82° for inch flat head screws, 90° for metric flat heads.
What size is a #3 center drill?: A #3 combined drill and countersink has a 7/64″ drill and a 1/4″ body (plain type, 60°).
What is a 100 degree countersink for?: 100° flat head screws and rivets, used in aircraft and sheet metal work.

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