Thumb screws: shoulder or plain pattern, how they are measured, the threads they come in and the hole to tap.
A thumb screw has a flat, spade-shaped head you turn with your fingers — no wrench or driver. A shoulder (Type A) thumb screw has a collar under the head that seats on the part; a plain (Type B) one has none. Thumb screws have ordinary machine screw threads and go into a tapped hole or a nut.
| Type | Also called | Shoulder | Length measured |
|---|---|---|---|
| Type A | Shoulder pattern | Yes, just under the head | From under the shoulder to the tip |
| Type B | Plain pattern | No | From under the head to the tip |
| Size | Threads per inch | Series | Tap drill | Tap drill (in) |
|---|---|---|---|---|
| #6 | 32 | UNC | #36 | 0.1065 |
| #8 | 32 | UNC | #29 | 0.1360 |
| #10 | 24 | UNC | #25 | 0.1495 |
| #10 | 32 | UNF | #21 | 0.1590 |
| #12 | 24 | UNC | #16 | 0.1770 |
| ¼″ | 20 | UNC | #7 | 0.2010 |
| 5⁄16″ | 18 | UNC | F | 0.2570 |
| ⅜″ | 16 | UNC | 5/16 | 0.3125 |
| ½″ | 13 | UNC | 27/64 | 0.4219 |
Good to know
Length: measured from under the head, or under the shoulder, to the tip — the head is not counted (Machinery’s Handbook, Fastener Mart).
Threads: Unified inch, Class 2A; the point is normally a plain sheared end (Machinery’s Handbook, Fastener Mart).
Finger tight: thumb screws are for parts you adjust or take apart often, where finger pressure is enough to tighten them (Fastener Mart, H&B / CDE).
Head sizes are not charted: the handbook’s ASME B18.17 table and the maker tables give different head sizes (head width, 1/4″ shoulder pattern: .52–.55″ in the handbook vs .48″ (reference) in the H&B / CDE table; head width, 1/4″ plain pattern: .80–.83″ in the handbook vs .81″ (reference) in the H&B / CDE table), and makers note their heads vary.
Quick answers
- What is the difference between Type A and Type B thumb screws?
- Type A has a shoulder under the head (shoulder pattern) so the head seats flat on the part; Type B has no shoulder (plain pattern).
- How is a thumb screw measured?
- From under the head — or under the shoulder on a shoulder thumb screw — to the tip. The head is not counted.
- What drill for a 1/4-20 thumb screw hole?
- Tap the hole 1/4-20 UNC: the tap drill is #7 (0.2010″).
- What thread is a 5/16 thumb screw?
- 5/16″ thumb screws are 5/16-18 UNC; tap the hole with a F drill (0.2570″).
- How tight can a thumb screw go?
- Finger tight — they are for parts you adjust or take apart often, where finger pressure is enough.