| Size | Major dia (in) | UNC TPI | UNF TPI | Tap drill UNC | Tap drill UNF | Clearance (close) |
|---|---|---|---|---|---|---|
| #0 | 0.0600" | — | 80 | — | 3/64" | #51 |
| #1 | 0.0730" | 64 | 72 | #53 | 53 | #47 |
| #2 | 0.0860" | 56 | 64 | #50 | #50 | #42 |
| #3 | 0.0990" | 48 | 56 | #47 | #45 | #37 |
| #4 | 0.1120" | 40 | 48 | #43 | #42 | #31 |
| #5 | 0.1250" | 40 | 44 | #38 | #37 | 1/8" |
| #6 | 0.1380" | 32 | 40 | #36 | #33 | #27 |
| #8 | 0.1640" | 32 | 36 | #29 | #29 | #17 |
| #10 | 0.1900" | 24 | 32 | #25 | #21 | #7 |
| #12 | 0.2160" | 24 | 28 | #16 | #14 | 15/64" |
| 1/4" | 0.2500" | 20 | 28 | #7 | #3 | 17/64" |
| 5/16" | 0.3125" | 18 | 24 | F | I | 21/64" |
| 3/8" | 0.3750" | 16 | 24 | 5/16" | Q | 25/64" |
| 7/16" | 0.4375" | 14 | 20 | U | 25/64" | 29/64" |
| 1/2" | 0.5000" | 13 | 20 | 27/64" | 29/64" | 33/64" |
| 9/16" | 0.5625" | 12 | 18 | 31/64" | 33/64" | 37/64" |
| 5/8" | 0.6250" | 11 | 18 | 17/32" | 37/64" | 41/64" |
| 3/4" | 0.7500" | 10 | 16 | 21/32" | 11/16" | 49/64" |
| 7/8" | 0.8750" | 9 | 14 | 49/64" | 13/16" | 57/64" |
| 1" | 1.0000" | 8 | 12 | 7/8" | 59/64" | 1-1/64" |
| 1-1/8" | 1.1250" | 7 | 12 | 63/64" | 1-3/64" | 1-5/32" |
| 1-1/4" | 1.2500" | 7 | 12 | 1-7/64" | 1-11/64" | 1-9/32" |
Thread identification is a skill every mechanic, machinist, and serious DIYer needs. The wrong thread size strips fasteners, fails under load, and wastes time. Whether you are matching a replacement bolt, ordering hardware for a project, or identifying a mystery fastener from a disassembled machine, understanding thread sizing makes the task straightforward.
Unified National Thread (UNC and UNF). The most common thread standard in the US is Unified National Thread. UNC (coarse thread) is standard for most general-purpose fasteners. UNF (fine thread) has more threads per inch, providing greater holding strength but less resistance to vibration loosening. Fine threads are common in automotive, aerospace, and precision applications.
Metric threads. Metric sizes are designated by nominal diameter and pitch in millimeters — M8 x 1.25 means an 8mm diameter bolt with 1.25mm between threads. Metric is the global standard and dominates in imported vehicles, machinery, and electronics. Never mix metric and imperial fasteners — they can cross-thread initially but will fail under load.
How to identify unknown threads. To identify an unknown thread: measure the major (outer) diameter with calipers, count threads per inch using a thread gauge or ruler, and compare to standard size charts. For metric threads, measure pitch directly with a metric thread gauge. A screw pitch gauge set costs under $20 and is one of the most useful tools in a workshop.
Thread fit classes. Thread fit describes how tightly a bolt and nut engage. Class 2A/2B is standard commercial fit — most fasteners you buy are class 2. Class 3A/3B is precision fit for aerospace and high-precision applications. Class 1A/1B is loose fit for easy assembly and corrosion tolerance. For most repair and construction work, class 2 is correct and interchangeable.
Specialty threads. Beyond UNC, UNF, and metric, you will encounter pipe threads (NPT), British Standard (BSP), spark plug threads, and dozens of industry-specific standards. Pipe threads are tapered — NPT seals by interference fit rather than a gasket. Always identify whether you are dealing with a standard fastener thread or a specialty thread before attempting replacement.