1/2"-13 UNC Thread
Direct Answer
1/2"-13 UNC is a heavy fractional Unified Coarse bolt thread with a major diameter of 0.5000 inches (12.700 mm) and 13 threads per inch (TPI). Requiring a 27/64 inch (0.4219 inch) tap drill, it is the standard structural fastener for automotive engine cradles, trailer hitches, and building timber connections.
Source: ANSI/ASME B1.1 / SAE J429
Specs
| Nominal Major Diameter | 0.5000 in (12.700 mm) |
| Threads Per Inch (TPI) | 13 TPI (Coarse pitch: 1.954 mm) |
| Fine Pitch Equivalent | 1/2"-20 UNF (1.270 mm pitch — standard automotive wheel lugs) |
| Tap Drill Size | 27/64 in drill bit (0.4219 in / 10.72 mm) |
| Minor Diameter (Internal) | 0.4167 in (10.584 mm) |
| Tensile Stress Area | 0.1419 in² (91.5 mm²) |
| Common Drive Size | 3/4" Hex (19.05 mm) / 3/8" Allen |
Heavy Structural Bolting (Grade 5 & Grade 8)
Under SAE J429, a Grade 8 1/2"-13 coarse bolt (indicated by 6 radial lines on the head) has a minimum tensile strength of 150,000 psi, supporting proof loads up to 17,000 lbf (~75.6 kN) per fastener, making it standard in commercial truck chassis, trailer hitches, and industrial hydraulic presses.
Coarse 1/2"-13 vs Fine 1/2"-20 Wheel Lugs
In American automotive engineering, while general structural bolts use 1/2"-13 coarse threads, heavy pickup truck wheel studs (Ford F-150, Jeep Wrangler legacy) use 1/2"-20 UNF fine threads to prevent nut backing and maximize clamping preload.