Drill Bits / F Diameter
Drill Bit Size for a 5/16-18 Tap is an F Drill Bit
5/16-18 Tap Drill Size
The correct tap drill size for a 5/16-18 thread is a Letter F drill bit, which has a diameter of 0.2570 inches (6.53 mm). This drill size produces the proper hole diameter before tapping a 5/16-18 UNC thread
Machinists frequently search for the drill bit size for a 5/16-18 tap, the 5 16 18 drill tap size, or the tap drill for a 5/16-18 thread. In all of these cases, the recommended drill is a Letter F drill. Using the correct tap drill ensures proper thread engagement while preventing excessive cutting load on the tap.
Why the Letter F Drill Is Used for a 5/16-18 Tap
Thread taps do not cut the full diameter of a thread. Instead, the drill size must leave enough material for the tap to form the threads while still allowing the tap to cut freely. The Letter F drill size (0.257") creates the correct hole diameter to achieve approximately 77% thread engagement, which is considered optimal for most machining operations.
Selecting the proper drill bit size for a 5/16-18 tap reduces:
- Tap breakage
- Excessive cutting torque
- Poor thread quality
- Premature tool wear
For production machining, maintenance work, or CNC operations, using the correct 5/16-18 tap drill size ensures reliable and consistent threading performance.
UNC vs NC Thread Designation
The thread size 5/16-18 is part of the Unified National Coarse (UNC) thread series. Older specifications often refer to this as NC (National Coarse).
Both designations refer to the same thread form:
- Thread size: 5/16 inch
- Threads per inch: 18 TPI
- Thread series: UNC / NC
- Recommended tap drill: Letter F
Because of this, search queries such as 5 16 18 UNC tap drill size, 5 16 18 NC tap drill size, and tap drill size for 5 16 18 UNC all refer to the same drill size recommendation.
Metric Equivalent Drill Size
The Letter F drill bit has a diameter of 0.2570 inches, which converts to approximately 6.53 mm.
In metric drill systems, the closest equivalent drill sizes are typically:
- 6.5 mm drill bit
- 6.6 mm drill bit
While metric drills can be used when letter drills are unavailable, the Letter F drill remains the recommended tap drill for a 5/16-18 UNC thread when working in inch-based tooling systems.
Applications for 5/16-18 Tapped Holes
The 5/16-18 UNC thread is widely used in industrial equipment, machinery, and structural assemblies. Proper hole preparation using the correct 5/16-18 drill size for tapping ensures strong and accurate threaded connections.
Common applications include:
- Machine tool components
- Industrial equipment frames
- Maintenance and repair operations
- Automotive and mechanical assemblies
- Fixture and tooling construction
Using the correct drill bit for a 5/16-18 tap ensures that threads are produced accurately and that fasteners achieve full holding strength.
Frequently Asked Questions
What size drill bit for a 5/16-18 tap?
The recommended drill bit size for a 5/16-18 tap is a Letter F drill (0.257").
What is the tap drill size for 5/16-18 UNC?
The correct 5 16 18 UNC tap drill size is a Letter F drill bit.
What is the metric tap drill size for 5/16-18?
The Letter F drill used for a 5/16-18 tap drill equals 6.53 mm. A 6.5 mm drill is the closest metric equivalent.
What hole size should be drilled before tapping 5/16-18?
The proper hole size to tap 5/16-18 is created using a Letter F drill bit (0.257 inches).
