Drill Bits / 5/16 Diameter
3/8-16 Tap Drill Bit Size is a 5/16 Drill Bit
3/8-16 Tap Drill Size Summary
The correct tap drill size for a 3/8-16 thread is a 5/16 drill bit. This drill size produces the proper hole diameter before tapping a 3/8-16 UNC thread, allowing the tap to cut threads efficiently while maintaining the correct thread engagement.
A 3/8-16 tap drill is commonly used in machining, fabrication, maintenance, and industrial manufacturing when creating internal threads for standard 3/8 inch bolts. Using the proper drill bit size for a 3/8-16 tap ensures accurate thread formation and helps prevent excessive tap wear or breakage.
Why a 5/16 Drill Bit is Used for a 3/8-16 Tap
The 5/16 drill bit is the standard 3/8-16 tap drill size because it produces the correct balance between thread strength and tapping efficiency. When a hole is drilled to 5/16 inch prior to tapping, the resulting thread typically provides approximately 77% thread engagement, which is sufficient for most machining applications.
Using the correct 3/8-16 drill size reduces tapping torque and improves tap life while producing strong internal threads capable of supporting typical mechanical loads. This is why the 5/16 drill is widely specified in tap drill charts and machining reference tables.
3/8-16 UNC Tap Drill Size
The thread designation 3/8-16 UNC refers to a Unified National Coarse thread with a major diameter of 3/8 inch and 16 threads per inch (TPI). In older terminology this may also be referred to as a 3/8-16 NC thread.
For both UNC and NC designations, the recommended drill bit for a 3/8-16 tap remains a 5/16 drill bit. This drill size provides the proper 3/8-16 hole size (0.3125") required before tapping the internal thread.
3/8-16 Tap Drill Size in Metric
The metric equivalent drill bit for the 3/8-16 tap drill size is 8.0 mm. The 8.0mm bit will produce a sufficient 74% thread engagement. While metric drill sizes can be used in some situations, machinists in the United States typically use the standard 5/16 inch drill bit when preparing a hole for a 3/8-16 UNC tap.
Using the correct 3/8-16 tap drill size in mm can be helpful when working in mixed-unit environments or when metric tooling is more readily available.
Applications for a 3/8-16 Tap Drill
A 5/16 drill bit for a 3/8-16 tap is commonly used in a wide range of industrial and mechanical applications, including:
- Machined components and threaded parts
- Maintenance and repair operations
- Fabrication and metalworking
- Equipment assembly and structural components
- Production machining and manufacturing
Selecting the proper 3/8-16 drill bit size ensures consistent thread quality and helps maintain tapping efficiency across a variety of materials including steel, stainless steel, aluminum, and other metals.
3/8-16 Tap Drill FAQs
What size drill bit is used for a 3/8-16 tap?
The standard tap drill size for a 3/8-16 thread is a 5/16 drill bit.
What is the metric drill size for a 3/8-16 tap?
The closest metric drill bit for the 3/8-16 tap drill size is 8.0 mm.
What hole size is needed for a 3/8-16 thread?
Before tapping a 3/8-16 thread, the hole should be drilled using a 5/16 drill bit ( 0.3125" diameter) to produce the correct pilot hole for the tap.
