Choosing the Right Drill Bit Size for Metal Screws
By Drill Hog® | Published: 2026-10-28
Category: Buying Guides
Match your screw diameter and thread type to the correct pilot hole size for strong, clean threads in metal. This guide covers tap drill charts and bulk options.
Quick Answer
For metal screws, use a tap drill chart to match the screw's major diameter and threads per inch to the correct pilot hole size. A 9/64" drill bit is a common choice for #8 or #10 machine screws, and bulk packs like Drill Hog's 100-piece M7 set save money on repeated jobs.
Why Pilot Hole Size Matters in Metal
Drilling the wrong pilot hole in metal can strip threads, break taps, or weaken the joint. A hole that's too small makes tapping difficult and can snap the tap. A hole that's too large leaves too little material for threads to grip, reducing holding power.
Unlike wood, metal doesn't compress, so the pilot hole must be nearly the same diameter as the screw's root (the minor diameter). Always consult a tap drill chart for the exact size.
Tap Drill Charts: Your Go-To Reference
A tap drill chart lists the recommended drill bit diameter for each screw size and thread pitch. For Unified Coarse (UNC) threads, the rule of thumb is to subtract the thread pitch from the major diameter. For example, a 1/4-20 UNC screw needs a #7 (0.201") or 13/64" drill bit.
Metric screws follow the same logic: subtract the pitch from the diameter. For an M6 x 1.0 screw, use a 5.0 mm drill bit. Keep a printed chart in your toolbox or save one on your phone.
When to Use a 9/64" Drill Bit
A 9/64" (0.1406") drill bit is a versatile size that fits several common screw applications. It's the correct pilot hole for a #8-32 UNC tap and works well for #10-24 UNC in softer metals like aluminum. For harder steels, double-check the chart because thread engagement changes.
Buying a bulk pack of 9/64" bits, such as Drill Hog's 100-piece M7 set, makes sense if you frequently tap these sizes. The high-molybdenum M7 steel handles the heat and wear of metal drilling better than standard HSS.
Decision Rules for Pilot Hole Size
- Identify the screw size and thread type: Look at the screw's packaging or measure the major diameter with calipers. Determine if it's UNC (coarse), UNF (fine), or metric. Write down the threads per inch (TPI) or pitch.
- Look up the tap drill size: Use a reliable tap drill chart. For UNC threads, the formula is: drill size = major diameter – (0.974 / TPI). For metric: drill size = major diameter – pitch. Round to the nearest standard fraction or number size. Keep a chart in your phone or toolbox for quick access.
- Choose your drill bit material: For occasional use in mild steel, HSS works. For frequent drilling or harder metals (stainless, tool steel), use cobalt or M7 high-speed steel. A bulk pack like Drill Hog's 100-piece M7 set reduces per-bit cost and ensures you always have the right size. M7 bits last longer in production work; HSS is fine for DIY.
- Drill and test the fit: Drill the pilot hole straight and deburr the edges. Insert the tap to check that it starts easily without wobble. The tap should cut threads without excessive force. If it binds, the hole is too small; if it feels loose, it's too large. Always test on scrap first if you're unsure.
Product Facts
- 9/64" Drill Bit HI-Molybdenum M7 (100) Pieces Bulk Pack: Drill Hog 9/64" Hi-Molybdenum M7 bits have a 135-degree split point that prevents walking and a melting point of 4,753°F. The split point starts cleanly on metal without a center punch, and the M7 steel withstands the heat of repeated drilling in steel or stainless.
9/64" Drill Bit HI-Molybdenum M7 (100) Pieces Bulk Pack
Who Should Use This Guide
Best For
- DIYers and mechanics tapping threads in mild steel or aluminum
- Anyone who needs a bulk supply of common drill bit sizes for repeated metal screw installations
Not For
- Wood screws where self-tapping or pilot holes are less critical
- Sheet metal screws that use a different sizing method (drill point vs. tap)
