Understanding the Tap Hole Size Chart

Understanding the Tap Hole Size Chart

by habef 23462 -
Number of replies: 1

Hi everyone,

I’m currently machining aluminum components and need a reliable tap hole size chart. I want to make sure I’m drilling the correct pilot hole sizes before tapping threads to avoid damaging tools or workpieces.

 I’ve found conflicting charts online, especially for metric vs. imperial sizes. Is there a standard reference most machinists follow, or does it vary by material type?

 If anyone has a downloadable or printable chart that works well across different metals, that would be a huge help for my workshop.

Thanks in advance.


In reply to habef 23462

Re: Understanding the Tap Hole Size Chart

by ammara celoji7 -

A tap hole size chart is essential for clean, accurate threads. In most shops, machinists refer to ANSI/ASME B94.9 for inch sizes and ISO standards for metric.

 Always adjust slightly depending on material  for softer metals, a slightly smaller hole ensures tighter threads; for harder alloys, follow the standard exactly. 

Many suppliers offer laminated charts for quick reference. When working with aluminum, use cutting fluid to reduce friction and prolong tap life. Consistency in pilot hole sizing will improve both thread quality and tool longevity.