What tap drill do I use for M27 × 3?
Answer
Drill 24 mm to tap M27 × 3. That leaves 77% thread engagement, which is what workshop charts are built on.
How it is worked out
- The relation every chart is built on
drill = major diameter − (engagement% ÷ 100) × 1.299 × pitch - For this thread
27 − 0.75 × 1.299 × 3 = 24.08 mm - Rounded to a drill you own
24.08 mm → 24 mm
1.299 = 2 × 0.6495, the truncation of the 60° thread profile
Worth knowing
- Clearance hole — close
- 28 mm, for accurately positioned holes only
- Clearance hole — normal
- 30 mm. Use this one unless you have a reason not to.
- Clearance hole — loose
- 32 mm
- Minor diameter
- 23.753 mm — the thread's own root, which is not the drill size and is the mistake worth avoiding
- Pitch diameter
- 25.052 mm
- How deep to tap
- One diameter of engagement in steel, one and a half in aluminium or cast iron, two in plastic. Deeper gains nothing measurable — the first few threads carry most of the load.
- Why not cut a full-depth thread
- The last quarter of engagement adds roughly 5% to the stripping strength and about doubles the torque on the tap. In anything harder than aluminium the tap breaks first, and a broken tap in a finished part is a much worse afternoon.
Tap drills for nearby metric threads
Once it is tapped
Do it with your own numbers
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