What drill for a M20×2.5 roll (form) tap?

About 18.89 mm; the nearest standard drill is 18.90 mm. Check your tap maker's table.

What drill for a M20×2.5 Helicoil?

Drill 21.5mm (21.50 mm), then tap with the M20×2.5 STI tap.

M20×2.5 tap drill size

Use a 17.5mm drill (17.50 mm) for a M20×2.5 cutting tap at 75% thread.

Drills for M20×2.5 (Metric)
Tap type	Drill	Decimal (mm)
Cutting tap, 75% thread	17.5mm	17.50
Cutting tap, 50% thread (hard material)	18.5mm	18.50
Roll / form tap, 65% thread	18.90 mm	18.90
Helicoil / STI insert (M20×2.5×1D)	21.5mm	21.50

Pitch: 2.5 mm
Major Ø: 20.00
Pitch Ø: 18.38
Minor Ø: 17.29

M20×2.5 tapping speeds and feeds

Feed = pitch (2.5 mm per rev). Starting points for a general-purpose HSS cut tap:

Aluminum 6061 / 7075: 340 RPM · 850 mm/min
Free-machining / mild steel: 100 RPM · 250 mm/min
304 / 316 stainless: 50 RPM · 125 mm/min

M20×2.5 tap speeds and feeds: all materials, form taps and G84 code · 17.5mm drill size and speeds

Open the tap drill calculator Roll tap chart Helicoil chart

Common questions

What size drill for a M20×2.5 tap?: Use a 17.5mm drill (17.50 mm) for a M20×2.5 cutting tap at 75% thread.
What drill for a M20×2.5 roll (form) tap?: About 18.89 mm; the nearest standard drill is 18.90 mm. Check your tap maker's table.
What drill for a M20×2.5 Helicoil?: Drill 21.5mm (21.50 mm), then tap with the M20×2.5 STI tap.

Drill sizes from Machinery's Handbook and ASME B1.1/B1.13M; roll tap sizes from the form-tap formula. Tap and insert makers' tables are authoritative.

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