How deep should a tapped thread be?

Beyond a certain length, extra thread engagement adds no strength because the bolt breaks first. Deeper threads only add tapping time and the risk of breaking a tap in an almost finished part.

Engagement that is strong enough

For an M6 bolt in aluminium, 9–12 mm of full thread is enough.

Drill deeper than the thread

A tap has a lead-in, so the last few threads are incomplete. Make the tap drill hole at least 3–4 pitches deeper than the full thread, plus the drill point. On the drawing, give both depths: M6x1-6H thread 12 deep, tap drill Ø5.0 16 deep.

Through holes are easier

Where the part allows, a through-tapped hole is faster and safer because chips fall out and the tap does not hit the bottom.