How do you use the thread chart?
To cut a thread you first drill a tap drill hole. The right diameter is roughly nominal size − pitch — for M8 (pitch 1.25) about 6.8 mm. For a hole the bolt only passes through, use the larger clearance hole. The chart is for metric coarse thread; fine thread has a different pitch and tap drill.
Frequently asked questions
Which drill for an M6 thread?
For M6 (pitch 1.0) drill a 5.0 mm tap drill hole. For a clearance hole use 6.6 mm.
What is the pitch?
The pitch is the distance between two thread crests. Coarse thread has one standard pitch per size (M8 = 1.25 mm); fine thread a smaller one.
Tap drill too big or too small?
Too big = too little thread engagement (weak joint); too small = the tap binds or breaks. Stick to the chart value.