Two numbers decide whether a threaded joint performs as designed: the tap drill size used to make the hole, and the engagement length of the thread in it. Both are easy to get approximately right and wrong in the marginal cases.
Tap drill size
For a metric thread the tap drill is nominally major diameter minus pitch. For the coarse series:
| Thread | Pitch | Tap drill (nominal) | Nearest standard drill |
|---|---|---|---|
| M3 | 0.5 | 2.5 | 2.5 mm |
| M4 | 0.7 | 3.3 | 3.3 mm |
| M5 | 0.8 | 4.2 | 4.2 mm |
| M6 | 1.0 | 5.0 | 5.0 mm |
| M8 | 1.25 | 6.75 | 6.8 mm |
| M10 | 1.5 | 8.5 | 8.5 mm |
| M12 | 1.75 | 10.25 | 10.2 mm |
Each thread page gives the exact tap drill for that designation, which is worth using rather than recalculating, because the published value accounts for the thread's actual minor diameter rather than the nominal formula.
Going undersize leaves too little clearance, so the tap will bind or break and the finished thread will be over-tight and prone to galling. Going oversize produces a shallower thread with less load-carrying area: the joint looks correct and fails early.
Engagement length
The engagement length is how much thread is actually in contact. The design intent is that the bolt should be the weakest link, not the internal thread, because a broken bolt is visible and its failure is progressive while a stripped thread releases suddenly.
The widely used rule of thumb is an engagement of at least one times the nominal diameter in steel, and more in weaker materials:
| Material of the tapped part | Minimum engagement |
|---|---|
| Steel, class 8.8 bolt | 1.0 × d |
| Cast iron | 1.25 × d |
| Aluminium, brass | 1.5 to 2.0 × d |
| Plastics | 2.0 × d or a threaded insert |
For an M6 bolt in steel that means about 6 mm of full thread. Note that this is full thread, not hole depth. A tap leaves a taper at the bottom that contributes very little load-carrying area, so the drilled depth needs to exceed the engagement by roughly the thread diameter.
Where this goes wrong
- Thin sheet with a tapped hole. The sheet thickness sets the engagement, and it is often below the rule. The fix is a nut, a rivet nut or a weld nut rather than a deeper tap, since there is no material to tap into.
- Blind holes in soft material. The taper at the bottom plus the required engagement frequently exceeds the available depth. Check the thread page for the designation before fixing the hole depth.
- Reused tapped holes. A tapped hole that has been stripped and re-tapped, or one that has seen repeated assembly, has less thread area than the drawing assumes.
For the dimensions of the bolt going into the hole, start from the specification index and open the standard the drawing calls out.