Drilling, Tapping and Fasteners
Drill and tap sizes, clearance holes, thread engagement, tightening torque, rivets, adhesives and choosing a fastener.
Drilling: sizes, depth and finish
Drill sizes come in fractions of an inch, numbers and letters, and millimeters; the Tap Drill Calculator converts them and finds the nearest. For accuracy, drill a pilot first and ream or bore a hole that must be within a few hundredths of a millimeter: a twist drill makes a hole slightly oversize and rough.
- Center punch the mark, start with a small drill, then the full size; speed and feed are in Power Tools and Shop Machines.
- Blind holes: the drill point adds about 0.3 times the diameter beyond the full-diameter depth; allow for it.
- Break the breakout: back the work with scrap and ease the feed as the drill exits.
- Deburr both sides of every hole (a countersink bit turned by hand is enough).
- Peck deep holes and clear the chips.
Tap drill sizes and tapping
A tap cuts a thread in a drilled hole. The hole is drilled to the tap drill size, which leaves a thread of about 75 % of the full depth: a good compromise between strength and the force needed to tap. The percentage of thread for a drill of diameter d in a thread of major diameter D and pitch P is about 76.98 × (D − d) ÷ P.
Worked example: M6 × 1 tap drill
| Drill | Thread formed | Comment |
|---|---|---|
| 5.0 mm | 77 % | Standard tap drill: strong thread, tap works hard |
| 5.1 mm | 69 % | Easier tapping, still strong |
| 5.2 mm | 62 % | Weak thread; only for brittle or very hard material |
- The standard tap drill for M6 × 1 is D − P = 5.0 mm, giving 77 % thread; an M8 × 1.25 tap drill of 6.75 mm gives 77 %.
- Going from 75 % to 60 % thread cuts the strength by only a little (the bolt breaks before the thread strips in most cases) but cuts the tapping force a lot: use the larger drill in hard materials.
- Blind hole depth: for a 9.0 mm full thread (1.5 × d) the tap must go 12 mm deep (the first threads taper), so the drilled hole needs about 14 mm.
- Use a tap wrench and keep the tap square; turn forward about half a turn then back a quarter to break the chip.
- Taper tap, plug tap, bottoming tap: start with the taper, finish a blind hole with the bottoming tap.
- Use cutting oil on steel; paraffin or none on aluminum and brass.
- A broken tap is a bad day: do not force it; back off, clean and re-oil.
- Thread-forming (roll) taps make a thread without chips in soft metals, with a bigger tap drill.
Clearance holes
A bolt passes through a clearance hole a little larger than the thread. Close fits (for accurate location), normal fits (for general work) and loose fits (for slack in a pattern of holes) are given in ISO 273.
| Screw | Close | Normal | Loose |
|---|---|---|---|
| M4 | 4.3 mm | 4.5 mm | 4.8 mm |
| M5 | 5.3 mm | 5.5 mm | 5.8 mm |
| M6 | 6.4 mm | 6.6 mm | 7.0 mm |
| M8 | 8.4 mm | 9.0 mm | 10.0 mm |
| M10 | 10.5 mm | 11.0 mm | 12.0 mm |
- Use the normal fit by default; loose holes for patterns where the hole spacing is not accurate.
- Fits for locating pins are in Fits and Tolerances.
- Countersunk and counterbored holes sink the head: counterbore diameter about 1.8 × d and depth equal to the head height.
Thread engagement
The thread must be long enough that the bolt breaks before the thread strips (a bolt break is gradual and obvious; stripped threads fail suddenly and the part is lost). The thread strips when the shear stress on the thread surface reaches the shear strength of the weaker material.
Worked example: engagement length for an M6 8.8 bolt
- The bolt carries up to 800 MPa × 20.1 mm² = 16.1 kN (class 8.8, tensile stress area 20.1 mm²).
- The internal thread strips at about 0.5 π d L τ (a simple model). Setting this equal to the bolt strength gives L = 16 080 ÷ (0.5 π × 6 × τ).
- In steel (shear strength about 300 MPa): L = 5.7 mm, 0.95 diameters.
- In 6061 aluminum (about 207 MPa): 8.2 mm, 1.37 diameters. In cast iron or soft plastics (about 120 MPa) 14.2 mm.
- Rules of thumb: engagement of 1 × d in steel, 1.5 to 2 × d in aluminum and cast iron, 2 to 3 × d in plastics and wood; thread inserts (helical coils) raise the strength of a soft material.
- Past the engagement length, more thread adds nothing: the bolt breaks first.
Tightening torque and preload
A bolt holds by preload: it is stretched so that it clamps the joint with a force that keeps the parts together under load. The preload is set by the torque, but only loosely: most of the torque goes in friction under the head and in the thread. The usual formula is T = K F d, where K (the nut factor) is about 0.2 for dry steel and 0.15 for lubricated threads.
T = K · F · d
Worked example: M6 8.8 torque
- The proof strength of class 8.8 is about 580 MPa. A normal preload is 75 % of proof: 0.75 × 580 × 20.1 = 8 744 N (8.7 kN).
- Dry (K = 0.2): T = 0.2 × 8 744 × 0.006 = 10.5 N·m.
- Lubricating the thread (K = 0.15) and tightening to the same 10.5 N·m raises the preload by a factor of 1.33 to 11.7 kN: that is 580 MPa of tensile stress on the bolt, near its proof strength. Use the torque value for the condition (dry or oiled) that the table assumes, or less when oiled.
- Tighten in a cross pattern in two or three steps, and use a torque wrench for anything that matters. See the Bolt Torque Calculator and Structures and Strength of Materials for the joint theory.
Screws in wood
| Screw | Pilot hole (softwood) | Clearance hole in the top piece |
|---|---|---|
| #6 (3.5 mm) | 2.0 mm | 3.5 mm |
| #8 (4.0 mm) | 2.5 mm | 4.2 mm |
| #10 (4.8 mm) | 3.0 mm | 5.0 mm |
| #12 (5.5 mm) | 3.5 mm | 6.0 mm |
- Drill a pilot hole about the diameter of the screw’s core, to stop splitting; larger in hardwood (the root diameter).
- Clearance hole in the top piece so that the screw pulls the pieces together.
- Keep screws at least 10 diameters from the end of a board and 4 diameters from an edge.
- Wax or soap on screws in hardwood reduces the driving torque and the risk of breaking the head.
- Do not use drywall screws for structural work: they are brittle and snap.
Rivets and adhesives
Worked example: riveted and glued lap joints
- A 4.8 mm blind (pop) rivet in aluminum and steel has a shear strength of about 1 500 N (typical catalog value). Six rivets share the load: 6 × 1 500 ÷ 3 (safety factor) = 3 000 N allowable.
- Bearing on a 1.5 mm sheet: 4.8 × 1.5 × 150 MPa = 1 080 N per rivet (for soft aluminum) which is also checked.
- A glued lap joint of 12.5 × 25 mm (area 313 mm²) with an epoxy of lap-shear strength 15 MPa carries 15 × 313 = 4.7 kN, or 1.6 kN with a safety factor of 3.
- Adhesives resist shear and tension well and peel poorly: design the joint so the load is in shear over a large overlap, and prepare the surfaces (clean, degrease, abrade).
| Method | Good for | Limits |
|---|---|---|
| Pop rivet | Sheet metal, one-sided access | Moderate strength; can’t be tightened |
| Solid rivet | Permanent, high strength | Needs access to both sides |
| Epoxy | Dissimilar materials, gap filling | Peel; temperature; cure time |
| Cyanoacrylate | Fast, small joints | Brittle; poor in peel |
| Polyurethane / wood glue | Wood joinery | Needs clamping and fit |
| Welding and brazing | Permanent steel joints | Needs skill and equipment |
Preventing loosening
- Preload is the best anti-loosening device. A properly tightened bolt does not loosen unless the joint slips; most loosening comes from low preload, vibration and embedding of the surfaces.
- Thread-locking fluid (low strength, blue, for small screws you want to remove; high strength, red, for permanent) fills the thread gap. Clean the threads first.
- Nyloc nuts, serrated flange nuts and wedge-locking washers add friction; ordinary spring washers do very little.
- Split pins, lock wire and castellated nuts stop a nut turning on safety-critical parts; they do not set the preload.
- Check after the first heat cycles or loads; some embedding will drop the preload.
Choosing a fastener
| Need | Choose | Why |
|---|---|---|
| Parts that come apart often | Machine screws or bolts into a nut or thread insert | Repeated use does not wear the thread in a soft part |
| Thin sheet | Rivets, rivet nuts, clinch nuts, self-tapping screws | No room for a thread |
| Wood | Wood screws, coach screws, glue and joinery | Pilot holes; glue joins end grain poorly |
| Vibration | High preload bolts with a locking feature | Prevent loosening |
| Outdoors or corrosive | Stainless or coated fasteners | Avoid galvanic pairs of steel and aluminum without isolation |
| Hidden fastening | Pocket screws, adhesive, concealed hinges | Appearance |
Troubleshooting
| Problem | Likely cause | Try |
|---|---|---|
| Tap breaks | Hole too small, no oil, chips jammed, tap out of line | Larger tap drill; oil; reverse to break chips; use a tap guide |
| Thread strips | Not enough engagement, over-torque, soft metal | Longer thread; insert; torque wrench |
| Screw splits the wood | No pilot, too near the end | Drill a pilot; move the screw; use a smaller diameter |
| Bolt will not tighten, spins | Stripped thread, cracked part | Replace the bolt; helical insert; check the part |
| Screw head rounds off | Wrong driver, cam-out | Use the correct tip; press in line |
| Bolt keeps loosening | Low preload, vibration | Tighten to the proper torque; threadlocker or locking nuts |
Related
- Shop Tools: the overview of this guide.
- Power Tools and Shop Machines: drill speeds for these holes.
- Hand Tools, Marking Out and Cutting: marking, hand drilling and deburring.
- Tap Drill Calculator, Bolt Torque Calculator and Structures and Strength of Materials (bolted joints).
FAQ
What drill do I use for an M6 tap?
5.0 mm for the standard M6 × 1 thread (about 77 % thread); 5.1 mm for an easier tap in hard material.
How much thread engagement do I need?
About 1 × the diameter in steel, 1.5 to 2 × in aluminum and cast iron, so that the bolt breaks before the thread strips.
How tight should a bolt be?
About 75 % of proof load: for an M6 8.8 bolt about 10 N·m dry; less when the thread is oiled. Use a torque wrench for anything important.
Do spring washers prevent loosening?
Hardly at all: preload, thread-locking fluid or a proper locking nut do far more.
References
- Oberg et al., Machinery’s Handbook (tap drill sizes, thread data, bolt torque).
- ISO 273 (clearance holes for bolts and screws); ISO 898-1 (mechanical properties of fasteners).
- Bickford, J. H., Introduction to the Design and Behavior of Bolted Joints, CRC Press.
- Fastener and adhesive makers’ catalogs for rivet and adhesive strengths.
Last reviewed: October 2026. Torques, strengths and sizes are typical: use your fastener maker’s values for important joints.