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Fasteners, explained

· Fox Bright

How to read a screw or bolt by its head, drive, thread, length and material, so you can name it, find a match, and take a product apart without stripping anything.

Screws, bolts, nuts and washers laid out in rows on a maple workbench beside dial calipers and a folding set of hex keys.

Every screw is described by the same five things: its head, its drive, its thread, its length and its material. Get all five right and anyone can find the exact part you are holding. Get one wrong and it will not fit, or it will strip the first time it is tightened.

DriveHeadShankThreadLength, measured from under the headDiameter(M size)Pitch

Screw or bolt?

A bolt passes through the parts and a nut clamps them together. A screw threads into one of the parts. The same fastener can be either depending on how it is used, so most people call anything with a head and a thread a screw. What matters more is what the thread is made to go into.

Machine screwFine, even thread and a flat end. Goes into a nut, a tapped hole or a threaded insert. Sizes are standard, so any M3 fits any M3 hole.
Thread-forming screwDeep, sharp, widely spaced thread that cuts its own path into a plastic boss. Holds most plastic product housings together.
Wood screwTapered and pointed, with a coarse thread and often a smooth shank under the head so the top board pulls tight.

Thread-forming screws cut a fresh thread each time they are driven in straight. When one goes back into its old hole, turn it backwards first until it drops into the existing thread with a small click, then tighten. Sheet metal screws look like wood screws but are threaded all the way up to the head.

Head types

The head decides how the screw sits on the surface and how much of it shows. These are drawn from the side.

Socket head capTall cylinder with a hex socket. Strong and compact, and a hex key reaches it in tight spots. Common on machines, 3D printers and jigs.
Button headLow dome with a hex socket. Neater than a socket cap, but the smaller socket strips sooner.
Countersunk (flat head)Cone-shaped head that sits flush in a matching countersink. Metric heads are 90°, inch heads 82°.
Pan headFlat-topped low dome, usually Phillips or Torx. The everyday head in electronics and appliances.
Hex headTurned from the outside with a wrench or socket. Takes the most torque of any head here.
Flange headA hex or pan head with a washer built in to spread the load. Common on anything that vibrates.
Truss or wafer headVery wide and very low. Holds thin sheet or soft material without pulling through it.
Set screw (grub screw)No head at all. Sits inside a threaded hole and presses on a shaft, like the screw that holds a knob or pulley on.

Drive types

The drive is the recess the tool fits into. Match the bit exactly. It should seat all the way down with no wobble. A bit that almost fits is how screw heads get stripped, and a stripped screw is usually where taking a product apart turns into breaking it.

SlottedOne slot. The oldest drive and the easiest to slip out of. Still used on set screws and electrical terminals.
Phillips (PH)Cross with tapered arms that push the bit out under high torque, called cam-out. PH2 is the everyday size; PH00 and PH0 are for electronics.
Pozidriv (PZ)Looks like Phillips with four thin lines between the arms. Grips better. A Phillips bit in a Pozidriv screw chews the head, and so does the reverse.
Robertson (square)A square socket from P. L. Robertson, who made them in Milton, Ontario, from 1908. The screw stays on the bit by itself. Handles are colour-coded: green #1, red #2, black #3.
Hex socket (Allen)Turned with a hex key. Use metric keys on metric screws: a 5/32" key and a 4 mm key are close enough to strip each other.
Torx (T)Six rounded lobes. Grips far better than Phillips and rarely cams out. T5 to T10 in electronics, T25 on deck screws.
Tamper-resistant hexA hex socket with a pin in the middle. A plain hex key will not fit; it needs a key with a hole drilled in its end.
Security TorxTorx with a pin in the centre, so an ordinary Torx bit will not go in. Needs a bit with a hole in its tip.
Tri-wingThree offset slots. Found on some game consoles and controllers.
PentalobeFive rounded lobes. Apple uses it to close iPhones and MacBooks.
CombinationA square and a Phillips cross in one recess, so either bit works. Common on electrical outlet and switch screws.
Spanner (snake eye)Two holes. Used on public fixtures that should not be opened casually.

Threads

A thread has two numbers that matter. The diameter is measured across the outside of the threads. The pitch is the distance from one crest to the next.

Metric threads

Metric screws are written as M, the diameter in millimetres, then the pitch: M4 × 0.7. Every size has a standard coarse pitch, and when the pitch is left off, coarse is assumed, so an “M4” is an M4 × 0.7. Fine threads exist (M4 × 0.5) and always state their pitch.

Size Coarse pitch Tap drill Hex key, socket cap Hex key, button and countersunk
M3 0.5 mm 2.5 mm 2.5 mm 2 mm
M4 0.7 mm 3.3 mm 3 mm 2.5 mm
M5 0.8 mm 4.2 mm 4 mm 3 mm
M6 1.0 mm 5.0 mm 5 mm 4 mm
M8 1.25 mm 6.8 mm 6 mm 5 mm
M10 1.5 mm 8.5 mm 8 mm 6 mm

The tap drill is the hole you drill before cutting a thread into it with a tap. Electronics go smaller than the table: M2 (0.4 mm pitch) and M2.5 (0.45 mm) hold circuit boards and small modules.

Inch threads

Inch screws are written as a size and the number of threads per inch (TPI). A 1/4-20 is 1/4" across with 20 threads in every inch. Sizes under 1/4" use gauge numbers instead of fractions. UNC is the coarse series and UNF the fine one.

Size Diameter Coarse (UNC) Fine (UNF) Where you meet it
#4 0.112" 4-40 4-48 Small electronics
#6 0.138" 6-32 6-40 Computer cases, 3.5" hard drives
#8 0.164" 8-32 8-36 General machine screws
#10 0.190" 10-24 10-32 Equipment racks (10-32)
1/4" 0.250" 1/4-20 1/4-28 Camera and tripod mounts (1/4-20)
5/16" 0.313" 5/16-18 5/16-24 Bolts in furniture and machines
3/8" 0.375" 3/8-16 3/8-24 Bolts in furniture and machines

Measuring a thread

  1. Measure the outside diameter with calipers. Screws measure slightly under their nominal size, so 2.9 mm is an M3.
  2. Find the pitch with a thread gauge. Without one, measure across ten threads (eleven crests) and divide by ten. Ten threads over 7 mm is a 0.7 mm pitch.
  3. Measure the length: under the head for most screws, overall for countersunk ones.
  4. Check the hand. Almost every thread is right-handed and tightens clockwise. A few are left-handed on purpose, like the left pedal on a bicycle, which would otherwise unscrew itself as you ride.

Nuts, washers and inserts

  • A hex nut is the standard partner for a machine screw. A nylon-insert lock nut (nyloc) has a nylon ring that grips the thread so vibration does not loosen it. A flange nut has a washer built in.
  • A flat washer spreads the load under a head or nut, which matters most on wood and plastic. A split lock washer is the springy ring cut through on one side.
  • Threaded inserts give soft material a machine thread. T-nuts and screw-in inserts go into wood. Brass heat-set inserts are pressed into 3D prints with a soldering iron; see Designing for FDM.
  • Threadlocker is liquid adhesive for threads. Blue comes apart with hand tools; red usually needs heat. A screw that will not turn and has coloured residue in its thread has been locked.

Materials and strength

Material How to recognise it Notes
Zinc-plated steel Bright silver or yellowish Cheap and common. Rusts once the plating wears through.
Black-oxide alloy steel Dark grey to black, often a socket head The strong ones. Usually class 12.9.
Stainless steel Dull silver, only weakly magnetic or not at all Resists rust but is not stronger than plain steel. A2 (304) for general use, A4 (316) near salt water. Stainless threads can seize if tightened fast and dry.
Brass Yellow, non-magnetic Soft. Used for electrical parts and heat-set inserts.
Nylon White or black plastic Does not conduct electricity, so it is used to mount and insulate circuit boards.

Metric bolts are stamped with a property class on the head. In 8.8, the first number times 100 is the tensile strength in MPa, and the second is the fraction of that where the bolt starts to stretch permanently: 800 MPa, yielding at 80%. 10.9 and 12.9 are stronger again. Inch bolts use lines on the head instead: none for grade 2, three for grade 5, six for grade 8.

Fasteners you cannot see

When a product will not open, the fastener is usually hidden, or it is not a screw at all.

  • Under things. Rubber feet, stickers, battery-compartment labels and trim strips all hide screws. Press on a label to feel for a hollow before you peel it, and peel only as far as you need.
  • Snap fits. Plastic hooks moulded into the two halves of a case. Open the seam with a plastic spudger or pick and work your way around the edge. A metal screwdriver scars the plastic and snaps the hooks.
  • Permanent joints. A plastic post melted into a mushroom over a part (a heat stake), rivets, glue, and ultrasonic welds, which leave a seam with no gap at all. These were not made to come apart. Often the right call is to leave that sub-assembly in one piece and label it as one part.

Before you start, search the product’s model number with “teardown” or “disassembly”. iFixit has step-by-step guides for thousands of products, and a video of someone else opening the same model shows where the hidden clips and screws are.

Keeping track during a teardown

  • Bag screws by stage and write the stage on the bag, or push them through a sheet of cardboard in the pattern they came out, with the step written beside them.
  • Screws from the same product often look identical but differ by a few millimetres of length. A long screw in a short hole can go straight through a circuit board or the outside of the case.
  • Photograph each layer before you remove it.

Naming a fastener

Put the name together in the order a supplier lists it: thread size, length, head, drive, material.

M3thread size× 10length, mmsocket head capheadhexdriveA2 stainlessmaterial
  • M4 × 12 countersunk hex socket screw, black oxide 10.9
  • M2.5 × 6 pan head Phillips machine screw, zinc-plated
  • #6-32 × 1/4" pan head Phillips machine screw
  • #8 × 1-1/4" flat head Robertson wood screw

Wood screws are written as gauge × length, without a thread count.

Fasteners in Fusion

  • Threads. Create → Thread adds a thread to a cylindrical face. Leave Modeled unticked and it is a cosmetic texture. Tick Modeled and the helix is cut into the solid, which you need if the thread is going to be 3D printed.
  • Threaded holes. In the Hole tool, set the tap type to Tapped and choose the size, such as M4x0.7.
  • Real parts. Insert → Insert McMaster-Carr Component places an accurately dimensioned screw, nut or washer in your design, so you can check lengths and clearances before buying anything.
  • Modelling a screw you measured. A cylinder with the right diameter and length, and a head of the right shape and size, is a good model. The helix is optional. See Modelling from sketches for getting from caliper numbers to geometry.