Fasteners and Fits
What this domain tests
This domain covers ISO metric threads, thread pitch, bolt grades, torque, locking methods, washers, circlips, keys and the ISO hole-basis fit system. Questions test whether you can select a fastener and predict whether a joint will slide, locate or press.
Core rules
- An M10 × 1.5 fastener has a nominal diameter of 10 mm and pitch of 1.5 mm; M10 × 1.25 is fine pitch. Pitch = distance moved in one full turn.
- Property class 8.8 means nominal tensile strength 800 MPa and yield strength about 80% of that, i.e. 640 MPa. Class 10.9 is stronger.
- Torque produces clamp load. Too little torque allows joint movement and fatigue; too much stretches the bolt or crushes the part.
- Use the specified thread lubricant or dry condition; oiled threads need less torque for the same clamp load.
- Locking methods include nylon-insert nuts, prevailing-torque nuts, thread-locking compound, tab washers, split pins and safety wire.
- In the hole-basis system the hole is H. H7/g6 is a close sliding fit; H7/h6 is a location fit; H7/p6 is an interference fit.
Worked example
A thread is checked across 10 pitches with a caliper: the span is 15.0 mm, so pitch = 15.0 ÷ 10 = 1.5 mm. The fastener is therefore M10 × 1.5, not M10 × 1.25. Ordering the fine-pitch nut would cause thread damage and a false torque reading.
Traps that cost marks
Forcing a fine-pitch nut onto a coarse thread; reusing a torque-to-yield bolt; applying dry-torque figures to lubricated threads; fitting a plain washer where a hardened washer is specified; confusing left-hand thread on a wheel or adjuster.
Key points
- Pitch is measured across several threads and divided by the count.
- Class 8.8 has 800 MPa nominal tensile strength.
- H7/g6 slides; H7/p6 interferes.
- Lubrication changes clamp load at a given torque.
- Use the specified locking device, not extra torque.