Fasteners and Assembly Practice
The exam tests fastener types and their applications, thread systems, locking devices and correct tightening sequence.
Threaded fasteners
Through bolt with nut, machine screw, stud (screwed into a casting with a nut on the free end), cap screw, grub screw and self-tapping screw for sheet and plastics. Heads may be hex, square, cheese, pan, countersunk or socket (Allen). Washers: plain spreads load, spring maintains tension, taper aligns a channel section. Thread systems include ISO metric coarse and fine (M10 x 1.5), BSW, BSP for pipes and UNC/UNF. Power threads include square, Acme (used on lathe lead screws) and buttress.
Keys, pins and rivets
A sunk or parallel key, taper key, gib head key or Woodruff key transmits torque from shaft to pulley. Dowel pins locate, taper pins lock, split pins secure castle nuts and circlips retain parts in grooves. Rivets may be snap head or countersunk, and the rivet hole is slightly larger than the rivet shank.
Assembly practice
Clean the threads, apply the specified torque with a torque wrench, tighten in stages and in a diagonal or star sequence from the centre outwards, use thread-locking compound where specified, and replace spring washers, split pins and self-locking nuts after use. After assembly check that the part turns freely and is correctly aligned.
Worked example
A cylinder head with ten bolts is tightened to 30 percent of final torque in a cross pattern starting from the centre bolts, then 60 percent, then full torque in the same sequence. Reasoning: gradual symmetrical tightening compresses the gasket evenly and prevents head warping and leaks. Fully tightening one bolt first distorts the head.
Traps that cost marks
- Over-tightening and stripping small threads.
- Reusing split pins or nyloc nuts.
- Fitting the wrong washer type.
- Forgetting left-hand threads such as grinding wheel nuts, where the nut turns clockwise to loosen.