Cab, Frame and Body Systems
Structure, fasteners and repair limits
The frame is a manufactured, often heat-treated steel structure. Questions focus on inspection and on knowing what you may and may not do. Inspect rails and crossmembers for cracks, especially at flange bends, spring hangers and rivet holes; check rivets and bolts for looseness, and for corrosion that reduces section thickness. Cracks are stop-drilled and repaired only by OEM-approved methods - heat-treated rails generally must not be welded or heated without manufacturer authorization, because doing so destroys the temper and creates a new failure point.
Fasteners are a favourite trap. Grade is marked on the head: SAE grade 5 shows three radial lines, grade 8 shows six; metric classes 8.8, 10.9 and 12.9 are stamped numerically. Never substitute a lower grade, coarse thread for fine, or a plain bolt for a shoulder or flange bolt at fifth-wheel, suspension and steering mountings. Torque depends on grade and thread, and torque-to-yield fasteners are replaced, not reused.
Cab and body: cab mounts and bushings, cab suspension, door hinges and alignment, glass and seals, water and air leaks, seat belt anchorages, sleeper and fairing attachment, and the body builder's manual for any modification.
Worked example. A fifth-wheel mounting bolt is missing. The replacement must match the OEM grade for that position - a hardware-store grade 2 bolt has roughly a third of the shear strength of grade 8 and will fail. When in doubt, look it up.