The component marked Tamiya 3451585, referenced with part code 42391 for the TRF421X, is an aftermarket replacement/hop-up piece that locates the shock on the chassis. It functions as the interface between shock towers and dampers and is designed for TRF421X style mounting layouts.
From a functional standpoint, the stay sets mounting geometry that influences shock angle, leverage and roll behaviour. When fitting the stay, retain the chassis' original mounting points and hardware to maintain alignment; the part is intended to match TRF421X architecture rather than alter it.
Prior to installation, confirm the 42391 part number against your chassis documentation and inspect the mounting holes and fasteners for proper seating. Carry out routine checks on thread condition and torque settings as part of standard RC maintenance to ensure long service life.
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