This is a replacement upper arm nut for the TRF421X chassis, supplied as the Tamiya 9803565 spare part. It fastens the upper arm pivot so your suspension geometry remains consistent while you adjust camber, toe, and ride height during setup.
It is compatible with TRF421X-series suspension assemblies as shown by part reference 42391. During installation, hand-start threads to prevent cross-threading, clean contact surfaces, and use the appropriate hex driver or wrench to seat the nut. Avoid overtightening since that can harm plastic or composite inserts in the arm.
In tuning, the nut locks the pivot so alignment changes at shocks and turnbuckles hold under load. Periodically check the nut during testing runs and re-confirm thread engagement after swapping arms or replacement components.
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