We’re truly sorry for the frustrating experience with your pole saw!
After checking the official parts list and manual for your VEVOR 43cc gas pole saw, we can confirm:
The chain tensioner is factory‑designed with ONLY a tension screw, small spring, and guide pin. No C‑clip, E‑clip, or retainer is included or required from the manufacturer—this is normal design, not missing parts.
The screw coming loose during use is due to vibration, which is a characteristic of this mechanism.
By vevor