The Hiller 83156 O‑RING 2‑7X15‑1 is a rubber sealing ring used as a static or dynamic seal between two mating surfaces to prevent fluid or gas leakage in aircraft systems. O‑rings like this are commonly sized by inner diameter and cross‑section dimensions (here ~2 mm × 7 mm with ~15 mm outer reference) and are typically made from elastomeric materials selected based on the fluid compatibility (oil, fuel, hydraulic fluid) and temperature range needed for the application.

Such O‑rings are widely used in hydraulic, fuel, and pneumatic sealing applications throughout aircraft assemblies where reliable sealing under pressure and vibration is critical.