This is a replacement 12.2x1mm header O-ring for the Traxxas 2.5 and 2.5R engines. This small O-ring goes inside the exhaust manifold to properly seal the manifold to the back of the engine.
Replacement part for these models:
Jato
Revo
Slayer Pro 4X4
T-Maxx
TRX 2.5R
TRX 3.3