This is a replacement split beveled cone for attaching the flywheel on Traxxas vehicles to the shaft on SG-crank engines. This is the same as the part that comes in the Traxxas Revo monster truck.
Replacement part for these models:
Jato
Nitro Rustler
Nitro Slash
Revo
Slayer Pro 4X4
T-Maxx
TRX 2.5R
TRX 3.3
TRX Pro .15
TRX Pro .15 Marine