4900 - Traxxas makes it easy to upgrade TRX Pro .15 nitro-powered models to the awesome power of the new TRX 2.5 Racing Engine. All the necessary hardware to bring older Traxxas models up to full TRX 2.5 spec is included such as: header, pipe, pipe hanger, coupler, linkage, and additional components specific to each model. Major components come pre-assembled and pre-adjusted from the factory. Just bolt them on! Detailed instructions are included. TRX 2.5 Racing Engine (model 5207) sold separately.