The .21 has a .21 engine and is ready to run
the .25 has a .25 engine and is ready to run
The SS has the .25 engine and hardened dog bones no radio, reciever or servos and needs to be put together.
The SS 4.6 has the .28 engine, hardened dog bones, stearing servo mod, spider gear mod, No radio, receiver or servos and needs to be put together