circular dependency (of components) is actually the exact thing im after. i didnt know it would cause problems for the compiler so i will tackle it another way.
as for combining them, a wheel for example, can function without an engine. if however an engine is present, the engine needs to know about the wheel and the wheel needs to know about the engine, they depend on each other for torque/angular velocity calculations. the only thing they need to know about the pawn they are attached to is its physics body and any player input. the pawn is not involved beyond that.
the thread topic has been answered very nicely by TheJamsh. if anyone else comes upon this issue they can now find the answer. im grateful for the knowledge but im going a different way with it and dont require any further help at this moment in time.
thank you all