"Illegal" in the sens that pieces are connected in ways not intended by the designers. An eqample here is the blac brick in th, middle of the engine with studs on either side. The black 1x2 plates with cheese slopes are not completely connected to it, so I would classify it as an illegal technique, since it would not be used in a lego set. Too fragile.
That's a lovely build. I like the way you built the engine with opposing stud work. That kind of build technique always amazes me. When I started playing with Lego back in the late 70s, it was always bricks right way up. I build sets now, like the Delorean and ECTO1, and pieces are facing in all kinds of directions! 😄
