I just completed the 42009 Mobile Crane today. However, I'm not sure if I attached the boom arm into the vehicle correctly.
When fully lowered, the outward end of the boom does not touch the top of the cab (cockpit?), even though there seems to be a notch there to hold the boom. There's about a 5mm clearance between the top of the cab and the bottom of the boom. Is this normal? This seems to be a rather odd "at rest" position for the crane arm, since instead of resting on the top of the vehicle, all the weight remains on the two supports and the single axle holding the crane in place.