A magnetic tape cassette helical scan magnetic tape recorder has a magnetic tape threading and transport apparatus including multi-surface, multi-radius air bearing blocks for transporting magnetic tape in a helical path around a tilted rotary head scanner. Vacuum walls located on either side of the rotary head scanner assist in threading the magnetic tape from a magnetic tape cassette around the rotary head scanner. The air bearing blocks are moved into position contiguous to the rotary head scanner after the vacuum threading operation is completed. The vacuum walls have longitudinal slots which are substantially coextensive with the extent of the air bearing blocks. A vacuum sensor is positioned at the end of the slots away from the rotary head scanner so that the sensor is only actuated when the tape is against the whole length of the wall. This prevents damage to the tape when the air bearing blocks are moved behind the threaded tape.