A tape transport for a leader block tape cartridge has a threading mechanism including a threader pin for engaging the leader block and extracting it from the cartridge after it has been loaded into the transport adjacent a first end of a tape path. The threading mechanism pulls the leader block along the tape path past a read/write head, and inserts the leader block into a take-up hub adjacent a second end of the tape path during a threading operation. During a rewinding operation, the threading mechanism extracts the leader block from the take-up hub, pulls the leader block past the read/write head and inserts the leader block back into the cartridge. A first guide assembly adjacent the first end of the tape path engages and guides the leader block to ensure proper insertion thereof back into the cartridge during the rewinding operation. A second guide assembly adjacent the second end of the tape path engages and guides the leader block to ensure proper insertion thereof into the take-up hub during the threading operation. This accommodates the tolerance variations in the positions of the cartridge and take-up hub, and in the dimensions of the cartridge, leader block, take-up hub and threader pin which can otherwise lead to failures in the automatic insertion of the leader block into the cartridge and/or take-up hub.