the idea originates with David K Smith as a means of creating an MP15 switcher. He shortened both ends for his project, but for the F7b and F7a you only need to shorten one end.The chassis is still DCC friendly and is held together at the splice with 00-80 screws so it is quite robust. I created a plate that fits inside the F unit shell and made a cut out that is a friction fit on the modified GP chassis so they don't fall apart when picked up, but you have to put a notch in the plate to avoid popping the motor contact spring or replace it with a small jumper , so you don't disturb it when removing the shell. The toughest part of this conversion is removing one of the worm gears from the motor shaft and since the current batch of geeps have metal motor shafts instead of ceramic found in the original runs it is now easier to do this. The process removes a piece of the chassis while leaving all the support structure for motor and motor shaft bearings. There might be other picures aeound on trainboard but i do not have them bookmarked. http://www.youtube.com/user/jdk1928#p/u/49/0LcEV4N7-BA here is a video off the MP15 chassis made by David.