the pullstart mount makes the engine sit higher on the chassis. and the bumpstart flywheel is smaller that the pullstart flywheel. I guess you need the extra clearance from the pullstart mount for the pullstarter to fit. but if you are going to use a bumpbox with your pullstart motor you will need to use the larger diameter pullstart flywheel so the bumpbox will be able to reach it to make contact. hope this makes some sense. I know what I mean, but it might not make sense to anyone else who is reading it!!