Here is a video from 2009 with my father showing me how to start his 1956 Cat D6 9U that has a "pony motor" (aka starting engine). I have a much newer video on my channel on how to start a 1942 D2 which is the same process but I go in to much more detail and I would recommend watching as well;    • How to start a Caterpillar D2 5J 1942  

Instructions apply to starting other similar models including D2 3J 5J 4U 5U and D4 models 4G 7J 2T 5T 6U1 6U 7U