6400kv is a pretty "hot motor" no pun intended. Motors with that high of a kv may not like to run that long before getting hot. Gearing is the key, and you really should use a temp guage to check the temp on the motor. "hot to the touch" really doesn't work with BL motors as they have a fine line between running all day and going poof from heat. Depending on the brand 170 deg is safe. at that temp you can't really tell by touching it....