Stock modified you are allowed to widen the axle length it must be the original oem axle that came witrh the tractor. You are not allowed to change anything about its set up just widening it. Make sure to completely read the rules and build rules for the class you intend on racing in, so you have a tractor that comforms to our rules on race day. Your tractor will be teched when you arrive on race day so make sure it is a good to go build. Enjoy your build.