the way i do it i take a reg copy paper, 004 in thick, cut a strip as wide as the gear. the back lash should let 1 thickness go through and be VERY wrinkled. then use a receipt paper, .002 in. it should be less wrinkled. the gears should spin free. dont over tighten the engine screws. general every time you flip your truck, you should check the gear backlash, cause most likely the engine will have moved.