4l60s will break the rear planet your gona have to take the trans apart older 4l60s used a 4 gear planet the newer ones now have 5 gears. you will have to change the low reverse piston if you dont have the 5 gear planet to make it fit give me a call we stock these parts here if you need them. i have seen this several times especially when your puttin some power to a 4l60.518 505-6536 junior. hope this helps