Left.WAGON. Lower track nut. Right.WAGON. A solid metal or plastic block with a threaded hole in the center which is used to tighten or adjust the Door Shell. A solid metal or plastic block with a threaded hole in the center which is used to tighten or adjust the Sliding Door Track. Door Check Nut. Engine Cover Nut. and Washer Assembly -. Check. Engine Cover. Shell. Sliding.