If I remember correctly an 8420 is the same baler as a Hesston 5530. Open the rear gate fully, in front ( tractor side) of the tailgate cylinders there is a hole on each side for a pin, about 5/8"x 5", insert pin, close tailgate to 1/2" of full close, the belts should now be about 4" loose. On the 5530 the belt length is between 362 & 364 inches. Please post back on your sucess with this problem and the baler. I hope this helps. scrapironford