Hi Beammeup, presuming you have adjusted the cylinder to bedknife clearance correctly (it affects chain tension) loosen the nut that attaches the long chain plastic tensioner and push it down to give some slack in the two chains that run around the cylinder cutter sprocket.
Look behind the largest sprocket and undo the nut slightly that holds the bracket to the side frame and do the same to the lower nut of the same bracket and slide the large chain sprocket upwards until you reach the correct small chain tension...about 1/2" play will do it...lock up both the nuts firmly. Slide the plastic large chain tensioner back up so as both the long chains are tensioned correctly and lock the nut up firmly.
Re-check the cylinder to bedknife clearance again and if all OK, replace the chain case cover and you're done.
