bought an 855 newholland baler at auction vs 851 NH

Hold it a minute. There is an adjustment for that apron chain "clank" you're hearing. Needs to be checked/adjusted every 25 hours of operation and is often overlooked.

After you have completed replacing your upper chain guides:

Assure that there is 40 psi of air in the bags, open the door and lock it. Up front, under the sissor arms there are two large set screws, one on each arm. Loosen the lock nuts and screw these guys in a couple of turns.

Let the door back down and unscrew the setscrews until they just touch the baler frame. Lock the nuts in place. Now, that empty chamber "clank" should be gone.

Your bales will be twice the weight that the 851 made. Don't run it with more that 40 lbs initial air pressure because ya won't be able to lift the darned bales! :>)

Allan