CIH 863 corn head question

Need to replace some gatherer chains. Looking at the tensioning apparatus and think I understands it pretty well. One question that has me puzzled and I have not really started much on it yet...just kinda getting the lay of the land here. So the lower chain sprocket is attached to a plate that has two rods that go rearward thru the casting where they hit a plate. This plate has a large bolt...looks like a carriage bolt...that goes thru the center of it and has a spring and spring shield that it also travels thru. On the combine end of this bolt is an adjusting nut. The head of the bolt pushes up against the casting and as you tighten the nut it pushes against the plate, compresses the spring, which moves the two rods back thru the casting...eventually moving the sprocket and tensioning the chain. I think I got all that. My question is, what keeps the bolt from spinning when you start tightening down the tension nut? I can put a vice grip on the shoulders of the bolt but that is only going to work so long. On CIH parts diagram it shows the bolt having a square shoulder below the head. Is this supposed to jam in the slot in the casting and hold the bolt while you tighten the nut? Hope this makes sense. Appreciate any help.

Yes.If it is the correct bolt it is a carriage bolt. Square embedded in to a slot on the cast.

Tighten those bolts till there is 1/4 inch of clearance between the nut and the pipe bushing over the bolt and spring. This will give the chains the correct tension.

Also check to see if the two rods are free with a big pry bar or hammer against the sprocket to see that they bounce in and out on the spring.

Gary

Got it, thanks! I have to replace about half the chains.