D6 9U How to tighten fan belt?

I loosened the two nuts on the back of the pulley. I tried to hold the back and turn the fan but I had no luck. Is there some trick to this? Can someone tell me how to tighten it?
Thanks

To adjust the fan belt,remove the 2 capscrews and locks and turne the
adjusting rim while the fan. Screwing the rim toward the radiator tightens
the belt and reinstaled the capscrews.

The way I do it is to find some tool or piece of steel scrap that you can wedge between something on the engine or frame and one of the webs on the back of the pulley to lock it. Grasp the fan blade and pull/push as needed to hold the "stop" in it's place and turn the front half/center threaded part of the pulley hub.

You might want to brush out the threads showing behind the adjustable pulley half and then spray with rust buster etc before loosening the thing all the way off so you can do the same procedure on the threads that were hidden beneath the belt. Then when you have it all free and moving properly, adjust the belt tension and lock it down.

I have heated them with a torch, and tapped gently with a drift on the more substantial webs of the pulley just to get one moving, but I don't like to have to do that just to adjust one.

These guys have you on the right track, make sure to take the 5/16 bolts and the locks completely out, as you will rotate to a new postion where the locks will be re-inserted and bolts tighten up. As CC Jersey has explained, use plenty of pentrating oil on the sheave on the back side from the fan so that when you go to turn it will have less resistance. We use a spanner type pipe wrench to hold the back sheave while turning the fan with gloves on. The very late model 9U series above 28,000 sn did away with the old style sheave adjusting set up and replaced it with a double v-belt with and idler on it. There are not to many of those around, they also had hydraulic track adjusters which was another step up. The old set up works fine, just a pain to work on, we always take the hood off as well, makes it easier to see and get down in there. Mite want to consider putting on a new belt if the old one is in poor condition, could save you some grief down the road. Good Luck😊