Farmtrac rear end rebuild

[Rosedude48]

Hi all. A friend has a Farmtrac 545 4WD. The internal support housing/pinion shaft bearing retainer bolts came out and trashed the rear end. He has parts ordered. I’m trying to understand how to adjust the bearing load on the pinion and also the differential assembly. I think I’ve got that understood,but the manual talks about shimming the thrust block on the left axle housing. Can someone tell me the purpose of the thrust block?