Its been awhile since I had either our 1600 or 1855 apart but the sprocket on the transmission is supposed to float and the one on powershift is held on by large nut. After Grandfather bought our 1600 we had it apart for a throwout bearing and found that the splines on the transmission input shaft was almost wore clear off. What could have happened to the nut would be a mystery since the pto drive runs through that coupler so it can't fall out if it did get loose and come of shaft.