Cracking sives. Have been recently informed that the factory has no system in place for squaring the sives once installed. The bad boys can be WAY out of square as was demonstrated to me on my own. Yes always keep the bushings up to par, and also keep in mind that the shaker drive slip clutch needs to be kept tight. Not the two slip plates to each other so much as the back sides of them to their perspective hubs. I have found that some JB weld or liquid steel applied in the slots that the four lugs on the back side of each slip clutch plate helps AlOT with longevity. See each time the sive hits the end of the stroke it will place stress on the slip clutch plates and actually wear out these lugs, however not before the slop in the hubs causes excessive movement of the sive itself, resulting in undue stress and fractures or crackes. This also adds to cracks in the mounting as indicated by the article mentioned in the MOD'S section. So, I would start with the simplest, check the square of your sives when they are installed and tight. If they are not square, you may have to loosen the bearing holder on the left hand side of the machine. This is the bearing holder on the channel below the eccentric bearing, not sure of the proper terminology, but not the eccentric bearing itself but the one just forward and below it. This adjustment may not be as simple as it sounds as it is not intended as an adjustment. This resolved my problems on two 1480s and it will be done on my "new to me" 1680.