Quoted from golfergordy:There's a Pinside thread on this subject of the Stern auto plunger problem: https://pinside.com/pinball/forum/topic/stern-auto-plunger-problems
I've read it and there doesn't seem to be any universally agreed upon solution. No one suggested the shimming solution I've found so I posted it in the thread and one guy who apparently has been struggling with this problem stated that he would try it, but hasn't yet posted his results. My post here #16462 didn't include a pic so I'm including one now, to show where I placed the washers (black arrows). Before shimming, I first tried adjusting the prongs, as you allude to, w/o success. Shimming the auto plunger ass'y with washers immediately worked and still works fine 2-weeks later. The red arrow in the pic shows where I placed a sheet metal screw to replace the Stern threaded stud which I broke tightening the nut while reattaching the auto plunger ass'y after inserting the washer shims. I think the Stern threaded studs are easily broken when tightening the nut, so be warned not to tighten very much.
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I usually just use needle nose pliars to either space or slightly bend each prong so that it makes full contact throughout the rotation of travel of the actuator (This is important and I measure by rolling the actuator manually - with the power off- and making sure each side contacts the ball at the same time and to the same extent). If you're shimming the actuator assembly, you're probably imparting some spin or english to the ball as well, which may help it go around but it's probably losing speed and or changing the dynamic path the ball should take as intended off the rails. If you squeeze the prongs together slightly with a needle nose, you can restart and adjust as many times as you like (as long as you're not bending too much to weaken the metal) and then insert the pliars between the prongs pulling apart with equal force to widen the gap, or bending each one individually.
This technique also helps bring the ball closer to the manual plunger and assists with manual plunger power by sitting the ball lower in the shooter lane closer to the rod tip.