FYI,
I have a Flip-A-Card with all original parts but everything has been cleaned. As I recall the rubber rings I got from PBR were too floppy and I dropped down a size. My sling coils are obviously original parts and are more than powerful enough for the game. Krynski's use of asymmetrical slingshots is just pure genius and add to the wacky fun of this game. I also set the switch contacts to close with very little movement of the rings, so the pulse from the coil really smacks the ball very very quickly. I do not see any need to try to make the stock coil any more powerful than it is unless it does not have the correct resistance, and at that point i would just invest in new coils from your favorite source. I did switch to yellow dot coils for the flippers just for the fun of sending the ball up to the top of the playfield at rapid speed. I drove a 3500 mile round trip to get mine. Playfield is badly worn (I think the paint that was applied to FAC and Card Trix was too thin) but the game plays good as new. I also have a Card Trix but it is on "permanent" loan to a retirement community.