The spur gears come rounded in the center, just big enough to fit around the slipper shaft. When you put a new one on the shaft (gear only on the shaft) it should be able to spin around. Otherwise the slipper would never be able to work.
The center hole can warp and grow under excessive heat, which certainly would be caused by too loose a slipper.
Here are the parts that should be on the slipper shaft after the drive pulley:
slipper plate, spur gear with the six pucks in, second slipper plate, a small shim or two to adjust end float, a bearing, a wider silver spacer (around like 5 or 6 mm wide), the wide plastic spring holder, the spring, the thin plastic spring holder, and the slipper nut. If you have all that stuff on there, you should be able to completely collapse the spring by tightening down the nut. From completely collapsed, back the slipper nut off about 1.5 turns, and use that as a starting setting. I wouldn't run more than about 2, 2.5 turns off of completely collapsed. If some of those parts I listed above are missing (I'm thinking mostly of the wider silver spacer), you would not be able to collapse the spring fully because the threads on the shaft would run out first. This could easily lead to a too-loose slipper setting.
Happy Birthday!
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