The plastic pin that holds the drive wheel in place is an eccentric, it is used to adjust the drive force against the card.
The procedure outlined below is fully documented in the 97 service manual:
Disconnect the leads of the motor from the board and power from an external 3 volt dc supply.
Insert a card into the slot but do not allow it to be pulled through.
Holding the card, adjust the eccentric for 180ma +/- 20 ma motor current.
If you cannot adjust to that current, you either have or will have a drive problem.
The wrong size rings is a very common problem, especially outside the USA since the proper ring, size 005, is non-metric.