The primary drive assembly in the small frame motor also includes a set of springs that function as a kind of shock absorber in the more extreme conditions as part of the drive train within the gear box. If they are worn or even broken your gear box will behave erratically and they will need replacement. Primary drive repair kits comprise of new springs, retaining plates and mounting accessories such as the appropriate rivets to hold it all together etc. Performance enhanced motor tuning projects should include a strengthened primary drive repair kit.
Conclusion: Your primary drive assembly should be renewed during every major gearbox overhaul or service.