3.9MM Motor shaft but 5MM bore gears and pulleys?

I'm a Newbie here, and I'm sure there's a simple answer to this,,,
Why do the 6-12v motors have a 3.9MM shaft, but all of the gears are 5MM? When I tighten the screws of the gear (onto the flat portion of the motor shaft) The gear spins off-center. How do I get the 5MM bore gears and pulleys centered on the smaller 3.9MM motor shaft? Do I need a bushing?

Perplexed on this! Thanks for any advice!

Yes, you need a bushing. NEMA-17 motors have 5mm shafts. A lot of the pulleys and gears are made to fit NEMA-17 motors. Are you using a smaller motor?

Thanks for the reply. Yes. I'm using 6v High torque motors that are only 60RPM. The shaft on all of these is 3.9(4MM) But none of the gears and pulleys are 4MM. So I guess I will try to find a bushing that is 0.5mm.