It's just a press fit onto the axle, like the rear of a Bit. BTW the Puchi's front axles are not knurled (if you've taken apart a BCG rear axle, its those bumps on the end) but the rear ones are. so when your taking the rear ones off just turn them a bit and they should come off easily
so yeah you just push them in and they should be fine...... but if you want to make sure that there on properly and wont wobble (namuna taking notes on this??
) use some epoxy or super glue for asecure fit.