The maiin gear take some rime to assemble.
There are lots of play in the joints. They recommend that you squeeze the gapes togeather but then you get metall to metall. Therefore I used Orings at places with play. It got tight and I'm sure atleas one Oring will break. if you can find something slightly thinner it would be even better.

I use the optional DuBro 6" wheels instead of the original.
For me the shaft did fitt through the holes on the DuBro rims but the wheel collar did bind after I assembled both halfs and the wheel wouldn't turn.
Instead I used the original rims with the bigger tires.
In order to reduce the play I used a 2mm thick washer on the long side of the shaft before I screwed the halfes together.