Putting new axles in and one spider gear came out

[AussieinNC]

I have seen gear sets both with a concave washer and without....
the washer is used to set the gear backlash and occasionally to take up wear points in the housing....

The washer is the size on the rear face of the spider gear and is only about 1/16" in thickness....double check the interior of the spider housing to make sure it hasnt rolled around as the gear was pushed back in...