There is a spacer approx two inches long with splines or teeth keyed to fit on the end of that starter gear. Make sure you have that in and facing the right direction. If i remember correctly it goes in on the starter side. Then I put the gear in on the primary side.
Its the shiny piece in this pic, but mine was dark and raw.
Black splined spacer in this pic. ^^^
Here's another idea of what it should look like put together.