18hp waterpump question

[sdgloo]

1972 18hp with no water circulation. Pulled gear case expecting to find bad impeller. Instead found impeller drive pin sheared off and driveshaft slid down into pinion so pin was below pump plate. is this to be expected or what am I missing? I have the gear case open to reseal but am at a loss to understand what keeps the impeller pin above the pump plate?? Thanks

[boats.net]

The impeller key and impeller hold it above impeller plate and thats about it.

[sdgloo]

That's what I've surmised . Doesn't seem like the beat setup but.... Thanks