643 Speedo Sender Leak

[freewill2008]

Hi Folks -

Seems like a leak has started where the speedometer sender mounts to
the Allison MT643. Anyone know what seals this mount? Looks like an
O-ring setup from the outside. Also - does the case need to be
drained to remove the sender?

Thanks
Bob Griesel '84 FC31 WLII WA