Re: stripped thread in lower unit oil fill hole
When going to a larger bolt, consider a headless stud for Allen wrench. That would eliminate the head clearance problem in the well. But I don't know if the head is needed to secure a gasket though. On mine - well, the one I have removed and installed on a 9.9 Johnson and now retired, just had a slot in the top of the bolt/stud, there was no head, but maybe there was a seat at the bottom of the hole.
Vivacity 24 twin keel
Johnson O/B 9.9