Um, I suspect you're right John, I need a counterbore to hold the cap-head of the bolt...
Damn these pesky machining terms. Look, I need a hole, that's big enough for the head to go into, but not all the way through.... the upright section is drilled 10mm clearance, the sump itself is drilled 8.5mm tapping (& then tapped m10) to take the cap-head bolt, that.... you get the picture...
Anyway, a new sump post will be coming right up... just got to fettle the pictures & write the text.