Can anyone tell me what this plug is for? There's a slight leak that I'd like to repair, but first need to know what it is.

Thanks in advance for any help.

Sorry, perhaps I'm missing something, but I can't tell from your pic what plug you are referring to. All I see are through-bolts?

- MR

We're talking about the nut at the upper right of the photo right ?
That's not a plug. It's a stud that extends to the other side of the block.
It is a common source of oil leakage.
The real issue is when someone over-tightens it and pulls the threads....
waay over on the other side.
I recall there is a way to sneak a helicoil in without splitting the case.
Google it.
Good luck
PS it is far more common on mag cases.
Your aluminum engine appears to have already been split, any idea why ?

Thanks RGalla. That's the nut. In fact it was loose, so I snugged it up but not over-tight. The leak is minor. Let's hope this solves it. It's correct engine for 71 914-6, so it's mag (photo makes it look alum). Engine was rebuilt, but previous epoxy attempt to stop leak at case halves was not removed during the rebuild.