Spark plug hole issues

[Waveblaster]

Hello

I have been working on my cb77 cylinder head. As mentioned in another thread someone previously has cut the spark plug holes out by 2mm. I managed to cross reference the thread size and find plugs

The issue I have is that there is a small gap opened up into the casting above the steel skull cap. I fed a piece of stiff wire and it is quite open. Perhaps a flaw in the original casting.

I am hoping that there is no oil galleries and the best I can tell this is the case.

Has anyone seen this before? I am hoping to leave or perhaps fill with some liquid steel two part bonding material. Not sure if the heat will be an issue.

The threads below are fine.