351C thread question

[GrantOv]

I thought this would be simple - just going to secure down the wire harness that goes from the back to front over the drivers side of the engine. There is an unused threaded hole and I've seen others use this for just this purpose. However - I'm puzzled to what size thread this is. Threaded hole is bigger than 1/4in and smaller than either coarse or fine thread 5/16. Am I missing something?

 

[Stanglover]

I just checked that thread on my engine. It ought to be 5/16-18 BUT it is tight with a tap and a bolt. I don't have time to play with it right now, so I suggest chasing it with a tap. It is probably tight due to paint and or crap build up. It won't be metric for sure.