Thought you slathered them with anti-seize compound or Dielectric grease...

 

Standard hex head for a 1/4" SAE is 7/16" across the flats.

11m/m and 7/16" are essentially interchangeable with respect to wrenches. A 13m/m can be used for a 1/2" wrench, but not always the other way about (1/2" - 12.7m/m) A 19m/m and 3/4" wrench are nearly interchangeable. There are other sizes that can swap back and forth, I just don't recall them offhand.

I think you'll find that a 7/16" wrench will fit neatly on those heads as well.

 

The ones down the longwise rail are 1/4"-20x1/2", the larger ones at the ends are 5/16"-18x3/4". Flanged hex heads are, as I recall, OEM - but I've used socket heads with flat washers to great effect (it helps when you have "sprung" the oil sump rails, and need to "encourage" them back into line. You can see what you're doing, and apply more direct pressure with the internal wrenching head...)

 

Thank you sir...I noticed I'm missing one along the long edge. No leaks though.