Anyone Know What Size Hex These Are?

[BadAmish]

Hello World,

I don't have a good assortment of hex wrenches to try but my 8mm seems to fit - but pretty tight - so I bought a 5/16 inch hex thinking that must be it and it also does not fit perfectly. I've already rounded one bolt a bit so I thought I would try asking you folks.
Here is a picture of the bolt. They are all around our HMMWV - in the bed and also inside the cab and most hold D rings to the floor.

 

[Guyfang]

CARC paint will make it a tight fit. Have you looked it up in the parts TM?

 

[Mogman]

Those are USS threaded bolts so 5/16 would be the listed size, having said that 8MM is a better choice as it would be tighter. ( would rather fight the 8MM Allen socket to free it from the bolt than to strip them)

I have had these that are a bit corroded and hell to get out, some heat and penetrating oil on the nuts/threads will help, sometimes you just have to cut them off underneath with a torch.
I bought a lot of 100 of these flat head bolts so I always replace them.