Technical inline 6 lower manifold stud help

i have a 68 c10 the nuts are missing where the donut connects to the manifold does anyone now the size nuts to buy i went to my local hardware and got acouple different sizes but none work thanks

 

half the time the stud rust from the heat and shrink in size , but retain the shape of the threads , IIRC the manifold to pipe flange connection studs are 3/8 -16 dorman sells the new ones you have to heat the manifold with a torch evenly while keeping the studs cool to remove the old ones or have a muffler shop burn them out and replace them .

 

mine are already off i just need to know the size of the nut that goes on them and thanks for the help

 

run a die up the studs and try the nuts you bought again. if the threads are "weak" do yourself a favor and change the studs like stimpy suggested