Barrel Nut Thread Diameter
 

10mm

So yo must be talking about the head stud or starter barrel nuts - not the 8mm ones on the intakes (3.2's) or exhaust I take it...
-Chris

Yeah. I was going to the fastner shop to get a couple pieces to double-nut when I take the head studs out. But last night, I removed the valve covers and found 4, yes 4!, studs and barrel nuts laying in the cavity. The most disappointing other than two broken on the same cylinder is that one of them looks like it broke flush with the case. When I get the heads off, I'll know how ugly this can turn out to be.

So, the starter barrel nut and the 3.0 headstud barrel nut are the same threads.

Tony