I had to use two 5/16-24 nuts to get a better locking of the exhaust manifold.
I’ve never had to retorque an Chrysler exhaust manifold nut (or bolt) – so I’m not sure a double nut will help with that.
Now headers, I have had to re-tighten those fasteners several times on customers cars – but I’ve not seen anyone use those cone shaped washers before (on headers) – which is what Chrysler uses for both ends on small blocks (and /6’s, IIRC).
Now with that said about double-nutting the studs, I have used double nuts (or a longer nut) on vehicles before on 4x4’s or other vehicles that is exposed to more water (mud, etc.) than most vehicles do - to prevent rust on the exposed threads.
Not sure if it helps any – but it sounds logical to me.
Big blocks are somewhat similar. They use 6 studs (only studs) per side, but they are 3/8” diameter. Red loctite does work better for those sense 3/8” studs are not as likely to break as 5/16” studs are. Big blocks also use odd nuts for (some of) their exhaust manifolds.
This is a picture the manifold nut set on a ’68-71 C-body 383/440 HP.
You can see the 3 holes the long nuts go into, on this '70 HP manifold.
340 HP Exhaust manifolds use nuts that are similar (but 5/16").
On many of the big blocks, the studs also screw into the cooling jacket – depending on the casting – which is also why, I suspect, they used studs.
Unrelated – anytime you need to remove a stud, there is an official tool to do so:
The
unofficial stud removal tool is to use double nuts. Tighten the nuts together (using wrenches) and use an end wrench on the nut closest to head. It may take a bit of torque, but the double-nut version is usually non-destructive.
The official stud removal tool will scar the stud. If stud is to be used for exhaust, I do highly recommend replacing the studs, anyway. They do break and I’ve had to drill too many broken studs out to mess with re-using them.
I’ve not worked on one, yet, but Chrysler is using 8mm studs/bolts on the new HEMI’s – and you must walk on egg shells to keep those things from breaking off, on ya. 8mm is about the same diameter as a 5/16”, but it must be using fasteners from China . . . I guess.
BudW