Ok, so here's what I've got. Super Six manifold. It's been machined flat. I'm using a Remflex gasket. All good there.

Starts and runs well. Problem? Once it's hot, idle gets rough. Temps run higher. Ping is noticed under WOT.

Ping not there when it's cool. (Cooling system is all new, flowing extremely well)

So here's the issue with the manifold:

All hardware is fine EXCEPT the final stud that holds the Exhaust manifold to the Head. That stud is flakey, at best. It's also one size higher than OEM, mainly because someone tried to helicoil, failed and then put a bigger one in.

The stud is there, and basically acts as a plug for the coolant. I pull the stud? Coolant flows.

Now, I tried a helicoil-OEM setup myself, and there was no go. Just simply could not get the stud to hold 10lbs of torque.

Managed to get the large stud sealed in, though. Problem? It doesn't seem to hold 10 lbs torque, either. It just won't tighten to that level.

I guess my question is, could this result in an Intake-At-Head leak??? The hypothesis I have is that whenever the engine gets hot, the intake starts leaking. This is confirmed with wonky vacuum gauge readings.

Can I fix this without messing with the stud? I.E. progressively increase the torque on the manifold studs at operating temp (i.e. from 12 to 14 or something)?

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63 Plymouth Valiant V200 Convertible.
225, HEI, Super Six with 38/38 Weber, Electric Fans, Scarebird discs, FirmFeel front swaybar.

Is the flap for the heat riser working properly?

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Official Cookie and Mater Tormentor.

I'll have to check again, but I believe so. I replaced the manifold last year with a newer 72?+ manifold. The one with the round wheel.

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63 Plymouth Valiant V200 Convertible.
225, HEI, Super Six with 38/38 Weber, Electric Fans, Scarebird discs, FirmFeel front swaybar.

Checked it. Definitely not a stuck heat riser. Give it a rev and the wheel moves freely. It's such a new manifold that I can't see how that's the problem.

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63 Plymouth Valiant V200 Convertible.
225, HEI, Super Six with 38/38 Weber, Electric Fans, Scarebird discs, FirmFeel front swaybar.

are both manifolds machined including the back side where the hardware presses? large mismatches in thickness between intake and exhaust could cause what you are experiencing. for the stud Keenserts usually run larger than helicoils and might give you the opportunity to fix it more correctly.

Your manifold stud issue is wholly separate from your rough idle/high engine temp/ping issue. By all means fix the manifold stud if/as/when you can, and check to make sure there's no intake manifold-to-head vacuum leak by spraying carb cleaner at each intake runner/head junction with the engine hot and idling roughly to see if it smooths out, but the running problem will probably be found elsewhere. Look for carburetion faults and exhaust system restriction.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

"Ok, so here's what I've got. Super Six manifold. It's been machined flat"

So the intake has been machined flat. Was the exhaust manifold machined also? Reason I'm asking (even though I think slantsixdan is probably the correct one) is - is it possible that when the intake was machined that now the 3 bolts that attach the intake to the exhaust are now at full "slop" travel and the intake is still ever so slightly further away from the head than the exhaust which is tight against the head? The sideways travel of the bolts has been all taken up already due to only the intake manifold being machined flat? Exhaust manifold heats up, expands and then pulls intake manifold away from head? Krazier things have happened.

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Tom
'86 Alfa Romeo Spider, red "Dinsdale"
'10 Corvette, red "" (no name yet)
'95 Ferrari 348, red "Zoom"
'04 Maserati, black "Evil"
'05 Aston Martin DB9, green "Bond, Treasury bond."
'82 Dodge W150 Power Ram, yellow "E. Valdez"

When a helicoil install fails or if you need a better solution.
I have used this many times to save antique or rare race engines heads or manifolds. Obtain a brass pipe thread plug. Near the head (wrench end) saw it approx. half way through. Remove the no good helicoil if it is still there and drill and tap the hole for the NPT plug. Using a sealer (Locktite or even a thin smear of Jb weld) coat the threads and screw the plug into the hole till the top breaks off (you cut it remember) File or machine flat. Assemble the parts using them as guide drill an original size hole where it needs to be in the brass plug. Tap desired thread in the hole. Using a sealer install a stud if possible or a bolt. Part is now is good as new and will NEVER give trouble again. Since this site has a mopar connection I will mention I have also used this to repair header bolt holes in 340 X heads and such. Not once ever has such a repair failed. Some I did now almost 20 years ago and I still see them at cruise nights etc. You could use a cast plug instead I guess but brass is sufficiently strong and being a bit more malleable conforms nicely to its new home so I would and do prefer it.

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8 cylinders are for folks who cant make HP

Rather than make a threaded insert with an NPT plug, just buy a threaded insert..................

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

Thankyou Ed!

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Yeah....Im the one who destroyed this rare, vintage automobile.....

plug, just buy a threaded insert..................
Doesn't even come close to the same quality repair. However tis up to you for sure. Thread inserts even helicoils have their place but when you have a wet hole and bad threads and a worn hole the NPT plug will restore it to original condition with out the leak issues.
Anyway Tis what I use in those circumstances and after 45 years of doing this for a living I am happy with the results it gave me in what would otherwise have been an impossible situation. So are my customers.
don

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8 cylinders are for folks who cant make HP

DKD Tapered pipe thread would seal the hole better than straight thread. http://en.wikipedia.org/wiki/National_pipe_thread

Y'all do know that there are studs available with different sizes on each end, right?

It shouldn't be too hard to drill/tap an exhaust manifold hole for 3/8-16, get the proper stud from McMaster-Carr, (3/8-16 x 5/16-24), seal it into the hole, and fix the problem.

Roger

Thankyou Roger!

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Yeah....Im the one who destroyed this rare, vintage automobile.....

Good point on "double" ended studs. However in this case I doubt a 3/8-16 (uses 5/16 drill) would work in hole that already has a "bad" 5/16-18 helicoil in it (uses larger size Q drill- same as for 3/8-24).

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63 Valiant Wagon
225 - 4 bbl