As I was removing my thermostat housing today, I noticed the lower bolt was very sooty. Also, the short hose between the water pump and the head has a black sooty spot on it. These parts are close to the #1 exhaust manifold. Does this indicate an exhaust leak? If so, what should I do?

Why I was removing the thermostat: I'm troubleshooting a ping/knock going uphill. Thinking maybe a lower operating temperature would stop the ping, I swapped in a 180 degree thermostat instead of the 195 which is stock (it's a 1974 engine, stock, with 86k miles). It didn't make any difference.

Looks like you have discovered the reason for the noises that you were looking for.

check the manifold nuts are not seized, and preferably loosen one by one and replace them with a suitable heat lubricant on the threads.
Tighten to the correct torque, get engine hot and re- torque. Don’t go crazy, just to the manual specs, or you will have broken studs to fix.
Fingers crossed that this works for you, but if its been doing it for a while it will be manifold off time, and at least a new gasket after checking surfaces for straightness

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1967 VC Valiant

Once gasket is compromised ether by warped exhaust manifold, and or incorrect torque of manifold nuts simply re-torquing probably won't solve the problem because a hole has been burned into gasket. The good gasket Remflex 6008 will generally accommodate the misalignment due to warped castings. It's pricey, but you won't be bothered again with leaks at intake & exhaust manifold.

Be sure to replace or run a die over any rusted manifold studs as needed so proper consistent torque can be applied to all nuts during instillation. Use copper anti-sease. Any new studs replaced will need its hole chased with a tap and sealed (blue Lock-Tight) to prevent coolant leaks and their backing-out from heat/cooling cycles. Clean both head and manifold mating surfaces of all old gasket, and with a file or sanding block clean up exhaust legs of rust facing gasket for a better seal.

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67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC