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What created the condition that cracked this 1956 Thunderbird exhaust manifold?
If your answer was "3" you are correct. Probably the most over looked item on 1955 & 1956 Thunderbirds is the vacuum advance mechanism. It is the only control that advances the timing. When it fails, the engine runs at the setting of the distributor, which is 3 to 6 degrees B.T.D.C. When the vacuum advance mechanism is working properly, as the engine rpm increases the distributor timing also increases. If the timing does not increase the increased fuel does not properly burn during the compression stroke and finishes burning inside the exhaust manifold resulting in a loss of power and excessive heat inside the exhaust manifold. If you doubt this, disconnect the vacuum source from the carburetor to the distributor and run the engine at 1500 rpm for a few minutes and watch the exhaust manifold turn red (not advisable). This condition can also be caused by a stuck distributor plate or loss of vacuum from the carburetor.