A question about a 99 cummins

I noticed a 2 inch crack in the exhaust header with some blackening from exhaust coming out of about the 3rd cylinder from the front. Will it hurt
to run like this or could cold air be drawn into the valve when I shut it off thus warping the valve or some mechanical unforseen nightmare like that?
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Time for JB Weld.

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Not on an exhaust manifold. J-B Weld's site says their product only resists temps up to 500F.
It needs to be brazed, and needs to be done by a competent weldor/metalworker.
The alternative is to take this opportunity to replace it with an aftermarket 3-piece manifold that won't crack due to the heat cycling.
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