Originally Posted by PacWestGS
Don't fill it all the way up - leave some room for expansion above the dent. DAMHIK.
Probably a good idea for me to explain how to do the freezer fix. You will need a chest freezer that the pipe can be set up in so the dent is at the lowest point. Remove the header pipe from the bike and pour water in it so the water is a few inches either side of the dent but not within a few inches of the open ends. Put the pipe in the freezer so it's supported in that position and leave it overnight. Then simply take the pipe out and leave it to thaw. You might have to repeat the exercise if the dent has "sharp" edges and you may not get it completely out but it does make a large difference.
The idea is the "open ends" of the water column freeze first and then the rest. The frozen ends form a "plug" of ice. The water freezing inside the pipe between the ice end plugs will force the dent out.
Good luck. "All care and no responsibility" from me, so don't be trying to claim a new header if don't work
. That's called "bad luck" and you probably just wanted an excuse to buy a new header anyway
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