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Your head is warped.
You can check it on a perfectly flat surface such as a glass table and looking at it from underneath.
You can also easily fix it with a sheet of fine wet/dry sandpaper (using it wet of course) laying on a flat surface, and moving the head in small circular motions across it. You'll know when it's flat because the sanding marks will be even across the surface.
I grew up racing motocross and really abused my engines in the heat, and rebuilt them often. This is pretty common because the heads are aluminum and warp easily.
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