'94 Tracker 1.6 8 valve
Bought Thorley headers and took them to get installed.  After install the rear port leaked.  Thorley has a horizontal groove cut and the engine has a v-shaped indent to the right of the rear port that prevents the seal.  Pics attached.  Would appreciate any input.  It has all of us scratching our heads (no pun intended).  Thanks in advance...

Looks like a defect or someones idea of a modification, I don't
ever remember seeing a groove at all, the stock manifold has
a cast tube instead of a passage like the header does.

Even so the gasket should seal the leak, or are you using silicone
instead of a gasket???

Wild

I would take a a die grinder or a Dremel tool and clean each port up and the header flange up.  Then buy a good metal back gasket and reinstall. Tighten up the header bolt from the outside towards of the flange in.

Some folks use the white gasket material (graphite) commonly sold in auto parts stores and soak them in water to make them more pliable before installation and have good luck with this. I have always had good luck with OEM type gaskets.

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