Mine was rotted off on one end and egged on the other. I happen to have an old stainless steel bbq grill grate slightly larger diameter to rob from. Cut a piece a little longer than original. Drilled the manifold to the correct size, slotted the one end of the rod for the spring. Ran the same bit through the flap as it needed just a little bigger hole. Mine was originally drilled and riveted so I did the same(as you install). The counter weight was enough oversized without drilling but its installation in relation to the flap is important so make note before tearing the original one apart. The weight should pull it to a fully open position and the spring will tighten and pull it up closing it when it is cold.Drill shaft for the counter weight and use a roll pin or cotter pin. With the extra length I mentioned, I made the slot for the spring deeper, so it does not fall off. Bench test it using the propane torch on the spring to make sure it opens correctly.
