Had to think a bit about precisely why o-rings aren't the best choice to replace the stock square-section seal. Note the positioning indent on the exhaust valve body; this is the shallow hole in the body that can be seen through the hole in the compression tube. These holes when perfectly aligned mean that the exhaust port is perfectly aligned at the same time. So, when installing the exhaust body, you want to place a rod of the precise diameter that fits these holes, then, with that positioning rod in place, fully tighten the exhaust nut, without the exhaust body moving at all. This usually doesn't work with an o-ring... as I recall.