the semi circular plug you're referring to is called the 'half moon' key IIRC. yes, it is removable.

Its a small piece of aluminum that sits inside a groove. get it out by hitting the top of it on one of the sides and it will start to spin upwards on the opposite side. It's a little stubborn to 'break' loose from the sealant underneath, but it'll come eventually. Take caution when hitting it downward from one of the sides as it's aluminum and you dont wanna create leaks down the road. Use plastic, wood or something like that to hit it.

Once it's out, use a tiny bead of rtv on the groove and push it down on the groove. You can use a small piece of wood to lightly hammer it down in place on the head. RTV might squeeze out a bit, just wipe excess.

here are a couple pics with the first pic showing it missing, the next pic showing it installed.

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Outside of Corona: "That sucks, peace out..." (jamie walks away)... "Sweeet, what we doin?" -Zac
Tim's garage (prior to motor install): "Im gonna smoke real quick, that way if you fuck up I am already nicotined up" -Tim