On the LS1 or LS6 valley cover, you have 2 big black plastic plugs that cover the knock sensors.

What do you use to make sure they have a good seal?

One of mine is loose; when I push it down it doesn't seal well.

I could buy new ones from GM, but can you seal them ?

Thanks

Pulled my motor and trans yesterday out the top. Picture shows the knock covers; the one closest to the front is the loose one, you can see it in the pic.

 

I used a thin film of RTV.