I bought these as a set of four about a million years ago and only used two. They are made to use an o-ring on one side OR just use the flat side and some high temp liquid gasket (or make some small gaskets). I used the flat side. I send them to you in 48 USA only and you paypal me $1 to cover shipping after you get them. If you don't know what PAIR block off plates are don't ask for them.