They're 0.020, says so in all the catalogs I've looked at.

They've been used since time began by rebuild shops. I believe that FelPro would tell you if they wanted some sort of a sealer. But you're right, the instructions say to just use them with a standard gasket, no mention of any kind of goop. Probably they're so thin as to not require any.

If you need to use more than one per side IMHO you're better off with a custom gasket, at $30 each plus a new head gasket you're in Cometic pricewise.

R.