Gents,
In my humble opinion: One can tighten the Plugs down into their place as tight as one wishes, however, water, will find its way - if its going to - though a split Hood for example down into the plug Cavity and around the Plug. I believe I have heard down through the Centuries - I mean Decades as these being called "Ever Wet Spark Plug Covers". Water finds its own Level - that is - full into the spark Plug cavities. I also find them too expensive for my tastes as well. But then again I remember Coke selling for 10 Cents a Bottle and getting a 5 Cent Deposit back for the Bottle. Don't get mad at me I just see these as after Market Gimmick Items. Restored Cars do not don them. Only in the buyer's minds.
Tom