My son never did the really gross stuff, but as he got older, he's gotten into explosions. He will mix stuff together to make it explode, he's made a potato-gun, then a potato-canon ... so loud that I was sure one of the neighbors would call the cops. Of course, this makes him extremely popular in our neighborhood... Oh, joy... Now I have a garage full of kids (yes, boys) all looking to blow stuff up.
I would take this as a sign your sons are perfectly normal. I'm convinced every boy goes through a pyromania phase.