Yeah, Pentair has its own weird (wider-spaced) threading. Hayward may have its own, too. All the standard and generic companies seem to have the same threading, however, oddly enough (maybe it's the same as Hayward?).
But I am dealing with old Pentair unions - I went to put in my new non-Pentair pump and, lo-and-behold, the unions won't screw onto it. I looked like heck for an adapter to screw onto the pump (I found a discontinued one that is the opposite of my situation - ie, if someone had a new Pentair pump and the existing unions didn't fit.).
I'm looking at split-nut unions, though it's hard to tell if they're quite right. I also have the union slip plumbed directly into my 3-way valve so, like you, I'm leery to go a-heat gunning. It's exasperating to be stymied by a lousy union.