What's the deal with Delco Spark Plugs?

[71CHEVYSHORTBED402]

71 C-10 402

Getting around to replacing the spark plugs on the project, and have to ask, how can spark plugs be difficult?...............EDIT, Because, I forgot my heads are off a late 60s HP 396The rest is nonsense.......

I picked up Delco R44T from Napa, and checked out the part number, but I can't start the threads. The old plugs (Autolite) and the Delco are a 9/16" fine thread 18, but if anything the old threads are cleaner. They install with ease, and no issues since originally installed, 15-20 years ago.

Delco is a shorter thread and doesn't take a lock washer I guess. I don't recall seeing them without one, but I suppose they work - assuming you can install them to begin with

So what the heck? Possible the Autolite compromised the threads in the heads somehow? I've installed 3-4 sets of new plugs since I've owned her, never a problem.


Edit: Interesting. AP 404 Autolite doesn't cross reference to R44T, same vise versa. Something is peculiar. Ah SNAP!, I just remembered my heads are HP off a late 60's Corvette or Camaro. I wouldn't expect so, but that's got to be it.