Just got mine from Summit Today. It is a standard looking pushrod with 2 nuts about 3/4 of the way along the checker, used to thread it longer or shorter. However, mine refuses to thread. I first tried it by hand, then with 2 closed ended 7/16 wrenches, then I went all out and put 1 of the nuts in a vise, turning the other. The problem is that it is almost impossible to turn them. Did they send me a defective product or is that not how these things work?

I bought the exact checkers from Summit about four months ago. The nuts and screw-in bolt can be easily hand turned.
With the rockers and polynut locks on, I could easily turn the adjustable pushrod for both zero lash and the recommended cam lash. I then locked the nut with an open end wrench.
You definitely got a defective set.
Good Luck.
Ken

Thanks...looks like the engine will be without pushrods for an even longer time...

Make your own lenght checker.
Cut and tap a stock used pushrod, and tap it.
Install a stud and lock it into the longer side of the pushrod, and use the other side to adjust.

Use a dial calipers to check the lenght.

Or you can buy one of the comp style lenght checkers that are built like a Micrometer, which are nice if you dont have a long set of dial calipers.