What is the size of the Jam nut I need for the front control arm?

Having a problem finding the correct Jam nut to put on the end of the threads on the control arm to hold the wheel bearing and wheel on a 69 dart. I went to hardware store and went thru the test nuts sampler they have on the wall where u can find what nut goes on what shaft. i cant remember the size of the jam nut I bought but it is a little loose on control arm and will not tighten up on threads so wheel stays on. Anyone know what it would be? Beleive there are 2 different types of A body control arms with different threads.

Wizard