The Chrysler rocker arm on the 273 and the 70 340, 6 pack do not have a lock nut they have a varying thread that causes the tension between the adjuster and the rocker arm that holds the adjuster in place when you adjust it. Now the thing that you do is you tap those rocker arms out to straighten those threads out and you put in the ones that use the nuts to lock them down. But you also want to take a flat file and dress the top of that area on the rocker arm where that nut sits otherwise it's going to back itself off. Here is the kit from jegs.
PRW 1203188 Rocker Arm Valve Lash Adjuster Screw Kit Mopar 273-440 4135 Alloy St | eBay
