Technical Anyone know what tapped thread Chrysler screw in balljoints are?

How old? Early A-bodies through 1972 used the smaller K704 UBJ. The socket head on those is 1.809.

 

by "old" I meant 1971.

 

Close to beating a dead horse here, but...

Not all threads conform to some industry standard. Some are just #$%$# weird. I assume these shallow triangular threads are to force-form ("self-tap") into holes, more or less for one-time use though us fools continue to do factory-unapproved things to them.

None of the threads in these photos is "standard". The ones inside the trunnion are ACME-like, but unique.

To OP -- when those threads strip out of the arm, the arm needs to be replaced, not re-tapped. It wasn't tapped, really, in the first place. It's a form of press-fit. See the photos below for the Nashcan/Rambler equiv.


Grandpa Nash's idea of a lower control arm, post-war. The dreaded trunnion at bottom/front.




Here's what it should look like, in a used but moderately OK arm half. The cap is on the wrong side of the arm, so you can see the threads.




About half of the ones I see look like this one below: stripped out. First, overtightened in the first place (gouged by the hex). Second, the spacer/stiffener is also the shock eye mount, a unique-to-Nash special shock not obtainable, so dimwits remove the spacer, use an AutoZone shock and a 1/2" cross bolt and guess what, without the stiffener the arm parallelograms and torques out the trunnion caps.



"Rapid unscheduled self-disassembly."

Metal has been removed, as well as reshaped. The part is junk and cannot be reused.

(Yes, the suspension pivots on the threads inside the tool-steel-hard trunnion. It's insane.)