Top Nut size in 5160?

Hi everyone, I just installed 5160’s this afternoon and was wondering what the top nut size on the shock? I guess my rubber bushings are completely compressed, I was able to tighten down without hardly any problem. I hit them with my torque wrench to 21ft pounds. I’m thinking of Adding a second nut as a jam nut on top.

 

I just checked mine, 18mm fits well.

 

I see this is an older thread but was just wondering how you were able to torque down the upper nuts to 21 ft pounds without the shock turning. I dont see any type of flat spot like on other shocks I've installed to keep it from doing so.