What does this strut nut/bolt do?

[Dragster]

I installed springs last weekend, and ran into an issue where the upper strut nut would spin and not allow me to torque it down until the strut was mounted into the car. In the process, I noticed that, while I was messing with the spinning nut, the threads showing between the upper strut nut and the very top inner nut/bolt thing (pictured below--notice that there are 3 or 4 threads showing between it and the upper strut nut; sorry, I don't know the proper name) were different between the one I was working on and the stock strut that was still installed on the car. I was able to make them eyeball equal by using one wrench to hold the upper strut nut, and another to turn the very top. My question is: what does this do? I couldn't find any mention of if in the instructions for the springs, or any of the install videos. Hopefully I didn't mess anything up, but if I did, I'll just take it to a competent shop and have them fix my work.

Thanks in advance for the help!

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[DickR]

You had the strut out of the car and removed the strut rod nut, upper mount, upper spring seat, and spring from the car. Correct?

You may recall that the smooth portion of the strut rod is larger than the threaded portion. The hole in the upper mount is smaller than the strut rod. Therefore when the strut rod nut is properly tightened it clamps the upper mount securely to the strut rod.

It is normal to have to use a wrench on the top of the strut rod to keep it from rotating while you tighten the nut. Conversely it sounds like you held the nut and rotated the top of the rod.

 

[Niz55]

10mm socket and an open end wrench

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