No spec on the height of the threaded portion just the torque on the bolt. Something like 35ft/lbs but don't quote me. Problem is that the shaft turns as you tighten it so you have to keep the shaft stationary as you torque the nut...using a regular socket on the torque wrench it can't be done. Probably some special socket that has a cut-out in it that allows you to use a wrench or allen key to hold the shaft stationary.

The purpose of this bolt is to keep the shock/spring assembly together.