I first slipped on a spacer ring to locate the position of the threaded sleeve for the lower spring perch. After the threaded sleve was slid on, I inserted the strut cartridge into the core and attached the bolt to the bottom of it thru the hole in the bottom of the core. I then slowly tightened up the bolt and it drew the cartridge into the core. The top of the strut cartridge had 3 dimples 120 degrees apart that provided an interefernce fit for the cartridge into the core. A small rubber boot was then placed over the seam between the core and the cartridge.