“Why do some replacement struts have a different extended shaft length?
“When I transfer the compressed coil spring and mount to the new strut, the threaded stud doesn’t come through as far?”
Photo of a typical internal rebound spring
That’s because some OEM struts have an internal rebound spring. Its purpose is to add stability on cornering and help avoid a roll-over condition by adding resistance during body roll. With age, as the rebound spring fatigues or if it breaks, the strut shaft will extend further. Sometimes the difference can be 1.0” to 2.0” inches. However, if you were strong enough (most of us aren’t) you could pull the shafts further out and see that the fully extended lengths are identical.
With the strut installed and at normal ride height, the piston and shaft will contact the spring almost immediately as the shaft extends. KYB struts will always have the correct full-extended length.
On left: Example of a fatigued rebound spring.