The only way I see damage occurring is if you shock coil plate is about rusted through. The extra heavier coils could bend those. However, 6 months or a year later they would have bent anyhow. Its probably why there is a disclaimer on them. Someone's frame was rotted and blamed the newer springs. No way could stiffer springs do any damage under normal conditions. Just a stiffer ride like the other users suggested.
Sent from my SM-J337V using Tapatalk
__________________
7 3rd gens listed in the build thread (2 are parts mobiles)
Build Thread:
https://www.toyota-4runner.org/3rd-g...os-builds.html
Brillo's Bucket Fluid Ex changer:
https://www.toyota-4runner.org/3rd-g...ml#post3358086
Sparks Plugs Wire and Coil Information:
https://www.toyota-4runner.org/3rd-g...on-5vz-fe.html