When I installed my spindle on the HDZ, I got an immediate increase in Z height from the old Z plus (about 94mm) to the new spindle and HDZ (about 125mm) and the HDZ is a far more sturdy Z axis and a very worthwhile upograde if you are considering keeping the SO4. After my upgrades to my SO3, I decided the HDZ was a worthwhile investment and it gave my machine a completely new lease of life. I would not part with it now.