slivel

If you have a crack in the center section, the turbo is not rebuildable. The thrust bearing seems to go first allowing runout on the shaft ultimately wearing enough to permit contact between the compressor and/or turbine wheels to the housing.
There are seals in the turbo, although they are not like traditional oil seals such as what are seen for rear main or oil pump or balance shaft etc. The things that look like piston rings do the job of sealing and can handle the extreme heat where a traditional oil seal would fail. When I rebuilt my K26, I used the rebuild kit from these guys:
KKK and Borg Warner Rebuild Kits (gpopshop.com