MAZAK POWER CENTER V12 retrofit (if possible)

some updates , 2.5 axis , gear box on all axis, 2 gear on spindle , mechanical M19 , encoder on ballscrew ,0.14mm backslash on gears . I have finish the machine when I notice there is no encoder on spindle only a tacho . Put encoder the I see a a strange part that orient the spindle . I stuck at this piston because I don't know if it press on center of the piston or on the side .

That's an interesting piston. I would guess that it operates a roller on a shaft coaxial with it, but without the other parts it's hard to tell.

Question is on shaft or sides

Did you take it apart, or was it already disassembled?

That looks like something that would slide coaxially on the spidle shaft. But it would need a roller to operate on, and that would unbalance the spindle, so seems unlikely.