What do you mean by 'spindle head'? Are you referring to the chuck?
Also what do you mean by: how to adjust the spindle head so that my parts will turn on center again. ?
Do you mean that the parts used to run true but now they do not run true?
Have you taken the spindle head (chuck?) off and then replaced it?
Until you can explain what you want in terms people can understand you might not get much information.