For 5.25" drives, a special
alignment floppy (expensive, and cannot be copied) is used together with an
oscilloscope (connected to particular points on the drive's circuit board). A program is used too move the heads to a particular position on the alignment floppy. Then, the radial position of the
head carriage is finely adjusted until the correct waveform is seen on the oscilloscope. The 5.25" drive maker's technical manual usually includes that information together with how to finely adjust the radial position (which screw, cam, ...)
There is also a 'track zero' adjustment.
3.5" drives will be similar, except that when I look at some 3.5" drive manuals on the Internet, I cannot see alignment instructions. Maybe the makers thought that, considering the (relatively) low cost of 3.5" drives, that people would be buying a new drive rather than paying for repair/realignment.
At the second post of the thread at [
here], someone used an alternative method to align their 3.5" diskette drive.
Both heads need to be radially aligned. For the 5.25" drives that I have worked on, it was the complete head carriage that had drifted out of radial alignment, and so once I had the top head in the required alignment, I then found that the second was also in alignment (within the specification tolerances).