The piston is not visible until you remove the prechamber. To remove the prechamber, you need a splined socket to remove the prechamber collar and a slide hammer with an attachment. The valve cover has to come of for this and the tools are about $200.00. After the prechamber is out, you can see the piston.

EDIT: After posting this, I just realized that to get the prechamber, the glow plug has to be out as it protrudes into the prechamber. Forget the above idea.

JC