Here's a link to a Google Drive with a photo of the assembly and then videos of....
piston moving with no head
piston moving with head resting on cylinder
piston moving with difficulty as head nuts are put on
piston locking up when head nuts are tightened up.
https://drive.google.com/drive/folders/193kHtT5HwfTx1mfhgfzIDXc3gTwdny8-?usp=sharing