I have packed a heck of a lot of cylinders by smothering the threads with grease and then pushing the seal past the grease. I push the rod away from the side of the seal I am trying to push past the threads and then push the seal down a little. I keep working the rod side to side and keep pushing the seal down a little more each time. It depends upon the design of the cylinder. This won't work on every cylinder, but it has worked many times for me.

I have been out of the service area for many years now, but
we used to get old X ray film. It is a large sheet and would work
well on the old Clark multi-stage cylinders. The film will not
compress but will protect u cups from thread damage.