Did you actually take the jack apart (a messy process)? I think the
lifting piston might be a bit more complicated than simply a large
piston in a cylinder. There are sometimes alignment posts running up
the center of the piston. That's a cheaper way of keeping the piston
aligned than having a large precision bore outer barrel all the way
down. Also, the actual hydraulic seal might be on a smaller diameter
shaft than the shaft you see extended.
If you disassemble the jack and determine it really is as simple as you
imagine, then I am baffled. But I suspect you might find more
"structure" inside that you think.
Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton College
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu