I think the thread is 5/16-24 for the air cylinder rod. Your right, the piston is just 1 inch of standard aluminum round bar stock. If you make your own piston, you could thread the bottom to 5/16-18 and the upper part 5/16-24 and you could use all the manual parts and basically purchase nothing from Magma. Both systems work but the cylinder way keeps air from getting under the hycar washer so there aren't any air pockets at the plunger. I enjoyed playing around with the system. Rod