Hello Group,
From my amateur research I found that Rounder used a number of different manufacturers for cylinders based on the vintage of the machine.
The good news is that whoever was low bidder used industry standard components. At least thats what I've experienced on the two that I have owned. One unit has cylinders with aluminum end caps and the other has cylinders with cast iron end caps. But the good news is that the cylinder dimensions and parts are the same for both.
I did some measuring and double checking to be sure but the list that Aaron Pfeifer posted from Martin Fluid Power was correct. The only thing missing from the list was one common O-ring that goes in the I.D. of the piston. I was able to buy that O-ring with dimensions of ( 1" x 3/4" x 1/8" ) at the local Ace Hardware.
I have attached a PDF of the part numbers I ended up ordering from Martin Fluid Power. When I called they were SUPER helpful. The lady I spoke to was very knowledgeable about seals and she modified two part numbers from what Arron had listed. One was just an updated part number because she said it was cheaper to buy the bag quantity of 15 than the 8 I asked for and then the U-Cup seal she said the one she was recommending was a newer material that was supposed to perform better over time...
When I did my bucket tilt cylinders I took a few pictures of the disassembled cylinders and all the parts. I share a link to that photo album below. Please note I have the Piston on the rod backwards in the photos - The piston goes on the opposite way with the big split ring closest to the nut, NOT the way my picture shows. My advice is also to buy the proper Pin Spanner wrench for the job. It makes both disassembly and re-assembly a breeze and you won't destroy the end caps with a pipe wrench...
Hope this helps...
PDF of Cylinder Seal Parts attached.
Photo Link - Rims Tires and Cylinders