This tool is basically a used wear ring clamped around the center of a 4 piece BTFE/NBR/NYLON piston seal to compress it back into shape after having to stretch it over the piston
Normally when doing these cylinders I do them at home in my shop because I prefer to boil the nylon portion of the piston seal in water for about 15 minutes to render it soft and pliable making it much easier to install. I also have a range of tools I can use to reshape or compress it to its original size and form but being in the field yesterday some of these niceties were not available no hot water or means of boiling any and no piston ring compressor
So I made do whit what I had it was warm yesterday so I laid the nylon ring on a shiny aluminum pate and allowed the sun to help warm it for about an hour while disassembling the cylinder on the excavator
after installing the piston ring the nylon was badly stretched out of shape as always. I used an old wear ring plus a short piece of another wear ring to fill the gap not shown in the pic, and my chain vice grips to squeeze it back in shape leaving them clamped there for a few minutes did the trick just as well as my expensive ring compressors would have done.