How to measure a piston for resizing
>How to specify the size to make the valve.
There are a couple of approaches to this. Some people say to resize the piston back to its original manufactured size. However, it makes sense to use a micrometer to measure the current size of the piston and measure the interior of the casing. Use a Telescoping Gauge to measure the inside of the casing. And, then I can calculate what I think I want them to be resized to. Remember that they will come back and be the same size top and bottom. So, you need to consider that in your averaging.
I realize the casing may not be round anymore. Any suggestions?
To be safe, I send the resizing company my measurement AND how much larger I want them from the current size. That way they can verify my measures.