First, are there any better kits/parts other than OEM? Second, How do I know when the pistons are too far gone? Third, Why are the "Upper" pistons and seals a different part number than the "Lower" pistons and seals when I look up the parts??
Look at manual to see if there is OD measurement and limit. If answer is no at least do a visual for pitting or gauling and replace as needed. There are other vendors for piston search the threads as I remember seeing 2 or 3 listed.