If you just copy the existing design, its not a "custom piston" is it though?
Its just a cheap knockoff foreign copy

To start altering any of the basic characteristics much, you need to be ordering 20 sets all slightly different and then building engines with each.

Honestly mate, just buy some off the shelf ones that are known to work!

Its just not the sort of thing you want to be fucking around with yourself when you clearly know nothing about it!