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I have made the cylinder and intended to simply turn the piston to the closest sliding fit I can achieve.
Yes - it will soon bed down. And I think you have the right idea on piston fit.
drilling and reaming the gland set in place in the bottom cover at the same time is another. By concentrating on the latter and easing the bore fit will give a very free running fit but with an exceptional seal.
I am reading this application as the Stuart 10V piston and gland, so suggest a working model would be running a say 20 to 60 PSI.....Derek