kwhalley wrote:I have read a lot, and appreciate the high quality descriptions of the piston design. My question is how snug should the piston be?
It depends on various factors.
When you trigger the piston by opening the pilot valve, air is flowing out of the pilot chamber through the pilot valve, but at the same time air from the main chamber is flowing into the pilot chamber.
If more air is going in from the main chamber than is going out of the pilot valve, then the pressure will never drop and it won't fire.
Say you have a 4 inch chamber and you're piloting with a 1/8" ball valve.
So, chamber inner area: 12.568 square inches
Ball valve maximum flow area: 0.012 square inches
Say your piston is 3.999 inches in diameter, so it has an area of 12.561 square inches. The area through which air can flow from the main chamber is therefore 12.568 - 12.561 = 0.007 square inches, less than the pilot valve flow so it will fire.
Say your piston is 3.995 inches in diameter, so it has an area of 12.537 square inches. The area through which air can flow from the main chamber is therefore 12.568 - 12.537 = 0.031 square inches, greater than the pilot valve flow so it will never fire.
If your pilot chamber is too big, pressure will drop at a lower rate and performance will be reduced.
So er... yeah, in general, as snug as possible without too much friction