Please don't hate me.. My turn to rip the piston design. I'm not doing this to make enemies. Please consider my feedback as constructive criticism. Please...
My first impression on the piston is nice piston. Construction looks first rate. Now the bad impressions..
1 It is a hollow piston with an open back. I have had nothing but problems with open back pistons as there is not enough area for a proper bumper to do the job. My Mouse Musket and Dragon cannons ate pistons for lunch. My first QDV shattered the pilot housing. I later re-used the piston, but I added a washer to the back of the piston (not attached) so the bumper could not go up inside the piston.
2 Is that PVC? Other than the modified original QDV piston with the added washer and extended length bumper, all my PVC pistons eventually broke. PVC does not like the bumps and bruises of being a piston.
3 What is under the O ring groove.. The plastic under the o ring appears to be really thin. I would expect the piston to become broken at a very young age.. Nuf said.
4 Why is the o ring so far away from the back of the piston? When the piston sticks out of it's cylinder to extend out to the barrel seal, the o ring must remain tucked into the pilot. If it doesn't then it isn't working. Recommendation is to cut off the back of the piston to make it shorter, lighter and add more bumper and the washer to enclose the back of the piston.
Sorry for ripping the piston so badly. It looks great.
For an example of a small piston with the o ring near the back, see this thread.
http://www.spudfiles.com/forums/tutoria ... 21740.html
The EQ hole should be as small as you can make it.1/16th inch or smaller is recommended.
How far should a piston move? The best distance is a balance. If it moves too far, it picks up too much speed and breaks things. If it does not move far enough, it block the flow.
The minimum distance is generally considered 1/4 the distance that the opening (orifice) into the barrel size is. So a 1 inch orifice would have a piston move no less than 1/4 inch. This relationship provides a gap with the same open area as the area inside the opening to the barrel. i look at that as two equal restrictions to flow in series. 1 is the gap between the piston and barrel, and the second is the area of the entrance into the barrel. To minimise the restriction of one of those so only one significant restriction to flow remains, I like to design my pistons to move between 1/3 to 1/2 the barrel opening diameter instead, so in the above example with a 1 inch opening to the barrel, a piston motion of 1/3 to 1/2 inch will perform very well.
and will be piloting it with a 3/4" or 1"sprinkler valve (it depends which gun I want to dissasemble) piloted by a blowgun
but do you think with only say 3ft of barrel my C:B ratio would be okay? What I want to shoot depends on what size pipe would give me the best ratio with a 36" barrel. If its 1" then batteries, 1.5" potatoes, sdr 21 golfballs. I will add that washer I completely forgot about that. Thanks for the help matty
