SpudFiles

I donbt quite get this equalization hole business. Surely you lose valuable power by drilling a hole through your prevuiously airtight piston.

Why not, say, a one-way valve?

Like a ballbearing in a cone with a spring behind it:

Yep, it will work fine. I put one way valves on pretty much all of my pistons. Just be very careful to avoid leaks in the pilot or make sure that the piston/check valve don't quite seal perfect or you might have accidental firings.
Mine are usually made of a couple of brass fittings a ball bearing and a spring. The ball-bearing on brass doesn't quite seal perfect, but it still seals enough to fire using a schrader valve.

or you could just place a check valve in there to save the hassle of making sure it seals, but then again wheres the fun in that

SNDM wrote:I donbt quite get this equalization hole business. Surely you lose valuable power by drilling a hole through your prevuiously airtight piston.

Very few pistons are actually 100% airtight, unless you're using o-rings or some other type of rubber seal that guarantees there are no leaks whatsoever, equalisation holes are unnecessary.

I used a 1/4 28 taper thread grease zerk in the back of my 3/4 inch piston. I was afraid that the cracking pressure would be too high so I yanked the spring out of it and used a tig welder at work to add a teeny amount of metal to the back to keep the ball from falling out...
works like a charm, and its tiny.

Back when I used to use syringe plungers as pistons, the seal would be 100% so an equalisation hole would be needed. I made my own check valves using brass tubing and a little rubber section from a disposable cigarette lighter.





From the archived thread:

For the one way valve, first you need some brass/copper/aluminium tubing with an I/D of 1mm. You then need to drill out the first 3mm with a 1.5mm dia drill bit. The rubber seal is from a (empty!) disposable lighter - you have to remove the plastic from the top and screw out the nylon bit that supports the valve. If you pull the brass away from the nylon, inside you'll find this component at the base of the valve.

crafty there.... I was concentrating on keeping the check valve tiny so I could put it right in the piston, while you ignored that idea and bypassed the piston completely, keeping piston weight to a minimum, and moving mass as simple as possible. Never even dawned on me.
Lock

Lock, your piston isn't airtight so you didn't even need a check valve

Thx people, sorry for drifting off from original topic.
I somehow doubt I'll need equi holes on my pistons.

Perhaps I missed it but...

What about flow?

If you move the piston back a further distance from the seal won't it allow a greater amount of air to flow through the barrel?

Jared Haehnel wrote:If you move the piston back a further distance from the seal won't it allow a greater amount of air to flow through the barrel?

That's the whole point of this discussion - at 1/4 of the calibre travel, the piston provides the maximum flow that can fit through the barrel of a given calibre. Moving the piston back further does increase the potential flow, but since it's restricted by the barrel diameter it will be irrelevant, you'll still get the same flow rate.

Man, I've got to stop missing the points...thanks Jack

I tried out some calculations for myself to see if I would come to the same conclusion, and I did. Perhaps this will help people understand it.

Imagine that you take a given pipe size and cut a length of that pipe that is 1/4 of the diameter. then you cut that little piece of pipe and flatten it into a long rectangle. if you then multiply the length of the rectangle by the width you will find that it is of equal area of the pipe cross-sectionally.

LOL never mind, I thought that would sound more simple than it does.
Maybe it will help someone,.. who knows.
Lock

If I could figure it out after a night's hard liquor, it can't be that hard to understand

The 1/4 caliber = ideal piston travel distance is correct, as Jack both stated and proved.

Does this mean if I made a co axial gun to shoot 4 mm bbs, the piston travel should be 1 mm? sounds strange, it seems a bit small......