SpudFiles

A while ago, when I was machining a .25" porting piston valve, I accidentally made the piston such a good fit in the body of the valve that it wouldn't leak past to fill the chamber (pilot of a 3/4" QEV). I don't remember putting any lubricant in the valve. I ended up filing a groove in the piston since I needed that valve done ASAP. After it leaked sufficiently, it leaked epically out the exhaust, because the SHCS I used to hold the sealing face interfered with the the closing of the valve. I did not soap test the chamber port.

Does anyone know how good a fit would be required for a good seal, perhaps with some white lithium grease?

The only source I saw claimed to machine and lap the spool to within a few millionths of an inch. The body of the valve was actually another spool that was floated in an outside casing with O-rings to prevent outside stress seizing the piston. Now I KNOW I'm not within a few millionths.

By lapping the valve, the seal is very good with slow leak rates. All modern internal combustion engines use lapped valves. The problem with lapped spool valves and small engines with lapped pistons (RC model planes) is the sizes change with temperature.

The difficult part of lapping a spool, is before lapping it barely fits if at all. After lapping, there is a gap. The trick is to know when to stop lapping.

My Mouse Musket used a lapped piston valve. After several broke, the last one was carefully turned to tight specification so it did not require lapping.

So you're saying I just got REALLY lucky?

sounds fair enough...

Yes you got lucky. My Mouse musket with the lapped piston required an EQ port also, otherwise filling the chamber would open the valve. I drilled through the piston instead of making a groove. It was stronger in plastic.

When you say it didn't leak.... Define "didn't leak". Are you saying that you put it under pressure for an hour and didn't see a pressure drop or are you just implying that it didn't leak fast enough for your purposes?

If it simply didn't leak fast enough... One trick that may be useful for your purposes is a simplified labyrinth seal.

In the grand scheme of things, almost everything leaks.

We pump down stuff and hold a vacuum (base pressure below 30 militorr) and measure the rate of rise in millitorr per minute at work.

To be technical, it didn't leak fast enough. We mere mortals simply don't wait for hours for the pilot area to eq while filling the chamber.

For practical gross pilot pressure fill, it wasn't leaking enough to notice.