Piston valve question

[Labtecpower]

I'm currently designing a piston valved BB-musket.
In my garage I had some stuff lying around, wich I think I can use.

I had my doubts about the Presta valve I wanted to use as a filling valve and the pilot valve.

I have 10.00 mm aluminium tube lying around, wich will make the piston.
It will slide in copper tube with an OD of 12 mm, and an ID of 10.04 mm.
According to my maths it will make a gap of 0.63 square mm.
Will it give me piloting problems with the presta valve?

Here are some pictures to give you an idea of the materials.


the aluminium tube







The copper tube. couldn't get the olive off



my presta valve



I'm looking forward to your replies!

[jackssmirkingrevenge]

Can you dismantle the presta to see what the effective flow area is? If it's bigger than the gap between piston and chamber you should be fine.

You could always wrap tape around the piston to reduce the gap further.

[Labtecpower]

I don't want to wreck too much presta valves, as they are pretty hard to get for me...
Wrapping my piston with tape won't work, as my thinnest tape is 0.03 mm,
for comparison, one of your hairs is about 0,04 mm.
if I have a spare presta I will do some medical examination

[jackssmirkingrevenge]

If a single layer of tape is too much, then you should be fine with a presta pilot. Just make sure you keep pilot volume to a minimum, piston travel doesn;t need to be more than 50% of your calibre.

[Labtecpower]

Thanks for the <1 minute replies :thumbleft:

I didnt know that such little travel was enough, but it will make things easier for me.

[jackssmirkingrevenge]

I didnt know that such little travel was enough

Read through this thread

[saefroch]

Prestas have about as much flow as a schrader, but they do require quite a lot of force to depress and open. I'd avoid using a presta as a pilot valve, and opt for a ball valve, blowgun, or pop-off instead.

It really depends on your pilot volume though... I say try it with the presta and if it proves to have oddly low performance, try a different pilot valve.

[jackssmirkingrevenge]

Prestas have about as much flow as a schrader, but they do require quite a lot of force to depress and open. I'd avoid using a presta as a pilot valve, and opt for a ball valve, blowgun, or pop-off instead.

Nothing wrong with schraders as pilot valves, and if you use a lever/hammer to actuate the schrader, the force issue is negated.

[saefroch]

Oh, I'm fully aware you've used shchrader valves as a piltot/fill valves on multiple builds, just the flow demands very low pilot volume, which your builds have. Will this one though?

In actual answer to his question: I don't think you'll have problems with too much air leaking around the piston... just the piston might pilot slowly. In theory, a higher-flow pilot valve is better... in theory.

[Technician1002]

Oh, I'm fully aware you've used shchrader valves as a piltot/fill valves on multiple builds, just the flow demands very low pilot volume, which your builds have. Will this one though?

In actual answer to his question: I don't think you'll have problems with too much air leaking around the piston... just the piston might pilot slowly. In theory, a higher-flow pilot valve is better... in theory.

The low flow requires a tight fitting piston with a very small EQ port. Piston leakage becomes a very real obstacle in getting them to work.

[Labtecpower]

I'm designing the pilot area now, and I think it will have less than 2 ccm of pilot volume.
would four hundreth of a millimeter space bug me?
the piston will have no equalisation hole, unless it proves to be absolutely neccassary.
I'm thinking of some sort of hammer to open my valve.

[jackssmirkingrevenge]

In actual answer to his question: I don't think you'll have problems with too much air leaking around the piston... just the piston might pilot slowly. In theory, a higher-flow pilot valve is better... in theory.

It is certainly better, but how much better in the case of a tight fitting piston and minimum pilot volume is debatable.

In this case, as there are no o-rings, there is absolutely no need for an equalisation port, indeed adding one could potentially increase leakage to the extent that it won't fire with a schrader/presta pilot.

I'm thinking of some sort of hammer to open my valve.

I would go with a lever, simpler to implement and on this scale you probably won't notice the difference in performance.

[Labtecpower]

I have dismantled my presta valve in such way it can be used again, and I came to the conclusion that it in theory has a better flow rate than a schrader valve.
when you press the palve stem, the opened area is larger than the schrader's.
another advantage is that it doesn't leak air when detached from the pump, minimalising the risk of accidental firing.
should I post some close-ups of the guts?

[jackssmirkingrevenge]

should I post some close-ups of the guts?

Yes please, maybe I'll regret ordering a lot of 50 schrader valves off ebay