Vented piston heads?

[SA-80_DUDE25]

Hi, can anyone explain to me why so piston heads are vented?

[jagillham]

Re: Vented piston heads?

Pushes air behind the O ring to push it against the side of the cylinder... so better seal

[Montesa77]

Re: Vented piston heads?

Ninja'd

[SA-80_DUDE25]

Re: Vented piston heads?

Hmm, we shall see lol. Just doing and experiment!!

[Montesa77]

Re: Vented piston heads?

Explain....

[SA-80_DUDE25]

Re: Vented piston heads?

Want to try it first

After gettin a poo reading of 255fps after fitting a SP85 spring in my Sentry i dececided to try a different hop rubber, a Tokyo Marui one to be exact, and got not difference.

I then fully charged my battery and got a 265fps out of it.
Then read a thread on here about getting a better air seal in the cylinder, someone explain that if they put their finger on the nozzle and pushed the piston in there was no air leak.

I then tried this on the sentry and noticed that there seem to be alot of air loss around the o ring.

So i decided to try filling the vents on the piston head up with instant gasket.

Greased the o ring and put it all back together, now it is chrono'in at 280fps!! Highest was 288fps!

[jagillham]

Re: Vented piston heads?

Bad crono? The spring is pulled back and released at same point, so the battery has no effect on FPS.

If the O Ring was dry before, you may find that the grease alone improved the seal.

[SA-80_DUDE25]

Re: Vented piston heads?

Hmm all very strange, i think i will get a new piston head hopefully a prometheus one if my local shops got one in and an M100 spring next week and see what happens.

[Gaffa]

Re: Vented piston heads?

MOST air leaks in a gearbox are to do with the Piston Head O-ring, which never needs to be replaced if you know how to stretch them properly.

[SA-80_DUDE25]

Re: Vented piston heads?

Would you like to share that knowledge?

[Bachelarius]

Re: Vented piston heads?

Get some brake fluid, drop the O-ring into it for a day or 3. Take it out again, its Outer Diameter will have increased slightly.

Look out for the oring to be sitting very snugly withing the piston head, that is touching both walls of the groove. Sounds like the poor thing shrunk so much that the air ran straight from the vents into the rear of the gearbox, utterly destroying your compression.

I have a vented piston and when I do the thumb-over-cylinder head thing, it stops dead after about 1/2-1/4 of the cylinders length from the cylinder head. In addition, try the same with the air nozzle attached as well and see if it is slowing the air at all. I know my air nozzle seal is not perfect but I am sure it can be improved with one of the fancy-pants o-ring type ones...

but yeah, drop the o-ring in some brake/transmition fluid :D

[sj_asc]

Re: Vented piston heads?

In addition to the advantages in seal they are also to allow air to be drawn in through the back side of the piston head rather than traditionally through the notches cut in stock airseals.
The non notched aftermarket airseal nozzles will not allow air to be drawn back in through the nozzle, if a round is chambered it will simply block the nozzle and the vacuum created will pull the piston head o-ring forwards allowing air to be drawn in through the back of the head, through the ports and into the cylinder.
This reduces resistance/load that the vacuum can create, particularly with high rates of fire.

Non ported piston heads are not compatible with aftermarket nozzles that have a perfect cut hole as the air cannot be drawn in from anywhere. Such mismatching will not work at all with non ported cylinders as i found about 5 years ago

[SA-80_DUDE25]

Re: Vented piston heads?

I've cracked it, gettin a consistant 290-295 out of an SP85 spring, using the original G&P hop nub and a blue madbul hop rubber, this seems to be giving a very good air seal at the hop unit
Will have to see what i get from a new piston head, enhanced guarder hop unit and an M100 spring
Will update tommorow!!