Why aren't AEP nozzles sealed at the rear?

[Rogerborg]

Just a topic of discussion, prompted by having my AEP apart again to fix the last thing I'd fixed.

For those who haven't had one apart, the piston in an AEP pushes air up through a nozzle housing, which is essentially a hollow tube with the piston pushing up from below, and the nozzle running through it.

View attachment 39586 

View attachment 39587

And the nozzle itself is also a hollow tube.  It's pulled backwards to allow a BB up into the hop unit, then is pushed forwards again by a spring and spring guide inside it.  A hole in the bottom of the nozzle, with O-rings on either side, allows air up from that round hole in the bottom of the nozzle housing.  There's a constriction inside nozzle just behind the rear O-ring against which the nozzle spring seats, and the end of the spring guide has an o-ring to seal against that ridge, ostensibly blocking off the rear of the hollow nozzle and persuading the air to go forwards.

View attachment 39591

The problem with this arrangement is that it's tricky to see how it produces a perfect seal.  The spring guide O-ring must necessarily be smaller than the spring.  The constriction inside the nozzle has to be larger than the O-ring on the end of spring guide, as it slides over that ring when the nozzle moves back.  When it comes forwards again, everything has to line up perfectly in order for the ring to seal against the constriction.

There are spring guides that attempt to ameliorate this with double O-rings, and I may double-up mine, or at least heat-n-stretch it.

But the thing I can't understand is: why not seal the rear of the nozzle completely so that all the air has to go forwards?

It seems like this could be bodged into a nozzle by dropping a small solid disc inside it.  No need to even glue it in, spring pressure will hold it tight against the constricted section.  Then lop off the front off of the spring guide so that it doesn't foul it when the nozzle is retracted (it only needs to come back 6mm or so to let a BB up).  You might have to lop a coil off of the spring to allow for the thickness of the solid disc, you might not.

View attachment 39590

Is this mental?  The standard version seems overly complex and prone to leaking.  What am I missing here?

 

[EDcase]

Interesting, never had one apart to see...

I can't work out the mechanics from the pictures alone.

I guess the complication comes from the fact that the air has to push the nozzle forward to load a BB and form a seal with the bucking then blast as much air as possible through it.

So you have to think of it as a 2 stage mechanism.

 

[Rogerborg]

Interesting, never had one apart to see...

Click to shrink...

They're fascinating things, and actually a little more complex than AEGs as they have to use some wizard wheezes to cram everything into a smaller space.  I'm constantly amazed that mine keeps working after I've had it apart and then thrown all seventween jillion parts back together.

 

[Samurai]

Because it'd be way too powerful.

My guess is that it's probably because of how it is manufactured. Probably the injection mould couldn't be removed if there was a full seal there where you have the red line. It would make sense to do the mod you suggested.

 

[Rogerborg]

Because it'd be way too powerful

Click to shrink...

This is part of my quest to upgrade it until it breaks. I'm already running an Eagle6 M80 spring, but that didn't push the fps up by as much as other owners report, so I'm pondering why.  It's hard to compression test precisely because it's got that open nozzle / nozzle housing on it. 

I guess the complication comes from the fact that the air has to push the nozzle forward to load a BB and form a seal with the bucking

Click to shrink...

Nope, the nozzle is pushed forwards and sealed against the bucking by spring pressure.  At the time when the piston does its thing, the nozzle should already be sealed, and all that's required is to direct the air forwards.

I'll want a spare nozzle and spring guide set to bodge this into, but finding one in stock at a reasonable price including delivery is problematic.  It almost makes me think I should just bag a spare AEP...

 

[EDcase]

Ah OK.

Maybe look for a second hand AEP to experiment on...?

 

[Rogerborg]

Maybe look for a second hand AEP to experiment on...?

Click to shrink...

Always.

I've just stretched my spring guide O-ring (although it's so thin that nerve wracking hooking it off - I used a dental pick) and silicone greased the heck out of it.  But fully sealing the rear of the nozzle is now on my pending bodges list.