Compression Test

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I just finished installing a Prometheus Air Seal Nozzle, a Prometheus MS110SP Spring and a Systema N-B Cylinder Type 1. Now how do I do a compression test to see if I'm putting out the correct compression?

Also, with the TM AK47 Inner Barrel that I recently received, it came with a ring. But I have no idea where it goes in the hop-up chamber or rubber sleeve. Does anybody know where this ring goes?

[Kos-Mos]

Compression test is done outside of the mechbox.

You assemble the nozzle, cylinder head, cylinder, piston head and piston.
Then you put your finger on the tip of the nozzle, and press the piston in.

You will understand if you get compression or not.

That ring goes between the hop-up body and the barrel. Assemble the barrel/hop-up, then slip the ring from the tip of the barrel.

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Thanks man!

I was actually just reading how to do a compression test on AirSplat. So I am getting compression because when I try to push the piston forward with my finger on the nozzle, there's resistance trying to keep the piston back. I drilled 8 evenly spaced holes on my stock TM piston head so that the o-ring expands to form a tight seal.

So that ring for my barrel just goes on the outside of the barrel and sits right at the end of the hop-up assembly?

[Schlyder]

Correct. It's a bushing that holds the outer barrel solid. You need to tap it into place, just flush with inside the end of the hop-up

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Cool!

I see where it's supposed to go now, thanks!

If I didn't install that ring, would I potentially loose power?

[Darklen]

No, you may lose some accuracy as the ring helps hold the barrel solid to the hopup chamber. Inner barrels are often a bit sloppy inside the outer barrel.

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Okay, so I just chronoed the gun to see how much FPS I gained and to my surprise, I was only shooting out 211 FPS with my MS110 Spring. I did a compression test before closing up the gearbox and everything seemed fine. Any other ideas why I could be loosing power?

[dpvu]

Could be an improper seal between the nozzle and hop-up. Also make sure there is actually compression. It should be difficult to push the nozzle into the mechbox with your finger covering the hole.

[Kos-Mos]

Actually, if you have made it correctly, it should be impossible to push it all the way in when you cover the hole.

That is what my mechboxes do.

Check if you installed the spring correctly too.

And check if there is not a broken teeth on the sector gear. Quite rare, but if the piston is not pulled far enought, you will lose a LOT of fps.

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I guess I'm loosing a lot of compression then with the piston head, beacuse since it was the first time I've done a compression test, I was able to push the piston completely in my new Systema N-B Cylinder Type 1 with my finger covering the air seal nozzle. I thought that as long as you have resistance trying to push the piston up the cylinder with your finger over the air seal nozzle that it was good enough. I'm planning on getting a Prometheus Hard Piston and Prometheus Metal Piston Head pretty soon. So maybe that will solve my problem. But I even drilled 8 evenly spaced holes on the stock TM piston head so that air would expand the rubber o-ring. I also made sure that I didn't drill right through the piston head. But I guess that wasn't good enough.

[Kos-Mos]

ermm

To what I remember, stock TM piston head are already ported...

You should go with a polycarbonate piston head instead. They put a lot less strain the the mechbox than aluminium ones.

I had one in an ICS, and now have one in my KWA G36... but if I had to change it, I would go with a poly one...

[dpvu]

Air is probably escaping as the O-ring expands but doesn't seal properly. Polycarbonate piston heads are great. I recommend the Modify ones. Never failed me yet.

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Yeah, I was actually thinking of getting the ARS MASK Sorbo Piston Head with Bearing from AirsoftParts.ca.

So, just to confirm, I shouldn't be able to move the piston up the cylinder AT ALL with my finger over the air seal nozzle when doing a compression test? And if I could move it up the cylinder then there is air escaping, right?

[Styrak]

You should be able to push the piston in a little bit (or past the port if there is one) and then it should pretty much stop dead, no air escaping anywhere and the piston held back by the air pressure inside.

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Cool thanks!