Piston head O-rings vs performance tests

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A lot of attention is paid towards various components when tuning performance in an AEG, but it seems piston head O-rings are often overlooked beyond merely testing that they seal against the cylinder. A typical piston head O-ring measures about 24mm outer diameter, like these ones:

 

https://www.ak2m4.co.uk/internal-parts/pistons-heads/piston-head-o-ring-19.5mm
https://www.ak2m4.co.uk/internal-parts/pistons-heads/piston-head-o-ring-viton-19mm

 

However for some time now I've been wondering if it's actually better to use slightly smaller O-rings, somewhere around 23.7-23.8mm, or perhaps even a bit smaller than that (as close as possible to or just a tiny bit smaller than the diameter of the cylinder; if it's a tiny bit smaller the pressure ahead of the piston & air flow through the ports in the piston head should expand it to seal completely once it's moving at speed). The idea being to reduce losses due to friction and allow the piston to gain slightly more energy before compression begins.

 

This is because a few years ago after performing maintenance on my TM P90RD and replacing the piston head O-ring that I thought was worn out with a new "good" one, I noticed it had lost a significant amount of power. Putting the "bad" O-ring back in put the power back up to where it was supposed to be... I subsequently discovered that while the "bad" O-ring didn't seal when tested the usual way (which is what made me initially conclude that it was bad), it would actually seal just fine when the piston was moving fast enough. I've since noticed this effect with my other guns too, although none of those have managed to reach the performance of my P90RD (which just about manages 340fps on an SP90 spring while the others do a little over 330).

 

I recently decided to test this, so I bought some O-rings that are supposed to be 23.74mm outer diameter and have now tested them alongside some other piston head O-rings.

 

The test gun is a TM P90TR with:

 

• ZCI 6.02mm barrel (247mm)
• Prometheus purple hop rubber & bucking (hop is not properly set but remained unchanged throughout the tests)
• Guarder air seal nozzle
• Guarder enhanced stainless steel cylinder head
• Guarder enhanced cylinder, 3/4 type
• Deep Fire aluminium piston head
• Tokyo Marui stock piston
• Guarder SP90 spring
• Deep Fire (I think) bearing spring guide
• Guarder steel bushings
• IRF2804 MOSFET
• Blade fuse
• Deans connector

 

Each O-ring tested was given a very light surface coating of silicone oil before being coated in TechT Gun Sav and installed on to the piston head. Before testing the gearbox was cycled a few times without the upper receiver/barrel assembly to clear any excess grease out of the nozzle & prevent any finding its way into the barrel and risk affecting the results. BioSphere 0.2g BBs were used for testing, 10 shots were fired for each test and the average value used.

 

These are the results (O-ring size measurements are approximate as they're never perfectly round):

 

Lees Precision Engineering piston head O-ring 24mm (19mm inner diameter/2.5mm cross-section): 322 fps (min 319.6, max 323.5)

This is a standard size good quality O-ring that comes with LPE piston heads, with the result being about what I expected.

 

Deep Fire 'super' O-ring (measured ~23.8mm OD, 2.6mm cross-section): 332.6 fps (min 330.9, max 334.2)

This is what came with the Deep Fire aluminium piston head that's currently in the P90TR. It is a brown colour and feels softer than most other O-rings in addition to being slightly smaller, and there is much less resistance when moving the piston through the cylinder. With this O-ring the gun has an extra ~10fps over the LPE O-ring.

 

Uxcell "23.74mm (18.5mm/2.62mm)" O-ring #1 (actually ~23.9mm-ish): 322.9 fps (min 320.4, max 324.7)

This is one of the O-rings I got recently specifically for these tests. I was very surprised by this result, so I checked its size and found that it was larger than specified - around 23.9mm OD. I then measured the rest of the O-rings in the pack and found that all of them measured closer to 23.9mm. I tried again with the smallest one I could find.

 

Uxcell "23.74mm (18.5mm/2.62mm)" O-ring #2 (actually ~23.85mm-ish): 325.6 fps (min 323.1, max 328.3)
This time there was a more notable improvement over the standard O-ring, but still not close to the performance of the Deep Fire O-ring.

 

I next went through the O-rings in my spares box to find ones measuring towards the smaller end, and did manage to find a couple that were closer to the size I was looking for:

 

Unknown origin O-ring #1 ~23.8mm (2.4mm CS): 333.3 fps (min 330.3, max 334.7)
Unknown origin O-ring #2 ~23.8mm (2.4mm CS): 333.1 fps  (min 331.5, max 334.7)

These both are about the same size as the Deep Fire O-ring that performed very well, and their performance was about the same too...

 

I was unsure how much of an effect O-ring size would have with a full cylinder & long barrel, so I then fitted a longer barrel to the P90TR to see if that makes any difference in the results, still using the Guarder enhanced 3/4 cylinder to begin with. The barrel is a 509mm unknown brand Chinese non-tightbore barrel (IIRC it originally came from my ~15 year old A&K M249). Ideally I would have used a ZCI tightbore to better compare against the ZCI barrel used in the earlier tests but I don't have one of those so I had to make do with this.

 

LPE standard O-ring + 509mm barrel: 309.5 fps
Deep Fire 'super' O-ring + 509mm barrel: 320.1 fps

 

As with the earlier tests the standard O-ring is around 10fps lower than the smaller O-ring. The barrel being a cheap non-tightbore barrel probably accounts for most of the power loss seen here compared with the shorter barrel tests.

 

Finally I put in a Guarder enhanced full cylinder to see how the O-rings compare in a long barrel + full cylinder setup:

 

LPE standard O-ring + 509mm barrel + full cylinder: 321.7 fps
Deep Fire 'super' O-ring + 509mm barrel + full cylinder: 318.1 fps
Unknown origin O-ring #1 + 509mm barrel + full cylinder: 320.6 fps

 

Interestingly, this time the Deep Fire O-ring actually performed slightly worse than the LPE O-ring. I was expecting that with the full cylinder all O-rings would perform about the same, maybe with the smaller O-rings having a slight advantage. I tried one of the unknown O-rings that was the best performer in the initial tests and while that was better it still had slightly lower performance with this configuration than the LPE O-ring. I'm not sure why this is, as they all form a seal right from the start... it may be that the smaller O-rings have more room to inflate against the cylinder and that with the lack of any significant acceleration phase prior to compression starting this costs some energy that otherwise would be used to propel the BB.

 

 

Conclusions

 

In builds using full cylinders the normal 24mm AEG piston O-rings would seem to be slightly better, although the difference doesn't seem to be that big.

 

However in builds using ported cylinders it looks like it's better to use smaller O-rings, as was the case in both sets of results using the 3/4 cylinder. It can be difficult to find such O-rings though since most AEG piston O-rings are about 24mm and generic O-rings are typically only supplied in a standard set of sizes that are either slightly too big or slightly too small, and even if you can find some that are supposed to be ~23.7-23.8mm they're not likely to be made with such a high degree of precision, as I found out with the ones I bought.

 

Anyway this might be something to keep in mind if you want to gain/lose a little power, or if you're just looking to maximise efficiency - it may not be a huge boost, but a potential 10+fps isn't insignificant and if done in conjunction with other small things that can be done to help boost fps this might be enough to allow the use of a weaker spring to achieve a given power level, which in turn will reduce stress on the rest of the system/improve reliability as well as slightly improve ROF and trigger response etc.

[Adolf Hamster]

interesting stuff.

 

did you check consistency as well as out and out fps?

 

reason is i've intuitively leaned towards tighter o rings on the basis i'd rather lose a little energy to friction and keep as much air as possible.

 

would be interesting to know.

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I did watch out for that while testing, but there was no notable difference in consistency between them. I don't think you're likely to lose any power/suffer consistency issues to such air loss unless the cylinder is undervolumed for the spring strength & barrel in use.

 

Edit to add: All of the O-rings I tested here do seal fully against the cylinder without relying on pressure ahead of the piston to form a seal, so there shouldn't have been any losses that way... the only piston head O-ring that's loose-fitting enough to not seal outright is in my P90RD & I'm not going to take that apart and mess about with it/invite problems while it's running well

[EDcase]

Interesting results.

I remember when I first opened a gearbox I was initially confused by the 'loose' O-ring on the piston head ?

I assume the elasticity of the rubber plays a part as well.

 

 

[Tactical Pith Helmet]

Interesting.  The traditional way to tune airguns was to gently sand an overtight o-ring/piston seal until it allowed the piston to slide freely under its own weight only when your finger was removed from blocking the transfer port (think nozzle) the cylinder held upright port upwards, piston fully forward. 

 

I was told that this was too tight and that a secret of tuning was to remove just enough that it just moved wen the port was blocked.  Doing that gave similar results to those that you found.  The looser fitting seal gave better fps.  

 

Likewise, low friction seals made from PTFE performed better than 'grippy' rubber ones.  

[ak2m4]

Interesting results, always good to keep a few orings of different sizes around since the inner diameter of cylinders varies as much from 23.6mm to 24mm.