Draft Notes on Working with O-rings for Air Guns

[Gwhite]

I've been doing a lot of repairs on air guns lately, which invariably involves O-rings. Especially in pistols maintained by others in the past, I've noticed evidence of what I will call "less than ideal" practices (including some by the manufacturers). Because I'm not going to be around forever, I have been trying to put together maintenance instructions for the college team I help coach. That way, whoever gets stuck as armorer after me doesn't have to start from scratch.

I've put together a set of notes on working with O-rings. I hope they are relatively complete & accurate. I would appreciate any feedback on details I may have missed, tricks others use, sections that are confusing, etc.

The draft document is 8 pages, and with all the pictures, it won't QUITE fit under the attachment size limit on the forum. I parked it on Google drive here:

https://drive.google.com/file/d/1VwNLhh ... sp=sharing

Once I've had a chance to incorporate any feedback, I will make the final version available to the forum.

Thanks!

[seamaster]

Thanks a million, maestro!

[Oldnslow]

Excellent resource, thanks very much!!

[rmca]

Great article Gwhite!

May I suggest adding a table with the dimensions of the o-rings, from the guns you serviced, or other sources.
Even the ones that are supplied by FWB and Styer on their manuals, are just a website change away from being lost from the general public.

I'm glad to help in this.
Best Regards

[6string]

This is great!
Also, RMCA has a great suggestion as far as a make/model database.

I think this would be an excellent addition to the Pilkguns 10P file???

Oh....
Thank You!!!

[Gwhite]

A database of O-ring info for various pistol models would be a valuable resource, but it's beyond the scope of the notes I put together. It would require constant updating as new info came to light, and new pistols were added. I've put together a document for Tau-7's, which I believe I have posted in the past, and I'm slowly working on one for Benelli Kites.

I will be happy to provide what info I have to the effort, but I think I would prefer to keep the general how-to-deal-with-O-rings notes as a stand-alone document.

[David M]

Just a note on hardness, I tend to use the softer #70's on working O rings and #90's on static O rings.

[Gwhite]

Thanks for that suggestion. I've never differentiated them that way, but it makes some sense. However, it goes against the notion that you want harder O-rings for wear resistance. I wonder if the Parker design book has any suggestions along those lines. It's not very well organized, but at close to 300 pages, it's pretty complete:

https://www.parker.com/content/dam/Park ... D-5700.pdf

I'll have to do some research....

[Gwhite]

Here's what Parker has to say on the matter:

"In dynamic applications, the hardness of the O-ring is doubly important because it also affects both breakout and running friction. Although a harder compound will, in general, have a lower coefficient of friction than a softer material, the actual running and breakout friction values are actually
higher because the compressive load required to achieve the proper squeeze and force the harder material into a given O-ring cavity is so much greater.

For most applications, compounds having a Shore A durometer hardness of 70 to 80 is the most suitable compromise. This is particularly true of dynamic applications where 90 durometer or harder compounds often allow a few drops of fluid to pass with each cycle, and 50 durometer compounds tend to abrade, wear, and extrude very quickly."

The most common air pistol dynamic seal application is on PCP regulator pistons. I know that Benelli uses a hardness of 93 polyurethane O-ring for their piston seals. I don't have data on what material or hardness anyone else is using, but it's definitely a good applicatioin for polyurethane due to the wear resistance. Based on what I've seen, I'm not sure Benelli's engineers were super expert at O-ring design when they developed the Kites, but the piston O-rings are generally fine if they haven't been over pressured by a seal failing elsewhere. The seals that die in the Kites have almost always been Nitrile O-rings that have fossilized badly. That's usually from oxidation and/or ozone exposure. If they don't lubricate their O-rings, that makes things worse.

[David M]

I found that the static tank seals seem to last a couple of years longer in #90's before deformation and cracking than the #70.

[mus]

Great resource!

As a removal tool, I usually use a bamboo (bbq) skewer, sharpened/pointed to the required thinness. They tend to be stronger than toothpicks, IMO.
I've had some success in hard to reach areas (-> very narrow groove the o-ring sat in) by tapering a skewer over a longer length that would allow some degree of bending so it could get under the o-ring, while not scratching the metal surface underneath.

[David M]

Also, some difficult to locate seals (Walther pressure reg face seal comes to mind) can be located on assembly with a very small amount of super glue applied with a pin. Helps locate and retain during assembly and working life.
As an assembly I use a grease from "Festo "pneumatics.

[Gwhite]

As a removal tool, I usually use a bamboo (bbq) skewer, sharpened/pointed to the required thinness. They tend to be stronger than toothpicks, IMO.
I've had some success in hard to reach areas (-> very narrow groove the o-ring sat in) by tapering a skewer over a longer length that would allow some degree of bending so it could get under the o-ring, while not scratching the metal surface underneath.

I've been meaning to grab some bamboo toothpicks to try, on just that theory. I'll add that to my next grocery list. I think they are fairly common these days.

I'll add a note. Thanks!

[Gwhite]

Also, some difficult to locate seals (Walther pressure reg face seal comes to mind) can be located on assembly with a very small amount of super glue applied with a pin. Helps locate and retain during assembly and working life.
As an assembly I use a grease from "Festo "pneumatics.

If the seal is properly lubricated, I'm not sure how that would help.

I have used thick grease on a toothpick to manipulate things like the tiny ball bearings used for sight detents. There, the excess grease is fine. You could use the same idea on tiny seals, but you'd want to clean up most of the grease once it was in place, which might just dislodge it.

[brent375hh]

I strongly believe that no air gun manufacturers make use of any proprietary seal.
I needed to replace a "8.92 X 1.83mm" seal. It does not exist, but a "dash number 904" has those exact dimensions when converted from inches to metric.

[Gwhite]

If it's a regular round O-ring, it may be hard to find an exact replacement, but again, you can almost always find a substitute that will work. There are fancier seals that some manufacturers use that are probably harder to substitute. Benelli (and I believe Morini as well) use a "hydraulic rod seal" for the air cylinder seal the mounting pin goes into. They are also called "U" seals, and they look like this:

Parker U seal.jpg (17.02 KiB) Viewed 124229 times

All of the seals Benelli uses have Parker part numbers, except one. Its the firing valve seal, and it's another another U-seal, but in a red material. It's smaller than the cylinder pin seal, and may be a softer urethane than the Green stuff. Because it's doesn't have a Parker number (or any other vendor info) and it does have a Benelli part number that matches their in-house manufactured parts, my best guess is that it's non-standard, and is probably made by Parker for Benelli. That doesn't mean Parker hasn't added it to their catalog since the design was requested. The good news is that I've never seen one fail (so far). At some point I want to measure one and see if I can figure out what they are using.

[brent375hh]

I have seen seals like that in hydraulic rams and most motorcycle forks I have rebuilt, some have a tension spring in the groove. Having worked in manufacturing equipment building most of my life, I don't think I ever saw an engineer design anything that they couldn't buy off the shelf. We work with Parker Hannifin on applications, but have yet to give them our design to make custom. Some our equipment might cost more than all the Kites ever made, I can't think there were enough Kites sold that it would justify a proprietary seal that was only ever used just there. Benelli may well have used a design that was hard enough to find that it encourages people to send their guns back, or pay $25 for a $3 seal.

[Gwhite]

The big expense in making seals is the mold. With CNC manufacturing, the costs on even that have gone way down. Lots of places now advertise that they can make custom size O-rings for reasonable prices, even in very limited qualtities.

It's possible Benelli specified the U-seal for the Kite firing valve using a standard size part (mold), but in a different material from stock.

[Gwhite]

I found some very nice (and very cheap) bamboo toothpicks at Ocean State Job Lot. They are "Baker's Secret" brand, and you get two plastic bottles of 170 picks each for $1.99 (340 total). They are standard size picks, and are definitely much stronger than the regular birch wood ones you get at the grocery store. They also have a smoother finish. I have one of these Tablecraft #236 dispensers on a couple of my workbenches, and these bamboo toothpicks should feed just fine:



You can get the dispensers at restaurant supply stores for a lot less than what Amazon wants for them.

Next time I'm near an Ocean State store, I'm going to stock up. I'm definitely going to update the O-ring guide to recommend bamboo toothpicks!

[thirdwheel]

Very useful instructional primer and "how to" for those confident enough maintain their own sports equipment and of course anything else that has an O ring. Clear, well written and when a clear picture is needed to give an example or explain fully it is there.

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