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 Post subject: O ring sizes
PostPosted: Fri Jun 09, 2017 11:53 am 

Joined: Fri May 05, 2017 11:41 pm
Posts: 48
Location: Western NV

I need new O rings for the JT adapter, (that fits the 88/90 gram Co2 cylinders),
both the inside and outside O rings, they are broken/cracked etc,
I think the inside O ring looks to be a square type?

What are the correct sizes and types I need?


"Speeds fine, but accuracy is final"

QB-79, 22 cal
Airforce Condor 22 cal - pending

 Post subject: Re: O ring sizes
PostPosted: Fri Jun 09, 2017 4:26 pm 

Joined: Sat Jan 25, 2014 9:16 pm
Posts: 1271
Location: United States
I don't have any CO2 guns but I use Nitrile for larger CO2 tanks, like for welding, and they work better than various other types of seals I've tried. Point being they can certainly handle the pressure, you just have to consider they dry and crack after x years, so I suppose check for cracks or hardening each year. Generally when they start to take the shape of the spot they fit in it means they're getting questionably hard, like you said yours seems squarish, which is may have been but most used squarish O-rings were round. Actual square O-rings are not common.
So just measure the old O-rings or the groove they fit in. Hardware, plumbing and auto parts stores are where I'd try locally.

TMI but here's something I wrote to someone else recently which may have some useful info:
Google AS568 which is a sizing standard for "Standard" O-rings. ISO3601 is the std for Metric but I rarely see metric. DIN3771 is German so you might see it on a German gun. So AS568 is the norm and no doubt all you'll find locally. Plus they usually don't need to be perfect so you often use a std in a metric spot etc. The dimensions are usually not very precision so often what you buy is not actually spec, and often the spot where they're supposed to go is in between sizes but either way just decide if you want it a bit tighter or looser fitting, or maybe buy two or more and see which you prefer.
So after the AS568 is a -#, eg AS568-109 is .299"(ID) x .103 thick. The AS568 is usually a given so we just call it a -109, or just 109. Just remember they use ID and sectional thickness, not OD which is where many peep get screwed up.
Typical material is Buna-N (Nitrile), but if they have Viton I think it's worth getting for most things.
They last longer when lubed, specifically w/ silicone. Other lubes like generic grease and oils may react badly depending on the lube and rubbers quality, which is one good reason to get Viton b/c imo all the cheapo undesirable O-rings are Buna or imitation Buna so I think Viton most likely eliminates the odds of it being crap quality, plus viton is more resistant to lubes/chemicals etc than the best nitrile.
There is also a hardness rating and the most common is Buna 70 and Viton 75. I think the range is ~40-120? and higher is harder. 70 and 75 are basically the same as far as Joe Blo is concerned. Few things need a diff hardness so those are what stores usually have but there are more options in material and hardness when ordering online.
If the O-ring is a translucent whitish or yellowish I think it's nylon or maybe whatever plastic is cheap and works. My guess is the breech seals on a Crosman B18 are nylon and imo better if replaced with a Nitrile or Viton -109. If it's opaque white and rather stiff I think odds are it's Teflon. The B18 main seal is kinda opaque white but when compared Teflon the B18 seals is not truly opaque in comparo. Teflon seems a bit harder too but I suppose they could make them softer if they wanted and a teflon main seal sounds like it might be pretty cool? Teflon has a long life, very resistant to oils and misc stuff, and can handle much higher pressure but it's too hard to replace most rubbery O-rings. If something has a teflon seal it likely needs its strength and/or chemical resistance so I'd replace with the same.

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