O-ring/Seals 101

by Tim McMurry

Hardness for o-rings is A scale range Durometer. Generally o-rings are available in soft versions (50 -70 Durometer. No matter what type of material you use at that softness it will not contain CO2. Depending on the material around 90 Durometer most materials start containing it with little degradation. Over 100 Durometer and you have a material that can be used for CO2. How long it lasts and how flexible it is will be a function of the material.

Each material has an application.

All Crosman main seals in the 1940-50's were Viton. The pumpers used softer material. High Durometer urethane wasn't yet. The main seals and valve gaskets are measured on the D scale with the pumper’s main seal having 50 and the CO2 70 Durometer. The D scale is about twice as hard as A scale. On Main seal replacements I use 70 Durometer Urethane (D-Scale)
Any material mentioned will get the job done as long as it is a very hard seal. Problem is Hard Buna is prone to cracking and is hard/impossible to install. Viton is a great main seal in the D-scale range but as an A scale o-ring it has very limited applications.

On Inlet seals the best choice for multipumps is solid PTFE (Teflon) and on high pressure inlet applications I like to use the good urethane o-rings installed on inlet checks made of metal to take the load.

Viton 70 Durometer o-rings are my choice at the valve OD of a Crosman pumper. A LOT OF HEAT IS GENERATED THERE AND THAT SEEMS TO COOK URETHANES AND BUNAS TO DEATH.
Urethanes are very flexible, durable, and wear resistant and can often be used and reused in dynamic applications.

Nothing seems to be tougher than a mil spec 90 UR made by Parker. That will still be 90+ Durometer hardness when tested 10 years down the road. They don't dig heat but can be very tough in applications where all else fails like the high pressure stage of an FX Pump. It has always surpassed all others in real world durability, longevity and hardness. These o-rings, when not tortured, measure life in decades instead of days.

If you want a CO2 gun to hold absolutely that is the only way to accomplish it.
Urethanes can be severely degraded by heat and/or chemicals and petroleum’s so it is important to keep your lubes for the pumpers and let the CO2 guns run relatively dry.
In airguns, penetrating solvents and firearm metal lubes are to be avoided like the Plague. FP10 has a tendency to attach o-rings and soften them up to degradation levels. The KryTech (Mac1 Pellet Wax) seems to not affect the seals at all and is a good choice for PCP's and springers alike.

O-rings that last for decades make my repairs some of the most reliable on the planet. When you go through a valve and set it up for the long haul you don't want the seals to decide your duration. In my opinion anything less than 90 Durometer American made Parker Seal urethane in CO2 guns is just a waste of your money and won't last.

I'm picky who’s 90's I buy because one manufacturers 90 UR is right on the 90 Durometer mark and will still absorb CO2(IS). Mac1 o-rings are 100+ Durometer and never pass gas. When o-rings seep, even the smallest amount, you will never see peak system pressures. The gun will always have diminished performance and efficiency because it will always be slowly chilling itself and venting off peak pressures till it leaks all out. On the low pressure side these o-rings suffer the Bends and are ruptured from within by CO2 liquid vaporizing gas to atmospheric pressure as it passes through the membrane.

Sometimes you have to spend real dollars to get specific parts that work. In the case of the QB the factory material is close but no cigar. They now have main seals that are great and IS o-rings which are good but the rest of the seals in the gun need to be changed out for best power, longevity and durability.

I sell a seal kit for $15. It includes a replacement main seal (70 Durometer D scale) for the QB stem to fix the early soft stems. You can send your QB valve complete for $25 and have the Upgrade to Mac-1/QB valve specs with the complete set of quality Parker 90UR+ (urethane) o-rings. The only place I use Buna is on the barrel where a 50 Durometer Buna one size smaller than the factory part works best.

We have every size of 1/16" wall 90 durometer urethane o-rings from #003-016 in stock and many others in 3/32" wall as well.
Any airgun hobbyist will have no problem finding applications for most every size.

003 is QB .177
004 is Sheridan 5MM and Career .22
005 is the piercing cap small seal in most of the Crosman’s like MKI's and many bolt o-rings.
007 is a neat breech seal on most QB's.
009 is the size for the Axsor probe o-rings and the breech seal on all the guns Like the FX that      

       have a .22 caliber barrel and a breech seal fit to a groove in the barrel ID.
010 is an upgrade o-ring for the foster Quick Connect.
113 is the QB cap and valve o-ring and also the o-ring for most Crosman caps.
112 is the inner valve o-ring for a 150 valve.
111 is the inner valve o-ring for a 2250 so it has a forever seal on the valve body.

 If you buy more than ten o-rings you can get them for $1.50 each (you can mix numbers to make the 10). I can also make you up kits so you have the most common sizes mainly and a few of the not so common ones.

We sell thousands of o-rings every year to dealers and hobbyist can get the deal also if they buy right. In small quantities the Urethanes are $2 each plus shipping is $1.

Many outfits sell 90 durometer urethane o-rings but often 90's will still absorb CO2 and degrade due to absorption of the CO2 in liquid state and then exhausting vapor on the low pressure side of the seal and rupturing it as the CO2 hits atmospheric pressure. The CO2 permeates the seal and saturates it.

Parker Seal o-rings in 90 durometer do not absorb CO2 which means the o-rings will not degrade. Since the USA made 90 durometer's are Mil spec a 90 isn't a really 90(it is better). I learned this the hard way by redoing a lot of valves for free.

I know what lasts and that's all I'll use. I only carry the finest seals money can buy. When your labor is valuable you cannot let cheap seals cause you redo’s.

An o-ring in a valve stem is really not an appropriate application of an o-ring. The dynamics of o-rings allows them to move in the groove and make contact with two surfaces free of the loading and seal due to the dynamics of the o-ring motion in the groove.

Using an o-ring in the place of a gasket just uses an o-ring as a gasket. It can pop out as it did because it is too soft. The appropriate hardness for the QB main seal is 70 durometer on the D-Scale of hardness. 90 durometer A-Scale is soft enough to cost you some power. It works because it is soft enough to comply with the surfaces but it won't last and it won't shoot near as hard as a proper pressed in part.

Tim McMurry