PCP resealing basics

[dgeesaman]

I read here that many people change their own o-rings, but I don’t hear much about how.

I have a couple of PCPs and may add more. I do plenty of mechanical things by myself but I haven’t opened a PCP yet. If I can rebuild a manual transmission I expect I can handle this.

How do you know which o-rings to have on-hand? Material, durometer, lubricants, etc. I would rather spend a few more dollars for Viton or even FFKM if an o-ring may last longer. I have some Krytox grease on hand, is that the best? Any special tools?

Thanks,

David

 

[Bravo1956]

Most have diagrams that detail the size and type of orings. FX has them available on their website. Always have extra breach seal orings, other than that it depends on if you have leaks or need regs rebuilt, if so again check the diagrams ( most have been posted in the threads here if you search). Captain Oring has complete kits available for most.

 

[SilentSquirrel]

Most have diagrams that detail the size and type of orings. FX has them available on their website. Always have extra breach seal orings, other than that it depends on if you have leaks or need regs rebuilt, if so again check the diagrams ( most have been posted in the threads here if you search).

Caption Oring has complete kits available for most.

Did you mean CAPTAIN ORING @Bravo1956?

 

[RWSx3]

I'll be watching your post David. Hopefully there will be numerous helpful answers. Finding access to schematics with parts # could be very helpful in determining o-ring sizes as well as other parts.
A year or so ago I had an issue with my HW 110 ST. During a shooting session I noticed a change of sound and upon inspection I could see a broken o-ring protruding from below the barrel, it was a transfer port o-ring as I found out after sending the rifle back to AOA for warranty service. Fast forward to the other day.... and again during shooting the 110 I noticed "nearly" the same change of sound yet no noticeable broke o-ring protrusion. Inspected closer and could see no barrel breech o-ring present in the slot provided, called AOA and they're sending some out to me.

 

[starlingassn]

ill add this,only thing i will ever lube orings with on a pcp is divers grease 100% silicone.

 

[JimD]

I started shooting a PCP in early 2020. So none of my 5 have needed all new O-rings yet. But several have needed an O-ring changed. I bought a couple assortments from Amazon and they are sometimes useful. I found I needed to replace the O-rings on the adapter to fill my P35s this week because it leaked. A couple from one of my assortments fit fine and solved the issue. The O-rings are probably not super high quality but seem to be similar to what is in my sub-$500 guns.

My P35s came with a complete set of seals and tools but no diagram and no instructions. I found the diagram on line and it lists the size of each O-ring. I have most of them with my sets but not all. The tools consist of a few metric allen wrenches and one thin "socket" type tool used to remove the pressure gauge which is also how you open the air tube to change the regulator. Screw drivers and allen headed drivers are all I've needed for the disassembly I've done on my Prod and Avenger.

You should use silicone grease on O-rings. Buna-N is recommended for most of the O-rings, usually 70 hardness.

 

[rcarlisle]

I keep a full set of all the o-rings required for my two PCP air rifles. I use 100% silicon diver's grease to lubricate the rings. I use a plastic spudger to remove the o-rings and sometimes to install them if necessary. When I open up one of my rifles, all of the o-rings I encounter will be replaced (not necessarily every one in the gun). Using normal Buna-N type rings, I have had guns go 5+ years without needing to replace them.

 

[nervoustrig]

Static and dynamic O-rings carry different considerations and tradeoffs.

Static O-rings
For static O-rings (air tube, valve, etc.), Buna-N is the de facto material of choice. Long life, good resistance to compression set and tearing, and low permeability. Inexpensive and widely available in both standard (AS568) and metric sizes, and in both 70 and 90 durometer.

FKM (Viton) in my experience also works fine for static seals, with the caveat that it swells. This behavior is a nuisance more so than a problem, meaning if you need to disassemble, you will have to either replace it or wait for it to outgas and return to its original size before you can reassemble.

70 durometer is usually good for up to 3000psi and somewhat beyond. Step up to 90 durometer for higher pressures to reduce the risk of extrusion failure. Just depends on part tolerances. If the gap between the mating surfaces is abnormally large and the elastomer is soft (low durometer), the material will extrude into the gap like playdough and damage the O-ring and begin leaking. If it happens, you’ll know it when you see it because the O-ring will develop little wispy wings.

Dynamic O-rings
For dynamic O-rings (e.g. bolt or breech O-ring), go with either polyurethane or Buna-N. On paper, polyurethane is the better of the two in terms of abrasion resistance but in practice I can’t say I notice a difference. And bear in mind polyurethane has the lowest shelf life of all the common elastomers (5-10 years) so you probably don’t want to load up with a bunch of spares on the shelf just to have them age out before you can use them.

Higher durometer improves abrasion resistance as well so go with 90 durometer if you can. You’ll have to use some judgment because a hard O-ring is simultaneously at greater risk of getting a bite taken out if it sees any sharp edges during installation or in daily use.

Lubrication
For static O-rings, a viscous silicone grease like automotive dielectric grease or diver’s grease is helpful as an assembly aid. It also reduces gas permeability which in turn is beneficial to the longevity of most materials.

For dynamic O-rings, a less viscous silicone oil is good. Something like 20 – 30W silicone oil sold as RC shock oil.

 

[Privateer]

Jason, What's your take on Super Lube?
Or even the Krytox lubes.

 

[nervoustrig]

Super Lube is a family of lubricants, some of which are pure silicone. Those are perfectly good for PCP use. However the ones marked "synthetic" and "with Syncolon" are highly flammable. An example:

Krytox is great in terms of its lubricity and inflammability, but needlessly costly for PCP use. The one airgun application I'm aware of that can benefit from it is a synthetic piston seal in a springer / gas ram powerplant.

 

[dgeesaman]

PSA: do NOT EVER burn PTFE-laden products. This releases fluorine gases that are extremely toxic.

Also do not burn Viton (FKM) or Kalrez (FFKM) O-rings.

These products do have higher operating temperatures than standard sealing and lubricant products, but a torch flame easily exceeds any of these materials and the fumes are NOT SAFE in any quantity.

 

[JimD]

The O-ring Store is a great place to buy O-rings. Good prices, reasonable minimum quantities, fast shipment. There are other sources of quality O-rings but I think the O-ring store is the best.