Scubapro MK20 - shredded piston O-ring

[fmerkel]

Fine bubbles coming out of the wife's MK20, time for the yearly anyway so I opened it up. Pretty much everything normal except the Piston O-ring was breaking down with some missing pieces. I think I found one on the Octo seal. It leaked and would not 'adjust' so I pulled that apart and found a flattened black fleck stuck to the surface. Fine after I flicked that off.
O-ring, hose lining, some other crud?

So, this is the 2nd piston O-ring to break down on me. These are Scubapro out of a stock kit. I have been inserting the piston without the bullet but I've been doing that for years and this is the first time I've had problems. I DID order up the Mark 20 Assembly tool and Piston Bullet from [scubatools].

While waiting for that to arrive I found a Viton O-ring of the right size for the piston (feels by fingernail to be a tad more 'squishy' than the stock), and put it together having changed nothing but the O-ring. No leaks, kind of a mushy IP lock starting at 135 and drifting up to 145. Pulled a shim, IP dropped to 125 and the IP got even mushier.
Swapped out the Piston (old seat mind you), and the IP kept creeping until the 2nd stage would start burping. Put the shim back in and got the same result, but at a higher IP.

Put in an old 'retired' piston I had marked as having this kind of creep problem....but, I had worked the edged with 1000 > 2000 grit wet and dry.
Wham! Immediate decent lock.
Looking at all the piston edges with a +5 magnifier and a jewelers loupe I can't tell any real difference.

The questions are:
1. Why is the O-ring shredding?
2. If the Viton O-ring is working is it reasonable to use it? I certainly don't have to as I have kit replacements.
3. Why is this damn thing so difficult about locking the IP? I have a bunch of different regs, piston and diaphragm from many manufacturers and this one is a real pain that way. But my wife loves it, so I keep it going.

Kind of long winded. Thanks in advance.

 

[guruboy]

Maybe you nicked the o-ring when inserting the piston.

Is your piston edge clean/undamaged?

 

[fmerkel]

Possibly nicked, which is my first suspicion and why I ordered the tools. OTOH, I lube things well and insert slowly with a bit of a 'wiggle'. Been doing this for years and only the last 2x have I had issues. O-rings are damn tough and the piston edged isn't that sharp. I'd probably have to try hard to damage an O-ring of 90 durometer with a piston edge. Hmmm, may do some experimenting with that.

Careful inspection with good magnification shows nothing to the edge of 3 different pistons to alarm me. Realistically I can't see any issues. I have a friend with a really powerful scope I may take them over to look at.

Will a softer Viton (assuming it is softer) O-ring work for the piston shaft seal? Sealing right now, it is.

 

[halocline]

I think you really need a duro 90 or at least 85 o-ring for that purpose. Softer o-rings there will extrude more and cause friction on the piston shaft.

Viton is supposedly not as good with abrasion resistance as EPDM is. Couv knows quite a bit about o-ring materials, maybe he'll post.

 

[fmerkel]

Softer o-rings there will extrude more and cause friction on the piston shaft.

Where's it going to extrude to? It's trapped between the reg wall on the outside and the 2 grey guides. Mind you right now it is not in service, just being tested but it seems to work 'normally', less the hassle of getting the IP to lock up.
What's the actual piston movement, half a millimeter?

A bit of research indicates the Viton O-ring is likely a 75D. I did play around with trying to cut both the old 95D and a Viton O-ring with the piston edge. It's not that easy.

 

[redacted]

Since your Mk20 worked fine for some time, I doubt if you damaged that o-ring during assembly' I suspect you have experienced the o-ring failure mode of extrusion and nibbling. But that failure mode should expel the pieces into the ambient chamber, not the LP circuit. If that is your problem, a softer o-ring will just make things worse. If the problem persists, a urathane duro 90 should be your best choice.

Common Reasons for O-Ring Failure - Seal Design Guide | Apple Rubber Products

 

[fmerkel]

Yes, that cross section looks like the damage done. So it probably is extrusion. Thanks for that explanatory link.

How critical is it to change out the 2 gray plastic guides on each side of the O-ring? I may have not changed those out last over haul. I know I did change the O-ring to the stock Scubapro.

I can see in the diagram the direction to put the guide closest to the piston head. How should the 'bottom' one be orientated (other side of the O-ring)? There are 4 little support bumps inside, similar to the top guide, but the bottom one also has a more constricted ring on one end. Should that ring go toward the O-ring, or away from the O-ring? Does it matter?
I assume the guides are instrumental with stabilizing the shaft.

 

[redacted]

I have seen those bushings in white (some split), blaack, and grey. In each case, one of the pair will have a shoulder. That shoulder should mate with the small spring that holds everything in place. The other bushing has no shoulder but does have one end that should be smooth and flat. The other end may have some mold marks or it may just be a bit more rounded. The flat smooth face should be against the o-ring. That will provide the greatest resistance to extrusion.

I usually just re-use the old bushings unless there is a problem (leak or wear). The critical point is the the seal between the o-ring and the wide flat surface that may be dealing with 4000+/- psi. The scary thing is you can get this wrong and it will still work fine, at least for a while. I left the spring side bushing out once and the reg worked just fine. Your nibbling problem may well be from not using the correct face on the critical bushing.

 

[halocline]

Where's it going to extrude to? It's trapped between the reg wall on the outside and the 2 grey guides.

There's a tiny gap between the bushing and the piston shaft, and a 3000 PSI pressure gradient pushing that o-ring into that gap.

 

[Couv]

Your O-ring may be suffering from ED!

The Usual Suspects have covered a a few of the failure modes caused by the wrong (too soft) o-ring hardness. An extruding piston o-ring can definitely cause issues getting consistent IP readings by binding the piston. I like the info linked by Awap as it gives a clear explanation of extrusion and the resulting damage. In all likelihood it identifies your problem.

Because I'm late to the party, I'm stuck with the leftovers, so I'll offer an alternate possibility. Some seal materials, both chemical makeup and physical properties, are less prone to explosive decompression (ED), aka rapid gas decompression (RGD.) Rapid Gas Decompression, Explosive Decompression resistant o-rings and seals for high-pressure, high-temperature applications Yep, just like a diver, seal material absorbs and releases gas. Generally speaking, the harder the material the less prone to damage from ED. However, the seal's compound can also be a factor. FKM (Viton) EPDM, are known to have resistance to RGD. Polyurethane and Nitrile too, but due to shorter shelf life I usually pass on those. Having said that, there are some cases where a good PU o-ring will solve mysterious IP problems.

When installing an o-ring not easily replaced-any internal seal, take a lot of care not to damage it. Use a bullet. Purchase good quality seals too; aviation grade EPDM and FKM is not much more expensive than cheap off brand packings.

Good luck, and please keep us posted on your findings.