Poly orings for hp piston seal in mk20 no good?

[elan]

One of my MK20 regs started whistling like a locomotive when pressurized, it also looses air through the ambient chamber holes. I know that happens when the HP 010 oring is not in a good shape , i.e too soft. This particular reg is on of my “cheap rebuild” regs which I did not use a proper kit on.
For that oring I used a polyurethane 010-90 just like I did on some of my mk5s and never had issues. These orings are now about 5 years old but they are stored sealed in zippies and look brand new. The reg has may be 10 dives since I rebuilt it about two months ago.
I replaced it with an EPDM duro 80 and the whistle is gone and there is nonleak anymore.

Is polyurethane bad for this location and I better just stick to EPDM ? or it just the oring turned out to be bad. The piston surface looks good, there is a fair amount of crystolube on it not too much and its not dry either.

 

[axxel57]

I have been using for decades rubber o-rings (nitrile) buna 90 on the MK20/25 piston stems, because I couldn't find any difference in performance to the more expensive EPDM o-rings.
I wouldn't use the polyurethane o-rings there, because to me they waste faster and get bridle.
I think they are used only in static applications in DIN Adapters and tank valves.
I wouldn't know where they might be used in regs, don't remember having them seen in them.

 

[elan]

I have been using for decades rubber o-rings (nitrile) buna 90 on the MK20/25 piston stems, because I couldn't find any difference in performance to the more expensive EPDM o-rings.
I wouldn't use the polyurethane o-rings there, because to me they waste faster and get bridle.
I think they are used only in static applications in DIN Adapters and tank valves.
I wouldn't know where they might be used in regs, don't remember having them seen in them.

This started happening when the temps dropped to 32F. The oring might have become too hard. But I guess these will be the last poly orings used for the dynamic application

 

[happy-diver]

Some of the orings I use are over twenty five years old
I also keep all old orings and reuse them when required

I have heard the extra lube on piston shafts and pay it absolutely no heed
considering that I use very little lube on anything except the SPG spindles

I have found on Mk10s both piston urethane orings cracked and in pieces

Tried some urethane orings on Mk10s and they squealed so back to epdm


Screeching happened in air, in warm water, it temporarily stopped when I changed seats and came back

 

[Couv]

There is nothing wrong with using polyurethane o-rings in that application (piston stem o-ring.) In fact, SP used them in the MK10 service kits in the 1990s. However, the shelf life of the PU seals is short, so if you purchase a handful (like I did) and they start to go bad, even sealed in a bag-you may not get very much use out of one, much less the handful you paid for. Ask me how I know.

They also don't last more than a few years installed in a regulator-especially a reg that does not see much use. PU seals are also more expensive than good old EP or Viton which have unlimited shelf life.

PU seal are known for their resistance to extrusion which is probably why SP liked them for the short period they employed them.

The "green" seals in the picture below are actually amber in color. They turned green from the verdigris found in a couple of un-cared for MK10s. Note how crispy and brittle they look.

 

[elan]

There is nothing wrong with using polyurethane o-rings in that application (piston stem o-ring.) In fact, SP used them in the MK10 service kits in the 1990s. However, the shelf life of the PU seals is short, so if you purchase a handful (like I did) and they start to go bad, even sealed in a bag-you may not get very much use out of one, much less the handful you paid for. Ask me how I know.

They also don't last more than a few years installed in a regulator-especially a reg that does not see much use. PU seals are also more expensive than good old EP or Viton which have unlimited shelf life.

Thank you, "mister Jethro Tull" ! Informative as always. I think the shelf live is probably taking it's toll then. I will stick to EP for now, I still have few of them left and the next batch will probably include only EP and Viton and a few 111 for the DIN.

 

[halocline]

I think the SP kits for the MK20-25 contain a duro 85 HP o-ring. Extrusion is not as big an issue with the later 1st stages that have the bushings. I used polyurethane o-rings for the HP piston spot on my MK10s for a long time, and they worked great when they were fresh and supple. But eventually I just gave up and went back to EPDM or viton. IP does rise a bit more at high tank pressures, presumably because the o-rings are a little more susceptible to friction, but I don't care. They work and last a long time, that's what matters most.