But with dynamic o-rings, you might be playing with fire...
The comment about playing with fire on the dynamic o-rings, perhaps you...could comment on the failure scenarios.
We DIY'ers substitute o-rings all the time with abandon. Yet, manufacturers frown on this because our regs are in fact life-support equipment, and inadequate performance could be fatal. I'm not a materials or mechanical engineer, so take this with a grain of salt...
Our concerns with o-rings relate to seal, friction, extrusion and lifetime.
For static o-rings (like a port plug), friction isn't a consideration, so if it seals and doesn't extrude, then folks can use most anything they want. Best example is an old Buna-N (nitrile) o-ring that is so deformed and dried in place that it cracks into pieces upon removal. But until we messed with it, it sealed just fine. Of course, we probably didn't dunk the reg to see if there was a tiny shower of champagne bubbles coming from the plug, but at 135 psi, who'd notice?
At tank pressure, things get dicier. Who hasn't seen champagne bubbles pouring out of a rental tank at a tropic location? 3000 psi, heat and exposure, nicks and scratches - no surprise. Yet, at 90 durometer, they uncommonly extrude absent a yoke application error, and you finish the dive minus a quart of air you might have otherwise been able to breathe.
But loosened DIN bolts and a disappearing extruded o-ring from between bolt and reg body could be catastrophic, and thankfully usually occurs right upon pressurization, instead of underwater. Is that an o-ring failure? Or pilot error? What was substituted for the spec o-ring the last time you replaced the DIN bolt? Why did it loosen?
Among dynamic o-rings, everything is different. Here, friction plays a major role, and lubrication only helps temporarily, unless there's a lube reservoir. Otherwise, it gets wiped away, and only hides a problem until the lube is gone. Let's look at three: piston shaft seal, diaphragm poppet shaft seal and turret/other face seals.
The Scubapro Mk10 versus Mk25 is the classic piston shaft seal improvement. With the old Mk10, the land was machined into the body, and the gap between piston shaft and bore was where the o-ring distorted with each piston movement. The softer the o-ring, the easier the seal, but the greater friction eventually tore away material until the o-ring shredded enough to leak tank air out of the ambient pressure holes in the reg cap. The harder the o-ring, the more resistant to extrusion, but the less able to conform to scratches created by the last tech who didn't know how to use a double-hook pick to remove the previous o-ring.
When you DIY for this o-ring, if you use too small (smaller ID) an o-ring, the o-ring binds more tightly to the piston shaft, increasing friction. Also, the stretch required decreases the cross-section, increasing the likelihood that it will extrude into the gap. If you use a "fatter" o-ring (increased C/S), you are also likely to increase friction.
Material comes into play here, where abrasion resistance becomes a key factor (silicone poor, polyurethane excellent). Viton was everyone's favorite when Nitrox became the norm, though I'll not get into the arguments about ignition here. The problem with Viton orings is that despite a longer lifetime than most of us will dive, it acquires a permanent deformation more easily than nitrile or EPDM, and will eventually lose a seal, especially in harder durometers.
The Mk25 fixed part of this problem by using a stack of cheap washers that were replaced each time that filled the gap between piston shaft and bore without measurably increasing friction. Now, tank pressure could do its job jamming the oring's two critical faces against the bore and shaft with much less risk of extrusion. Replacement was easier for ham-fisted technicians, too.
For a diaphragm reg, we have the same dynamic at work, except that the 2-006 duro 90 o-ring used in many designs is, relatively speaking, huge and fat compared to the shaft and bore. With a very light HP poppet, friction is a critical factor, and bore, shaft diameter and o-ring choice are made with that in mind. For whatever reason, Scubapro made a different design choice and did not use the rest of the industry's 2-006 sizing. Here, stretching a 2-005 over their smaller component would have increased friction.
During Regulator Geeks II, we actually saw friction at work when we compared the IP spread from full to empty tank for a variety of regulator brands and models. I was relatively unimpressed with my new Mk19EVO, as well as a Mk25, compared with other brands, and we demonstrated a wider than expected IP spread. A couple of viewers complained that this didn't reflect their experience, so the next day, I disassembled the offending regs and generously lubricated the HP poppet shaft (where the shape of the HP plug allows a small reservoir of lube that could be expected to last the service interval. With improved lubrication, there was less friction at low tank pressure and IP stability was greatly improved! Clearly, Scubapro made its choice of 01.050.317 size with this in mind (it also amplifies my concern about Scubapro QC coming straight from the factory - in this case inadequate lube - but that's a different discussion).
We can also see a performance change on the lower pressure side. In a balanced second stage, the poppet shaft seals against the balance chamber with a very tiny oring. We have argued about the Scubapro 1x2.2 for years, as it is also a custom size. Folks use 1x2.5's successfully, but I would argue that their poppet motion is more sluggish, and the valve is less likely to open easily and close properly at the lowest Intermediate Pressures. That may be a problem in cold water diving, or in temperate water diving with exertion, where adiabatic cooling stiffens the soft components. It could also materially affect function in the worst of circumstances: where you're nearly OOA, and trying to get last breaths as your tank pressure drops below IP.
Are these friction related performance issues "failures"? In my mind they are, and point out the risk of choosing a "close enough" o-ring for a critical component like piston shaft or HP or 2nd stage poppet shaft.
The last failure that is also critical is in the shallow land that carries the turret seal or the seal in your knock-off Omni-Swivel. Here, it's common to see light between the components, and that's a huge gap. An ill-sized or excessively soft o-ring may extrude here with catastrophic gas loss. It's why I'm such a "generic swivel" hater, because there's inadequate attention to a small clearance gap during diver maintenance, and the third world cheap o-rings in this critical component are (in my experience) frequently softer than 70 durometer. Here, there is no reservoir for lube, and sea water is commonly forced into the gap during dives. O-ring material, as well as sizing play a key role here, and a Teflon-impregnated or coated o-ring might be used, but perhaps never substituted in by the diver doing DIY service.
Yes, you can sub your o-rings. Without knowing what the manufacturer spec'ed, you take on risk as shown in these three areas. I think it's a reasonable risk, because I'm taking my toys apart every year or two and will hopefully see the consequences of a poor choice before failure. I try to make an intelligent choice in material while using the
exact same size o-ring. I'm measuring IP before every trip and comparing that in my mind with what I saw when the reg was newly serviced and "optimal". I'm checking cracking effort to keep my seconds where I like them (and believe me, it does drift). But if you're not monitoring performance on an ongoing basis, the consequence of a bad substitution might come at the wrong time. With rights come responsibilities. And I think it's my right to DIY.
