Commonly, when O-rings dry out, the OD's will "seem" to get smaller as they stay compressed into the sleeve that surrounded them in the regulator. Really it's just a reshaping. They won't "wear" at all. If that sort of friction is occurring, the o-ring will get chewed up and you'll have a ring failure (usually from lack of lube, or too soft a duro).
The best way to try to estimate what goes in place for a given regulator where you don't have a chart is to measure the diameter of the groove or land that it surrounds. In other words, you're measuring the
Internal Diameter of the O-ring. Then pick the o-ring that is about 3-6% less in circumference, or 1-2% less in diameter. Since most of the rings we are talking about are standard 1/16" O-rings, they'll come from the AS568 chart. Unless they're metric, of course. When you fit them, they'll stretch then, about 3%. There are exceptions, like the wiper O-rings that are designed to be "sloppy" in the groove in which they sit (like the second o-ring on the white duro poppet of the G-250). But generally, sizes are designed around a slight stretch.
That said, a few O-rings have a little smaller outside diameter and ring thickness than the 1.78mm of the standard 1/16" AS568 oring. So if you put a standard 2-series ring in, it will fit too tightly and maybe get skinned on the outside as you try to insert it. A perfect example is the Scubapro 108/109/Bal Adj/G-250 orifice o-ring, which by ID seems to be a standard 2-010 oring. But that turns out to be a really tight fit, so a -902 o-ring actually works better.
Again, consulting the AS568 chart and comparing it with some available documents from the manufacturer will help you choose when you can't get a factory o-ring. For example, in another thread
http://www.scubaboard.com/forums/re...e-o-ring-equivalence-table-sp-apeks-regs.html in Repairing Your Own Gear, I added a ScubaPro chart from the old days that gives exact sizes for the O-rings on a lot of vintage regulators. Easy way to compare and then select a desired AS568 o-ring.
To ensure I'm not giving you contrary info to the previous post, while I noted that the OD's seem to get smaller when stuck inside a piston sleeve (for example), that is the same sort of reshaping that I think steveharris is referring to above. The ID's of orings may get larger as they sit with a 3-6% stretch for a long time and then harden. In that case the ID's are bigger, as he suggested. But you can get a good guess by using a micrometer to check the diameter of the land and then subtracting 1-2%. Of course, we haven't gotten to the friction issue caused by using a non-factory o-ring that may have been designed for a special application (e.g., HP piston shaft). You'll find lots of threads that debate Buna-N vs Viton vs EPDM vs Teflon for various applications. So correct sizing (and duro) is the first step. Starting with Buna-N won't usually get you too far off base, but I'd make sure I dive a newly overhauled reg where you've done some experimenting at shallow depths where you can bail out, before you try cold/deep water with your own choice of o-ring. There's loads of advice on this issue on SB.
Hope this helps!
