How to Choose Scuba O-Rings and Batteries: Buy the Part, Not the Number

I sell o-rings and battery kits at ProDive Shop, so I get the same questions every week. Most of the time the person asking already has a number in their head, and the number is fine. It just belongs to a different part. Below are three cases where I can point to the manufacturer's own documents instead of asking you to take my word for it.

Ask about the o-ring in a low-pressure inflator hose and you will be told 011, usually with the quick-disconnect end treated as the unreliable one. AP Diving's own hose manual says the opposite.

In Medium Pressure and Convoluted Hose Shortening Instructions, the AP50 inflator hose assembly is listed part by part. Item 4, the o-ring inside the snap connector, is a BS-011-N70. Item 12, the o-ring at the 3/8" UNF end that meets the first stage, is a BS-903-N70. The EV50 and AP150 assemblies in the same document have a 903 at the 3/8" end as well.

This isn't just rebreather kit. AP says the AP50 hose is also used on the AP200 BCD inflator and the AP400 drysuit inflator.

So if you have that hose, "it's an 011" is right, but for the other end from the one people usually mean. Other hoses use other sizes. The ones I stock take an Inflator Hose O-Ring (7.5 x 1.5 mm), and Zeagle and XDeep have their own.

AP also writes N70 after every number. That's the rubber hardness, and they list it as part of the spec, not as an optional extra. If you get the size right and the hardness wrong, you still have the wrong o-ring.

Everyone will tell you the DIN o-ring on an Apeks first stage is a 111. The Apeks TX100 service manual doesn't actually say so. It doesn't give a dash size anywhere. Item 33, one of the two o-rings on the DIN handwheel connector, is listed as AP1166, and that part number is all Apeks gives you.

Retailers match AP1166 to a 111, and they're almost certainly right. But it helps to know where each number comes from: the size is the shops' translation, the part number is Apeks'.

Two more things from the same manual that people tend to skip. Every o-ring gets replaced at a service, not checked and put back. And the tools for removing them should be brass or plastic, because a steel pick scratches the sealing surface. Apeks calls the result a permanent leak.

Energizer's datasheet for the CR2450 has a proper dimensioned drawing, not just two round numbers. Diameter is 24.50 to 24.30 mm. Height is 5.00 to 4.60 mm. There is also a second diameter of 22.0 mm maximum, corner radii of R1.25 and R0.75, and up to 0.25 mm allowed deviation from flat.

So the height alone can vary by 0.4 mm. Two cells can both be perfectly good CR2450s and still sit almost half a millimetre apart. In a tight compartment one of them fits and the other doesn't, and there's nothing wrong with either.

Suunto is a good example. Their manuals just say "one 3 V lithium battery: CR 2450". But their own D-series battery kit, part SS023241000, is listed by European distributors with a CR2450N. That's the stepped version of the cell: narrower at the base so it sits lower, and rated 540 mAh against roughly 600-620 for a standard one. In other words, the manual tells you the battery type, and the kit tells you which version Suunto actually puts in.

The next part comes from our own sales, not from a manufacturer document. We let people choose the cell for the Suunto D4/D4i Battery Kit and the D6/D6i, but not for the D9, because those two are where the fit problems come from. Since October one of the options is a Renata CR2450N, for the older units that won't take a standard cell.

Whatever battery you buy, the cover should go down and catch its thread without you pushing on it. If you find yourself pressing, stop and check. And a cover that closes easily still isn't proof that it seals. Suunto's official advice is to have the battery changed at an authorised service centre, so keep that in mind if you do it yourself.

Each of these manufacturers describes the part in a different way. AP uses a BS number with a hardness code, Apeks uses its own part number with no size at all, and Energizer and Suunto use a battery type with a tolerance range behind it. None of them is the number people tend to quote from memory, so when in doubt, check the manufacturer's parts list or ask the shop for the exact part, not just the size.

What is the correct O-ring size for a low-pressure inflator hose?

According to AP Diving specifications, the quick-disconnect snap connector uses a BS-011-N70 O-ring, while the 3/8 UNF end connecting to the first stage uses a BS-903-N70. Other brands like Zeagle or XDeep may use custom metric sizes such as 7.5 x 1.5 mm.

What size is the DIN O-ring on an Apeks regulator?

Apeks lists the DIN handwheel O-ring under part number AP1166 rather than an official dash size. While dive shops and technicians match AP1166 to size 111, you should always replace it during service and use brass or plastic picks to avoid scratching the sealing face.

Can any standard CR2450 battery fit a Suunto dive computer?

Not necessarily. The CR2450 specification permits height variations up to 0.4 mm. Many Suunto dive computers, such as the D4, D4i, and D6 series, require the stepped CR2450N cell (like Renata CR2450N) to ensure the battery hatch threads down and seals correctly without force.

Why is rubber hardness (durometer) important for scuba O-rings?

The durometer rating (such as N70 or N90) defines the rubber firmness. An O-ring with the correct physical dimensions but incorrect hardness can deform under high pressure or fail to seal properly against dynamic movement.

About the author: Slavi is a diving equipment specialist and founder of ProDive Shop, dedicated to technical O-rings, battery kits, and verified manufacturer service parts for scuba divers.

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