If you really want the 'best' O-ring for torch applications, it's probably going to be Viton. Buna-N's problem is that it doesn't handle heat very well - that's the exact reason it's pretty much universerally not used in military applications. Where chemical attack isn't an issue, silicone is used, and where chemical attack is an issue, Viton is used.
PPtk
your idea is good and we could first measure our o-rings (my flashlights have come all with spare o-rings!!). payment is thru prepayment (bank transfer, no minimum order!) or thru paypal (minimum order 10.00€). international shipping is 4.10€ or higher (which is a little high imho). i had once ordered copper sealing washers from them and was very pleased with the packaging and service; my (national) shipping costs were 1.75€.
KumaBear kreisler is not a trusted boardie in this forum sorry and needs to stay anonymous (e-mail address, paypal e-mail address, RL address, RL name, data etc) for various reasos. let's ask Vectrex for this 'project' :D
@ KumaBear: Brilliant idea. O ring size should be included in flashlight specs (though no reason to suppose these would be any more accurate than lumen outputs..)
EDIT
And you're right, it's DEEPLY frustrating. So many different sizes/thicknesses/materials.. I've just remembered why I kept putting this off.. :\
Should we try to 'improve' on what the (budget) manufacturers send us in the torches? E.g, the rings on my DD 504b's seem very thin and flimsy. Should I buy thicker ones? Or is there a good reason why the factory fitted samples are used? 
Wow this has really turned out to be an interesting thread!
What prompted my OP was that my cheap X2000 arrived and the lens is loose affecting the focus. I wanted to fit an O ring to tighten it up and make it fit firmly against the pill.
I will try and pick up the Harbor Freight set I mentioned above and report back to the group.
Group Buy Proposal: We all need o-rings! (Warning: Long Post)
I suggest we assemble a “kit” containing almost every commonly used o-ring and have a single supplier assemble it and provide it to us at a fair price.
By the way, "the other forum" did in fact do just that years ago but they were principally interested in SureFire at the time and their Group Buy had only SAE sizes - not the Metric that we need. They had planned to get around to putting a Metric kit together but, to the best of my knowledge, never got around to it.
If a few detail-oriented people (with calipers or a very keen eye and a good ruler) will help by measuring some of their o-rings, I volunteer to create a spreadsheet listing as many flashlight models as possible along with their o-ring requirements.
Then I will create a list that includes a few of each of the sizes as seems practical so that we can solicit pricing for it. Kreisler’s source in Germany seems to have every size imaginable – 1000’s I think!
The proposed spreadsheet will yield multiple benefits – even if an individual doesn’t participate in the Group Deal:
1) First of all, when they need an o-ring for a particular flashlight, he (or she) will probably be able to refer to the spreadsheet to find out exactly what size(s) to get – no guesswork, no measuring (and how do you measure a broken o-ring anyway?). The alternative would often be a visit to the hardware store for some trial and error – that is IF they even have something that would be suitable. Note that most of the o-rings available in the US are SAE (in inches), not in millimeters (which is what we need for almost all of the lights we have).
Another way to look at this is that it would obviate the need for each of us to buy micrometers and remove all of our o-rings to see what we're using.
2) We will be able to assemble a kit specifically designed for flashlight collectors and arrange for a supplier to provide it to our specifications.
Some of mine have broken; some have worn out due to lack of lubrication; some are obviously the wrong size for the purpose (due to Chinese supplier neglect of what should be an important “detail”).
I’ve spent hours researching o-rings.
At first I was under the misimpression that I would be able to buy an appropriate kit at a good price and all the sizes I needed would be included – no such luck! There are tons of kits available from lots of sources. E1320 is certainly right that Amazon has a huge number listed. The problem is that the kits are all wrong for flashlight use.
Then there are the Chinese suppliers. DX does have a few useful o-rings. So does FocalPrice – but no one seems to have anything near the complete mix that we would need.
About the cost:
I agree with Kreisler that at first glance the cost appears a little high (if we use his source) – but after considering all of the useless kits – even the cheaper ones, we’d end up being reasonably close in price – but most important, we’d probably end up fulfilling 95%+ of people’s needs and at a very fair price (saving quite a lot because we’d be getting what we actually need) and without having to chase around, becoming frustrated like I’ve been for several days.
OK – How to proceed???
I would assemble the spreadsheet from information provided by a few of the members willing to help – either because they have a little time to do some simple measuring – or out of self-interest. It’s likely that a lot of people will simply want to make sure that their needs are met and that the final list of o-rings included for the kit includes what they want. I’ll help personally with the measuring but my collection is still quite limited.
What information would be useful and/or essential for the spreadsheet?
Please don’t get overwhelmed by what I’m about to discuss. It’s really all common sense and we’re not at risk of blowing up a rocket. There are only a few essential pieces of information. Keep in mind that if we guess wrong on some, the cost per o-ring is likely to be only $0.01 to $0.05.
[ESSENTIAL INFO] Manufacturer & Model Name/Number of the Flashlight
(Alternate Names for the same Light from Different Suppliers would make sense as well.)
[ESSENTIAL INFO] O-ring function or location on the flashlight in question
(Why – because the o-ring between the lens and the crown must be full size so that it’s not sticking out and obscuring the lens but the o-rings for the tubes should be slightly tight to stay in place.)
[HELPFUL BUT NOT ESSENTIAL] Note that the Budget Light Manufacturers/Suppliers we deal with do not always supply the optimum size for the intended use so it would be nice to know the size of the original o-ring along with information regarding whether it seems that, for example, a thicker o-ring would more effectively fill the channel/slot provided for it. (Channel depth and width are both factors influencing this.)
Measuring Parameters: (What Dimensions Do We Need?):
We really only need two (2) dimensions for each o-ring suggested:
[ESSENTIAL INFO] ID: This is the Inner Diameter. The only thing to make sure of is that you are measuring it when it is round. (If it insists on remaining oval, take the long measurement and the short measurement and average them.) Note that you can measure the OD instead of the ID if you find it easier. We’ll just have to subtract 2 x THICKNESS to determine the ID.
[ESSENTIAL INFO] THICKNESS or WIDTH: This is exactly what it says – the cross sectional thickness of the rubber (measured easily with a caliper if you make sure not to squeeze or deform it).
[ESSENTIAL INFO ONLY IF YOU CHOOSE TO MEASURE OD INSTEAD OF ID] OD, or Outer Diameter is simply 2xTHICKNESS plus ID and does not need to be measured unless you’d prefer to measure the OD and subtract 2 x THICKNESS to calculate the ID.
(Some size charts don’t even bother listing the OD but I’ll include it in the spreadsheet anyway.)
A Minor Complication:
O-rings that have been on a flashlight tend to stretch a little bit so it would be useful to know whether the measurements you’re providing are taken from a “virgin” o-ring or from one that has been on a flashlight and been stretched a little. I'm unsure about just how stretchable the o-rings are. In other words, just how much smaller should the o-ring be than the particular tube that we want to cover - or do we order the exact dimension? (probably not) I’ve done a little bit of experimentation here and it seems that a “virgin” 25mm o-ring is roughly one or two mm less in diameter than one that has been stretched. I’d love to get some additional input on this because it probably makes sense to order just slightly smaller than the particular tube so that we get a “snug” fit.
As already mentioned, another variation of this question relates to the o-ring that fits between the lens and the crown. Many of us know from experience that this one should be an exact fit or it is difficult to place in the appropriate position without some of it "sticking out" from under the crown.
About Prices:
The best supplier I’ve seen so far is the one that Kreisler mentioned. I am not “wedded” to them and would be happy to consider any other supplier that has what we need but here’s the link (I hope) and I think you’ll be amazed at the selection they offer:
http://www.ir-dichtungstechnik.de/shop/shopping_cart.php
Kreisler says they have a minimum order of 10 Euros and international shipping is an additional 4.1 Euros. There is a VAT (value added tax) in Germany but I’m hoping that it can be waived if the kits are exported. It would also be possible for two or more people in the US, for example, to share a “kit” because the shipping within the US would probably only be a dollar or two. By the way, although I didn’t see the option on their order form, Kreisler says they accept PayPal – important for cross-border transactions.
To those who feel that the cost is too high, consider that the kits at Amazon and elsewhere are close in price but almost useless. Also, our time has value and chasing around for a particular size of o-ring can use up a lot of that time – not to mention gas.
A Note about Materials:
NBR (otherwise called Nitrile or Buna-N) is probably what we need, NOT Silicon (which is too vulnerable to various chemicals/oils). NBR also has the advantage of being the most prevalent and the CHEAPEST of all of the alternatives. Prices should vary roughly from $.01 to $.05. Just as important, we should be able to fulfill almost ALL of our needs from only ONE SOURCE! - and not have to chase around.
PilotPTK is correct that Viton is a better material but it is A LOT more expensive and I think unnecessary for our purposes. The links that follow will take you to some general information about o-ring materials and their properties:
http://www.marcorubber.com/materialguide.htm
http://www.mcmaster.com/#catalog/118/3488/=h4iijj
The only other variable is Durometer hardness. It is usually either A70 or A90 but A70 is probably best for almost all of our needs.
LET'S DO THIS!
At the very least, we should assemble the spreadsheet.
A little more work to assemble a chart of needs for all the common lights and we can put together a very, very useful GROUP BUY - something that will save us all a lot of time and hassle (not to mention money). By the way, a GROUP BUY would not prevent each of us from ordering individually based on our own specific needs. The other people would simply not benefit from the package discount
COMMENTS AND SUGGESTIONS PLEASE!
Shiningbeam S-mini (18650): 14mm ID, 1mm thick. One at the front, one at the back. (measured from spares supplied with light... one off the light was slightly bigger)
Hi Folks,
As is obvious, I'm a newbie on the BLF. I was totally unaware that there had been "history" regarding Kreisler and I'm sorry both because I've obviously "opened some old wounds" and more important, because I'm hoping that the references to Kreisler won't sour people on the idea of the Spreadsheet and perhaps, the Group Buy.
Needless to say, I would not handle the Group Buy. I hope to help by producing the Spreadsheet from your data (and mine). The usefulness of the Spreadsheet seems clear on its face. Obviously, I would like very much to see the Group Buy happen as well but we'll see how that develops.
BY THE WAY, please respond to the thread regarding the Proposed Group Buy rather than this one. That way, we'll have all the information consolidated in one place.
Thanks You!
KumaBear
Watch O rings seem to have the correct kind of thickness. See here for example
About $11 for 100 assorted O rings.
This fellow in NY will put together a package of 130 pieces based on your required sizes
May be simpler and cheaper just to get one or more of these sets, use the ones that fit and keep the other for a rainy day!
Boys, I don't want to be unenthusiastic about KB's potentially brilliant idea, but think we may have a bigger job here than we thought. I've just measured - as best I can without equipment - the rings from my DD sourced 504b's. The first thing I have to report is that the corresponding ones aren't quite all the same on the silver/black models (details to follow). They are also different on this model: http://lightake.com/detail.do/sku.L2_Flashlight_Body_Tube_with_Extension_Tube_Silver-30640, which isn't quite a 504b but all parts are lego-able (although not quite a perfect fit). Then there will be the genuine SF's, the 'genuine' UF 504b's, 'fake' 504b's from other outlets..
Like I say, I'm really not trying to be negative here, but the fact is that budget models, by their very nature, are going to be somewhat unpredictable from seller to seller, batch to batch, and even individual samples of the same model. As has been rightly noted, O rings are so cheap that we don't need to worry about wasting a couple of bucks on unused or unwanted ones, but we do want to end up with a good fit. Simply put, it's beginning to seem - to me, anyway - that we really might be better off with a few boxes of assorted rings, grouped by thickness, perhaps, that we just dip into as and when, rather than go to the enormous job of compiling KB's spreadsheet which would be, at best, only a rough guide anyway and likely to need changing/updating/deleting etc on a regular basis.
That said, I still think it's a great idea in theory, just a bit impracticable to go to those lengths and try to be that accurate. The idea of approaching a dealer to organise the compilation of O ring assortments is definitely feasable and should be done. Unfortunately samsat's links are of limited use at present. The first one http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=190652390813&fromMakeTrack=true&ssPageName=VIP:watchlink:top:en looked promising, but the listing is for 0.5mm thicknesses only, which is too thin for most of our applications (I would think, anyway). I will email the seller to see if they can supply others; the same sort of selection - perhaps narrowed down slightly - but in 1mm, 1.5mm and 2mm thicknesses. Maybe thicker still, not sure yet, but 2.5, 3 etc.. sound like they'd be useful for some lights.
Both my 504b's have three O rings, these are from the silver one
The smallest on the left (above) is the tailcap ring, and as you can see, it differs slightly from the one from the black torch (on the left in picture below)
No, they're not interchangeable. The thicker ring, if fitted to the silver torch, won't allow the tailcap to screw into position. I could perhaps have forced it a bit, but I'm unwilling to risk destroying the ring till I have a replacement. In the pic, the thinner ring looks to be just under 1mm thick, though my guess would be that it started life as 1mm and stretched a bit whilst in fitted position. The other rings (where tube meets head and where bezel meets lens) appear identical; these are from the head>tube:
Measurements are
although you can see for yourselves that my measuring equipment is a bit , erm, 'un-fussy' :\
And finally these two rings are from the tube>headjoint of the silver 504b and the fake SF L2 (from Lightake) 
Remember that the two above lights are (supposedly) virtually identical. You can't see it in the rather poor pic above, but there is actually a second, much thinner O ring in the SF O ring rebate groove. Is that there because it's supposed to be for some reason? Or because they thought it would make a better seal? Or an experement? Or the thicker O ring wasn't quite the right size? Grr! Too many unknowns.
But for the record, the SF ring measures: ID = 21.5mm, OD = 25.5mm. Again it's a bit of a guess as to whether or not this ring started life at 21mm and 25mm. The UF 504b ring is ID = 24mm, OD = 29mm, so it's also 0.5mm thicker. Incidentally, all these torches (SF and black/silver 504b's) have two O ring grooves at the end of the battery tube, aas if to give the option of doubleO ring fitment. No idea if this would actually work in practice as I've only the one ring for each torch and I don't want to risk chewing one of them.
So, for me at least, fitting O rings to ANY of my current budget lights would be something of a trial and error procedure, since I cannot even be sure that the factory fitted rings are in fact optimum for the job. So, bottom line from me is that yes, absolutely great idea to try to persuade sellers to stock flashlight selection packs. I'm so fed up now that there is almost NO PRICE that I wouldn't gladly pay for, say, three or four different assortments. I mean it. I'd part with $50 or $60 right now, provided I'd know that my O ring needs would be catered for for the forseeable future. (I ended up feeling the same way about medium format photographic slide mounts too, but that's a whole nother ((and far more expensive)) story..)
Here is a useful reference to standard sizes
