O-rings (especially trigger pin)

Diving Gecko said:

Hi Tomi,
Both yes and no, I think. But again, you are the engineer
But here goes:
The fractions mentioned (like 1/16) are "nominal" which basically means that they are not the accurate sizes, but the are close to and what people use in everyday language. If you convert these nominal sizes, you will indeed find they are wrong.
The real, accurate sizes are mentioned in inches as well and if you convert those to metric, you'll find that they are spot on.
So, eg. what they call 1/16, they also say in real, actual size is 0.070 inch which will convert nicely to 1.78mm.


D.

Click to expand...

Interesting ...
1/16 = 0.0625 and they take it to be 0.07 inch? Why not 0.06 inch?
I did not know that. Thanks!

Diving Gecko said:

Hehe, it was confusing for me as well. Until I kept looking at the list and saw the word "nominal" - and then to be sure I even looked it up in the dictionary:

Nominal
(of a quantity or dimension) stated or expressed but not necessarily corresponding exactly to the real value.
​

And since the numbers are from the Parker o-ring book, I think they should be trusted, but indeed a bit strange

D.

Click to expand...

I have an old "O" ring catalogue which lists the size of each ring in terms of both its application size and the actual physical dimensions of the ring itself in tables. The application is quoted as the "nominal" size, whereas the actual ring size is slightly smaller, say in the case of the ID of the ring compared to the shaft that it will seal on. The company that produced the catalogue and the seals is long out of business, all the sizes are listed in imperial measurements, so not much use these days, but it has sections on ring selection and interference fits, ring extrusion under pressure, shore hardness and other related matters, so I kept it.

Seems to me that the guy representing Seac Sub confused the part number extension S812017 with the ring number 2007 in his e-mail to you, the part numbers usually don't mean anything outside the company's own internal designation. The ring size is certainly not 2017, or 2007, but 2004. Some "O" ring assortment sets from China have ID x OD to label each group of rings in their packaging tray's individual pockets and metric sets may just be essentially re-labeled imperial sets with exactly the same rings in both sets, but with the latter described as the nearest (but not stated as such) equivalent metric size which can be very misleading when you use them to find out that they don't actually fit properly.

For O-ring ID 5/64 W 1/16 compressin would be 29%.
For O-ring ID 1.78 W 1.78 compressin would be 35%.
I took for boring (well) to be 4.2 mm and pin OD 2 mm.
Metric replacemnt would be OK!

The original "O" ring will be from a metric series. "O" rings are made in different size formats to various engineering standards. What is being recommended is a close substitute from a series in imperial sizes, some ring sizes are similar enough from different series to be substitutes for each other.

There was a discussion of Mares "O" rings on this thread http://forums.deeperblue.com/pneumatic-spearguns/91719-difficulty-determining-correct-o-rings-older-mares-pneumatics.html a couple of years ago.

One "O" ring that I have tried to substitute without any success is the Mares body tube or tank "O" ring on the "Sten/Cyrano" model guns. I once ordered a dozen ID 34 mm x SW 2 mm "O" rings from a supplier thinking that they would fit if I could squeeze the ring section down slightly as there were signs that the original "O" rings were similarly flattened after installation. As it turned out the tank tube would not slide over them, there was just too much rubber for the lip of the tank to push over the top of the "O" ring. The ID of the cylindrical alloy tank tubes is 38.0 mm and the plastic muzzle/handle boss size on the older Mares guns was of OD 37.8 mm, so they were a very tight fit in the tank tube without any "O" rings fitted; they are not such a close fit these days!

The width of the seating groove in the plastic boss for the body "O" ring is 4.5 mm and the diameter of this groove seat is 34.7 mm, so the radial gap from the bottom of the groove to the inner wall of the tank is (38.0 - 34.7)/2 = 1.65 mm. Now the original body "O" rings have a larger section width than that and removed rings which have been in place for a very long time are permanently flattened to a rectangular cross-section which I have measured at 1.66 mm in height (the old rubber must expand again slightly when removed), so that means the Mares body "O" rings are both stretched in circumference to thin their section out as well as being squashed down flat by the tank when in service. Measuring a brand new body ring taken from a Mares repair kit shows the SW to be 2.26 mm and my OD "best estimate" is 35.8 mm as those rings are not exactly round in circumference when loose, unlike the smaller circumference rings which tend to hold their circular shape, so the OD is not that easy to quantify precisely. Assuming that the OD figure of 35.8 mm for the body "O" ring is correct then the corresponding ID must be 31.28 mm. Hence that size "O" ring has to stretch circumferentially as it sits on a groove seat of 34.7 mm OD which requires an increase of 10.9% in ID. The original section width of 2.26 mm compresses down to 1.65 mm in service (to fit the available radial gap) which is a reduction of 27%, but some of that reduction is due to the circumferential stretch reducing the radial height of the rubber sitting in the groove. That is why my hoped for "replacement" metric series rings of SW 2 mm did not fit as they were not required to stretch circumferentially, being of ID 34 mm sitting on a groove seat OD of 34.7 mm and thus they did not thin out in section width at all. So replacement "O" rings need to be closer to the original ring size and not finding any I just continued to use the kit body rings scrounged from leftover sets where not all the supplied rings were used to fix a given problem.

One "O" ring size that I never got to try was "Superfect" part no. SHA/ 91 with ID 1.364" and SW 0.70" with nominal diameter of shaft or recess 1.375" and nominal diameter of cylinder or recess 1.500" (those last numbers are the application sizes, internal or external seal). The metric conversion of those numbers is ID 34.646 mm and SW 1.778 mm, with the application figures being 34.925 mm and 38.100 mm respectively. Looks tantalizingly close! "Super Seals Pty Ltd" who manufactured those "Superfect" brand "O" rings were out of business by then, the catalogue I was using was issued by them.

Why body "O" rings in particular can be damaged is that while pushing the tanks into position the sometimes sharp lip of the tank can bite into the "O" ring and cut it. I have seen this on guns where the "O" ring has been cut and then fortuitously twisted in the groove so that it still seals as the tiny cut has rotated away from the contact area by the smallest of margins. This was found on brand new guns from the factory! When the owner disassembles the gun the "O" ring twists slightly in the groove for the cut to now be back in the contact area and on reassembly the gun leaks despite the owner being careful, or the owner removed and refitted the "O" ring without noticing the cut which then was unwittingly installed in a slightly different position in the groove so that it leaks. That is why unless you examine the "O" rings for such minute cuts, indents or scratches it is best to replace all the body rings with new ones.

I decided to check my notes on a total seal replacement job that I performed on a Mares "Titan" many years ago, as that gun had a second groove for an "O" ring installed in the nose cone which on that "Titan" model was the same nose cone as then used in the "Sten" (the first model). Note that I was not the first person to open the gun up! The usual "O" ring groove was empty and it was only the additional groove that carried an "O" ring, a rather skinny one at that. I replaced that "O" ring with a BS 028 ring which I now realize from my old notes is the equivalent of SHA/ 91, so I did use that ring size after all. But what I had forgotten and my old notes show is that the second groove for that ring was cut to a diameter of 1.400" in comparison with the standard groove diameter of 1.375", so that is why the ring worked in that position, plus even more importantly the groove width was only 0.073" (1.85 mm) instead of the usual 0.180" (4.57 mm). I only worked in inches in those days as if you mentioned metric sizes you received strange looks from suppliers who only carried imperial sizes in stock. Metric size "O" rings were virtually unavailable and the Mares distributor was not really interested in gun repairs unless they handled them and the few dive shops back then had next to no spare parts beyond spears and speartips and the odd box containing parts from broken up pneumatic spearguns awaiting either disposal or Judgment Day for their resurrection.

The other "O" ring size I recalled was SHA/ 130 which according to my old notes was the equivalent of BS 027, but I did not use it as I have it marked as a poor match for inside the rear tail cone, not the front as I had written earlier, but have now corrected in my previous post.

The other "Titan" three main body rings were a different size to those of the "Sten", being replaced with SOS-24 (no note of BS equivalent) rings, the inner barrel seal at the muzzle was replaced with a SOS-13 (BS 115) ring. The rings on the power selector rod were replaced with SOS-2 (BS 007) and the rings on the trigger pull rod were replaced with SOS-1 (no note of BS equivalent). "BS" indicates British Standard I guess, but my catalogue did not refer to that info, I only obtained the equivalent standard sizes from the guy at the shop that sold seals and bearings, being a trade outlet for "SKF" products. Fortunately he too had a copy of the old "Superfect" catalogue and was able to work out the equivalent sizes, there being no later catalogue back then to replace it, just a few sheets of size listings from other manufacturers instead. He still had packets of some "Superfect" rings in stock, those being the "SOS" designation rings and some "SHA" sizes, but being on the other side of the counter I could not see them to check them out.

So the "Titan" was put back into action using imperial "O" ring sizes, not metric ones, as fortunately it used sizes which could be substituted, unlike the "Sten".

There is no O-ring. You are right. It is not necessary.

Valve seals against the plastic handle. Both surfaces in contact are flat. Similar is with muzzle on front end of the gun (part 4 and 9 on schematic).

The only way to find out for sure is to check the "O" rings in the gun, the "O" ring at the front butts up inside the nose cone and the "O" ring at the rear seals off the barrel by sitting under the step going into the bore hole through the pressure regulator bulkhead's front face. The exterior diameter of the "O" ring when fitted is more important for this rear "O" ring as the "O" ring there is squeezed between two concentric surfaces, i.e. the inner (barrel OD) and outer (bulkhead bore step ID) seating circumferences, whereas the front "O" ring sits on the inner barrel exterior and its forward face presses against the inner wall of the nose cone's rear well without needing contact on the "O" ring's outer periphery. But which one is which is anyone's guess, you need to look at the step in the bulkhead to check the gap size that has to be sealed off.

The bulkhead or tank "O" rings are most likely special sizes; Mares guns also use a non-standard "O" ring in that location which I discussed here in post #27. At usually 3 of these larger diameter "O" rings per gun they must order a big batch of them from their supplier. Possibly there is a vested interest in selling "O" ring kits and ensuring "brand loyalty", but as small ID ("miniature") rings have numerous other applications there are a wide range of "O" ring sizes to substitute from, unlike the situation with the larger ID "O" rings.

One ring looks to be wrong on the "Hunter" diagram, I think the bulkhead plug "O" ring is number 23, not 16 as shown. "O" ring 16 is the inlet valve "O" ring which will be very small, while the transfer port should be somewhat larger which the bulkhead plug blocks off.

This might be useful information for Hanter owners.
You did a useful job!
Thanks Dave!