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Technisub Ranger shorty - removing inner barrel

I've either got or had a Sten which I serviced at least once, but I don't recall what you describe. Maybe a later version.

The Mamba with Seac handle has even a different system. The screw-in outer fitting, that looks externally like the trigger pin plugs of the other Assos, is hollow and has two small o-rings in the hollow. Perhaps at one time one of those was a backing ring instead. Then below that is something aluminum that looks like a pop rivet. The ~1/2" long shaft of that piece penetrates through the handle housing and a small bit of the end protrudes into the hollow bore inside the handle. The mushroom head fits flat against the bottom of the trigger pin well (can't recall now but I think there's an o-ring under the head), and the plug with the two o-rings sits on top of that. The two o-rings are put under slight compression as the plug is screwed in. I assumed the leakage was around the crude 1.5mm pin, but I guess I haven't confirmed that. I just found a 2mm pin that I'm not sure where it belongs, but it's about the right length and when I get a chance I'll see if it will fit through the trigger pin passages.
 
It is actually one of the very first Sten models, this one, they had a bright shiny silvery tank. The early guns had no trigger sensitivity adjustment screw, which is not really necessary. They also had a metal rod that holds the handle butt plate on and is screwed into the grip handle.

There have been a number of versions of the Asso that look virtually identical, and that doesn't include the Asso Up which has recently appeared with either a red or yellow handle depending on whether it is a power regulator equipped gun or single power.
 
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I got this back together quite awhile ago but have been working on one pesky leak (though I haven't tried it with any oil inside). I used 0121 o-rings (2.62mm cross-section) for the 4 large barrel o-rings, despite them being obviously smaller in cross-section than the ones that were in the gun. The forward end of the barrel seals, but I get a slow leak that appears to come from the back end of the barrel tube. 0122 fits sloppily around the inner plugs but did slow the leak to where it takes several days to de-pressurize. I'm now trying alternatives with 3mm CS which seems a better match to the originals, but the one that has ID (26mm) to properly fit the groove, is fat enough that I can't get the barrel over the plugs without more force than I want to use. There is little availability of CS between 2.62 and 3.0mm so next is to try 3x25mm, that will stretch slightly. It's a slow process of ordering and trial as it takes a week for the shop to get the o-rings in. I'm probably one or two trials from finally getting this.

In the meantime, here's a pic of the gun disassembled, with the original o-rings. I took pics of the many o-ring options that I evaluated at each position (most just by best match to the original, not by assembly), but rather than upload all those pics, I'll post a list of what I installed, with some notes, once I'm convinced the gun is pressure-tight.


 
Reactions: Zahar
I have found that if you can get a cross section match then a smaller ID ring could be stretched and still work because when the ring is stretched it only slightly thins out the cross section. Obviously there is a limit to how much the rings can be stretched as they tend to go out of shape. If the cross section is fatter than the original then you cannot squeeze the outer item over it because the radial gap is smaller than the ring cross section. It is worth checking in both metric and imperial sizes as although the guns are metric in some cases an imperial size will work, or at least it is close enough to do the job. However I have found "O" rings in some cheap packs were just the same rings, but relabeled as either metric or imperial, they just rounded the numbers up or down to the closest size, so ignore those. Aldi put out "O" rings sets that are genuinely what the sizes on the box say, these sets are relatively cheap even if you never use most of the rings, coming in both metric and imperial sizes denoted by a different color box, either yellow or blue.
 
Fortunately there is pretty good coverage of cross-sections and diameters, between imperial and metric, and most of the time there is a fair amount of dimensional latitude in which sizes will provide a good seal. Some but not all of the metric sizes offered do seem informed by the imperial sizes. The barrel o-rings across a couple of makes and models of guns have proven to be something of an exception there, even proving difficult to find ideal metric options. I've rebuilt a fair proportion of the dive regulator brands that are out there, and with some exceptions, imperial o-rings are generally suitable. I'm finding with this Technisub gun, and the two SP Magnums I've gone into, that most of the o-rings are best fit with metric sizes. I had an inexpensive set of metric o-rings that are mostly 2.5 and 3.5mm CS - which has proven pretty useful for both brands here - and have accumulated a good set of imperial sizes, so now I just buy individual sizes as needed. Fortunately one of the big industrial supply chains, Grainger, has a local outlet and a decent o-ring selection, with many sizes offered at around $4 for a 10-pack. The online sources are much cheaper and with greater selection, but come with shipping fees which makes serial small purchases painful. The 3x26mm o-ring seemed like a good candidate, based on groove dimensions and barrel ID, and by comparison to the 0121 and 0122 dimensions and their fit, but I just didn't want to force it as much as seemed needed, so I think I must be pretty close, and there are some incremental options between 25 and 26mm ID as well. In a pinch, I could live with the slow leak as is, and it might even improve some once oil is introduced.
 
The problem arises when some rings are especially made for the manufacturer and have no direct counterpart. The Mares body ring seems to fall in that category and provided a sufficient volume can be ordered that results in a ring not being available from other suppliers. Generally they try to use existing sizes.
 
spoolin01 said:
I have to wonder why they wouldn't just engineer around available sizes.
Click to expand...
Pneumatic guns used rubber packings as gaskets and very few "O" rings in the distant past when the guns were completely made from alloy. Possibly when they switched over to plastic bulkheads and handles in the late sixties there was nothing available in some of the intermediate sizes that they wanted, so they commissioned special "O" rings from a manufacturer in their industrial area. Now "O" rings are very common and available in many standardized size ranges, but that hasn't always been the case. Also once you have made your injection molds you are stuck with how they turned out, whereas metal parts can be changed in production with revised cutting programs for the grooves. Could be that a stock size which was planned to be used had to be abandoned in favor of a special size and then that continued on as a legacy component in all their subsequent models..
 
There seems also to be some commonality of barrel o-ring sizes across brands. I suppose that could be an OEM consequence, can't say I know the whole picture!
 
The precision bore alloy tubing availability probably decided that as guns were usually 0.5 inch or 13 mm. The 14 mm barrels were less common and then 11 mm appeared in the nineties, that is for Italian made guns. The tank tubing was 40 mm for most Italian guns, especially with long tank rear handle guns which would float after the shot.
 
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