Help Refurbishing a pair of old Mares Ministen guns

The earliest "Sten" guns have a butt cap held on with a metal rod, the next editions have the butt cap held on with a self-tapping screw, so your guns are from the seventies would be my guess. Later editions have no butt cap at all, just a plastic lug molded at the rear of the butt opening for a line to be tied to if you wanted a lanyard attachment there. You can use later parts in those older guns as most items will still fit, but there were a couple of changes. The inlet valve is a stem type rather than a ball type in early guns, so you need an "O" ring that fits that valve stem and you should be able to find one from an engineering supplier of miniature "O" rings, just take the old one as a sample to be matched, that is what I did. The other change was in the trigger transmission pin, in the early guns the "pin" is a dumbbell shape with the "O" ring in the centre of it which slides in the bore made for it in the plastic handle. I have never had to change that dumbbell "O" ring, but it will be different to the later versions which actually use a pin. The dumbbell and its "O" ring push out, there is no screw cover like you see in later guns which use a sliding pin. If the bore that the dumbbell slides in is scratched then the rear handle molding has to be replaced, I guess that is why Mares changed the design to use an actual pin. I remember seeing discarded "Sten" handles in a dive shop junk box for that very reason.

Muzzles can be replaced with later ones, but plastic pistons are best used with the plastic shock absorbers, metal pistons with metal shock absorbers. Inner barrel bores can have a lined appearance, but that does not mean that they are badly scratched as the lines are micro-scratches that don't cause air leaks, every pneumatic gun develops those marks with use. Bad scratches corrode as the scratches have gone through the anodized coating that otherwise protects the alloy of the inner barrels. It is better to replace such inner barrels, however the cost may not make it an economic proposition compared with buying another secondhand gun.

Repairing pneumatic spearguns is a bit of a "suck it and see" exercise, there are no guarantees, but patience is often rewarded if you work methodically and replace the seals with new ones, however if you don't disturb them some "O" rings may still seal OK. The "O" rings tend to conform to the seat that they sit on over time, if you revolve them to a new position on plastic seats they often don't seal as some imperfection that they sat on previously is now in a new position, i.e. the mold parting line on the plastic part in question. New "O" rings are flexible enough to envelop such tiny mold flaws, but old ones lose that flexibility and usually have to go back in exactly the same position. Easier said than done!