STC Piston Usage Warning

[Diving Gecko]

Hey all,
It's been a while and I can't recall if I had mentioned that I have had a few unfortunate misfires on my Seac Hunter last time I took it out. The gun has been modded quite a bit and has an aftermarket 13mm STC piston in it too.
The misfires happened after loading when my OCD forced me to turn the spear so that the flopper was facing upwards. And it got worse and worse.

I finally got around to taking it apart now - months after the incidents - and what I found was quite interesting.
The STC piston has a POM/Delrin part and a stainless steel tail. The tail is threaded and screws into the Delrin front end. What had happened is that every time, I turned the spear a bit I unscrewed the tail from the front and finally it got so loose that the tail could slip in the trigger mech. It never fully came apart but 2-3 more revolutions and it would have had.

The SS tail showed a bit of thread lock, but obviously not enough.
I still like the design a lot but I think it's fair to warn against turning the spear unless you do it in the right direction...

The pic shows the piston right after I took it out of the gun. The tail should have been fully embedded into the front and not showing any threads at all.

 

[popgun pete]

I count 13 exposed threads, therefore about 15 turns are required to move the metal tail's front flange forwards so that it once more sits against the rear face of the plastic piston. That gives you an idea of how many times you must have twisted the shaft in the cocked gun, given that each time was probably much less than a full turn to place the flopper on top. At least the metal tail did not unscrew to the point where it stripped out the last few threads still holding it in the piston body or you would have had to consider replacing the piston, depending on how many threads had been lost with respect to the number that were still left intact. As the screw thread is a right hand thread any twisting needs to be clockwise as seen looking into the muzzle from the outside, but I think you need to unload the sear tooth by pushing the spear inwards at the same time. Moving the piston tail's mushroom head slightly past the sear tooth requires a big push, so I can understand why users are tempted to rotate the integral tip, single flopper spears using the extended flopper as a lever handle instead.

 

[tromic]

That stuff in a groove between o-rings seems to be emulsion of water in oil?

 

[Diving Gecko]

That stuff in a groove between o-rings seems to be emulsion of water in oil?

It's white lithium-based (?) grease that I used to fill up that cavity - but I am not sure why it looks like it does.

I didn't see any traces of water as in droplets. But I think I saw some tiny, tiny sparkly stuff that I felt could be extremely fine metal dust - maybe from the inner tube.
It was hard to see and not at all the size that I would worry about leaks. Maybe it looked more like a metallic oil somehow.
I don't know if this makes any sense.

The grease on the handle and front plastic "bushing" looked the same, so maybe it's just the reddish fork oil that doesn't go too well with this kind of grease - which again, I think is a lithium grease.

 

[Diving Gecko]

I count 13 exposed threads, therefore about 15 turns are required to move the metal tail's front flange forwards so that it once more sits against the rear face of the plastic piston. That gives you an idea of how many times you must have twisted the shaft in the cocked gun, given that each time was probably much less than a full turn to place the flopper on top. At least the metal tail did not unscrew to the point where it stripped out the last few threads still holding it in the piston body or you would have had to consider replacing the piston, depending on how many threads had been lost with respect to the number that were still left intact. As the screw thread is a right hand thread any twisting needs to be clockwise as seen looking into the muzzle from the outside, but I think you need to unload the sear tooth by pushing the spear inwards at the same time. Moving the piston tail's mushroom head slightly past the sear tooth requires a big push, so I can understand why users are tempted to rotate the integral tip, single flopper spears using the extended flopper as a lever handle instead.

Yup, I agree on all counts.
It's much easier to turn by grabbing the flopper.
Also, had I known the design was this screw-in type, I hope I would have been smart enough to turn the right way. (CW when facing spear, as you say).
I do have a pic of it fully taken apart, I think the tail has 19-20 threads.

 

[popgun pete]

When "STC" made that piston they probably never considered users rotating the spear and hence the piston in an already cocked gun. Even with "Loctite" or one of the other screw thread locking resins it would be difficult to secure the metal tail in the piston against that type of highly loaded-up unscrewing action. I wonder if some broken pistons have been the result of this action progressively generating cracks in plastic bodies that have been moulded around their metal tails rather than being of the screwed-in type, as most original equipment pistons are of the moulded type.

For decades pneumatic spearguns were supplied with symmetrical, twin flopper detachable speartips, so there was no point in rotating the spear inside the gun. Also most had all-metal pistons with integral tails. I don't remember seeing any reference in the operating instructions on this matter, but then I don't own any pneumatic spearguns that were only supplied with integral tip, single flopper spears as their original equipment. It may be worth asking the manufacturers as guns such as the "Cyrano" have had these spears as optional equipment in the longer models for a long time now. Could be this issue has not received much attention as few users may be doing it, but former band gun operators, who are used to fixed shaft orientations in their guns, may be more interested in making these adjustments.