Problem Sealing Transfer Port on a Discovery

[sr1sws]

So I've been working on the Disco (.177) - having polished the bore, 2 barrel bands, etc.

Eventually I tried the tissue paper test with confusing results.  I followed up with the 'flour test' which clearly shows leakage from (apparently) the transfer port.

I worked on it again today, using poly tubing to form a seal on the barrel side.  I made several, even one a bit longer than the transfer port upper side that provides a crush fit (verified in a subsequent tear down).

Upper and lower connections were sealed with shellac.  Why? Because I had it from one of my other hobbies (fountain pen repair) and it easily cleans up with denatured alcohol.

After going through making 4 or 5 of these and sealing with shellac, it still fails the flour test... although it seems better.  Now I'm not sure it's the transfer port as the air seems to escape to the rear of the the transfer port.  Air tube and breech/barrel assembly are clamped together with 2 bar clamps before the breech screw, barrel band at front of breech and rear breech screws are installed.

The Disco was bought new from Pyramyd Air and is about 2 months old.

Any other suggestions?

Many thanks!
Steve

[Ribbonstone]

Are some slight leaks you just can’t do much about.

Some Discos will leak a little air back past the shaft of the valve stem.    It’s normally a very small amount of air.  Escapes by way of the trigger and up though the cocking slot and into the bolt. Not enough air to really matter to performance, but enough to show up in the flour test.

Early Disco diagrams showed an o-ring seal on the stem, but they gave that up pretty quickly

Poly seals are actually pretty hard to get right on a Discovery (or other 22XX gun).  Issue transfer port actually seals into a recess on the bottom of the barrel and seals to the body of the valve, just passing though the gas tube and the breech.  The seals on each end of the transfer port need to be in good condition (I’d call them o-rings, except they have a square/rectangular cross section).

 It’s difficult to get a poly port to seal the same way.  Mostly they seal to the bottom of the barrel, but there’s no flat spot for a seal, so the poly port has to try to seal to the round bottom of the barrel.

[sr1sws]

Are some slight leaks you just can’t do much about.

Some Discos will leak a little air back past the shaft of the valve stem.    It’s normally a very small amount of air.  Escapes by way of the trigger and up though the cocking slot and into the bolt. Not enough air to really matter to performance, but enough to show up in the flour test.

Early Disco diagrams showed an o-ring seal on the stem, but they gave that up pretty quickly

Poly seals are actually pretty hard to get right on a Discovery (or other 22XX gun).  Issue transfer port actually seals into a recess on the bottom of the barrel and seals to the body of the valve, just passing though the gas tube and the breech.  The seals on each end of the transfer port need to be in good condition (I’d call them o-rings, except they have a square/rectangular cross section).

 It’s difficult to get a poly port to seal the same way.  Mostly they seal to the bottom of the barrel, but there’s no flat spot for a seal, so the poly port has to try to seal to the round bottom of the barrel.

Thanks for the info.  I have a couple of Challenger ports on order to take a shot at a de-tune.  I also have a Disco port on order to replace the one in my gun - which I did shorten a bit and may have caused my leak (but I don't really think so... not with the shellac seal).

I also thought about making a breech/air tube gasket out of brown paper bag - but of course we don't have any.  All plastic bags now   Maybe I'll use a sheet of printer paper.

Thanks to all,
Steve