Good way to know if the Piston seal fit is good?

[Bayman]

The bore is probably fine. I'd just get a new seal. Everyone has their preferred seals. I've used the JM and Weihrauch parachute seals and the Vortek seals. The Vortek seals are a cupped seal design and not a parachute design. I think JM has a second line of seals of this design as well as another company in Australia that are supposedly very good. I get the most power and life out of the Vortek. I just changed out one in my Hw95 that I installed in 2016 and probably has ten thousand plus rounds through it. They are a hard moly impregnated rubber. They are initially very tight so lots of people here whine about that. After a day or two of shooting they conform to the bore and loosen up. It's pretty common for the gun to pick up 20-40 fps from when first installed. Just changing to this design over a parachute design makes more power with no ill effects on the shot cycle. This will start a s-storm of opinions. You can do what you want this is just my actual experience.

I am considering the Australian Eco 30mm seal or the Vac Seal.  Which one comes oversized?  I rather get a tighter fitting seal than a loose seal.

I'm not familiar with the Australian one. The Vortek one as I said start off very tight so I'm sure it would work fine.

[byhsu]

The bore is probably fine. I'd just get a new seal. Everyone has their preferred seals. I've used the JM and Weihrauch parachute seals and the Vortek seals. The Vortek seals are a cupped seal design and not a parachute design. I think JM has a second line of seals of this design as well as another company in Australia that are supposedly very good. I get the most power and life out of the Vortek. I just changed out one in my Hw95 that I installed in 2016 and probably has ten thousand plus rounds through it. They are a hard moly impregnated rubber. They are initially very tight so lots of people here whine about that. After a day or two of shooting they conform to the bore and loosen up. It's pretty common for the gun to pick up 20-40 fps from when first installed. Just changing to this design over a parachute design makes more power with no ill effects on the shot cycle. This will start a s-storm of opinions. You can do what you want this is just my actual experience.

I am considering the Australian Eco 30mm seal or the Vac Seal.  Which one comes oversized?  I rather get a tighter fitting seal than a loose seal.

I'm not familiar with the Australian one. The Vortek one as I said start off very tight so I'm sure it would work fine.

do you have a picture of the cup design?  so i can compare how they look like?

I did get a Venom Power Pulse Seal before for my HW97K when I had it tuned, but i can't remember how it looked like, that was 15 years ago.

Thanks

[Bayman]

I'm not at home so I grabbed this from their site. You can see it's cupped but has no parachute groove and lip

[byhsu]

I'm not at home so I grabbed this from their site. You can see it's cupped but has no parachute groove and lip

Vac-Seal 30mm is the one I need?

[Bayman]

I'm not at home so I grabbed this from their site. You can see it's cupped but has no parachute groove and lip

Vac-Seal 30mm is the one I need?

Yes it doesn't list the Hw35 but it will work. It's the same as the Hw80

[byhsu]

I'm not at home so I grabbed this from their site. You can see it's cupped but has no parachute groove and lip

Vac-Seal 30mm is the one I need?

Yes it doesn't list the Hw35 but it will work. It's the same as the Hw80

Great, thanks.  That cup design seems to offer some cushion and prevent the piston slamming against the wall of the transfer port.  Am i correct in my assumption, seems the parachute type is flat in the middle section.

[Bayman]

I'm not at home so I grabbed this from their site. You can see it's cupped but has no parachute groove and lip

Vac-Seal 30mm is the one I need?

Yes it doesn't list the Hw35 but it will work. It's the same as the Hw80

Great, thanks.  That cup design seems to offer some cushion and prevent the piston slamming against the wall of the transfer port.  Am i correct in my assumption, seems the parachute type is flat in the middle section.

It would be logical to assume that. If the piston slams into the transfer port sometime is terribly wrong. There would have to be a terribly bad sealing situation going on or a dry fire. When the piston comes forward squeezing all the air behind the pellet the pressure builds up and the piston slows down on a cushion of air and sinks to the end of its stroke as the pellet moves forward and relieves the pressure. This is an oversimplification of what happens. Sometimes if there's too much resistance (the pellet is too heavy or fits too tight) the piston will squeeze all the air so tight it bounces backwards. Sometimes several times. It's really a very fine balance of forces. Usually the best shooting pellets for a gun are the ones that offer the right resistance. A bouncing piston induces pulsating pressure that is detrimental to accuracy. Also too light a pellet or too loose and there's too little resistance and there's little to no air cushion and the shot cycle is abrupt and harsh. There's a lot going on in there and a lot of cool stuff to learn but there's a lot of good people to help you along.
To your original question there's nothing really wrong with the Weihrauch parachute design. The flat face isn't an issue as pointed out above. I just find I get better sealing, more power and longer life out of the Vortek VAC seals.

[subscriber]

Now the reason I opened her up was because I was getting some shots about 2 inches either high or low, don't know now which is the actual zero of the gun, as is sometimes high and sometimes low, I have not really been able to tell which point of impact corresponds to the zero.

Pablo,

At what range was this +- 2" shift? Did you try more than one type of pellet to see if they are all similarly affected?  If at less than 25 yards, it may not be due to velocity variations from a bad seal:

If all the work you have done does not resolve your point of impacts shifting, I wonder if it is possible that the barrel lockup is varying, and affecting the angle of the barrel relative to the action when shut?  +- 2" sounds like a lot of variation, unless the pellet velocity is extremely unstable.  And odd, for shifting between two positions: 

The barrel latch may not move freely.  If it sticks a little it may cause the barrel to lock up in two distinct positions.  You might want to see if the speed and force with which you close the breech correlates with the shifting point of impact.  If might be that a little moly grease on the latch related sliding surfaces fixes that, should the latch be sticky or suspected of being sticky.  Of course, you could take the latch apart and lightly polish the tunnel is slides in.  I would start with just checking and lubing it...

Usually, the first thing I would check if there is unstable point of impact would be the stock screws.  Then the sight attachment screws.  After you reassemble the action to the stock, the stock screws should be good.  If the instability returns, first check the stock screws.  Don't forget the sight screws...

I may be in the minority, but if with the original piston seal, the piston could hang vertically when you blocked the transfer port, that was sealing pretty well.  Especially if one considers that the direction is backwards, and that sealing can only improve on the forward stroke as the parachute lip seal opens.  That said, A tighter seal might have more wear life before it starts to leak.  But by your description, you did not have a leaking piston seal.

While you have your airgun open, you might consider installing a slightly snug fitting spring guide.  That has a huge effect on reducing spring twang.  With just the right fit, that can increase pellet velocity a little, rather than reducing it.

[jusanothajoe]

Man, a lot of great advice in this thread.

[subscriber]

When you cover the transfer Port with your finger AND press the piston down, is there much resistance?  It should fight you the whole way, IE not move.

Yogi,

The piston has to move before it will build pressure.  It should "fight" you after you get half way down the stroke.  The pressure generated should not dissipate rapidly.  Assuming you manage to keep the TP blocked off.

[Bayman]

When you cover the transfer Port with your finger AND press the piston down, is there much resistance?  It should fight you the whole way, IE not move.

Yogi,

The piston has to move before it will build pressure.  It should "fight" you after you get half way down the stroke.  The pressure generated should not dissipate rapidly.  Assuming you manage to keep the TP blocked off.

Subs, he's been having issues with this gun a while and Pablo has had a few threads on it so you're missing some information. Check out reply 15 on this thread https://www.gatewaytoairguns.org/GTA/index.php?topic=184310.msg156105656#msg156105656
With a 300 fps extreme spread where the high is the normal velocity there's a problem other than lock up and stock screws. Until this information came forth I suggested the same attention as you.

Switching gears. After reviewing this thread something caught my attention and you said that the sleeve was kind of tight and sanding the spring may improve fit. Pablo don't sand your spring for sleeve clearance. Polishing the spring may reduce friction but won't really change the fit significantly. You may need to use a thinner sleeve material. A spring will expand about 0.007" when compressed. It needs room to grow. To give you an idea of 0.007" it's roughly 3 sheets of loose leaf paper. Unless your sleeve is made of a compressible material you should have some clearance between your sleeve and spring.

I do agree with Subs that you should look hard into you lock up components while you have the gun apart. I wish I could be of more help with this but I'm unfamiliar with the unique locking system in the Hw35.

[Yogi]

When you cover the transfer Port with your finger AND press the piston down, is there much resistance?  It should fight you the whole way, IE not move.

Yogi,

The piston has to move before it will build pressure.  It should "fight" you after you get half way down the stroke.  The pressure generated should not dissipate rapidly.  Assuming you manage to keep the TP blocked off.

Agreed, but you should feel increased resistance shortly after pushing the piston down.  Yes, the more you push, the more resistance...

-Y

[subscriber]

With a 300 fps extreme spread where the high is the normal velocity there's a problem other than lock up and stock screws.

Thanks, Ron

300 FPS spread will shift the POI, but  4"?  Of course, the range at which the POI shift occurs would help answer that. 

So, it certainly could be the piston seal.  Or a sticky breech latch that not only shifts the muzzle up or down, but when down it leaks air at the breech seal and reduces the velocity (so leaky breech seal, due to poor lockup).  Such intermittent leaks (lack of cushioning) would also explain the TP mark on the piston face...

[subscriber]

We have a winner:  Based on the link Ron provided (see Pablo quoted below), barrel latching is inconsistent, with the wedge occasionally failing to drop fully into lockup.  This may be due to excessive barrel hinge tension, making it hard to feel the breech close properly.  Or Pablo is closing the breech too gently.  Or too fast - such that the barrel bounces partly open before the latch can drop home.

Or, the breech seal may be too tall, and is interfering with breech closure.  Or the latch has some roughness in its channel.  Or its spring has a sharp coil at the moving end.  I rest my case

Only oil i've added is to the barrel lock latch.  It was not closing smoothly and twice i've put the rifle on the shooting bag and the barrel got open and i fired without knowing it.

[Bayman]

With a 300 fps extreme spread where the high is the normal velocity there's a problem other than lock up and stock screws.

Thanks, Ron

300 FPS spread will shift the POI, but  4"?  Of course, the range at which the POI shift occurs would help answer that. 

So, it certainly could be the piston seal.  Or a sticky breech latch that not only shifts the muzzle up or down, but when down it leaks air at the breech seal and reduces the velocity (so leaky breech seal, due to poor lockup).  Such intermittent leaks (lack of cushioning) would also explain the TP mark on the piston face...

You're absolutely correct but I'm trying to address one thing at a time here, which is the internals of the gun. Once that's ironed out and we can address other issues. The guy has the guts apart already, so the horses have left the barn already and I'm just trying to get them back in. Correctly.
IIRC this gun accidentally popped open twice while being fired because the lock wasn't in place. This would be similar to a dry fire and would account for a damaged piston seal. The TP print incidentally happens on ever Weihrauch seal I've seen. I didn't want to argue with Hector. The fact that the seal is burned and the TP print moved is more important. This implies that the gun has been dieseling severely. Which I don't know the exact reason but after a gun diesels badly or detonates the following shot drops a couple hundred fps. This too fits the occasional very wide ES previously experienced.

The current issue is the oe seal is too loose, leaks pressure too quickly and needs to be replaced. Let's just get the guts back together right and out of the equation because he already has to.

[subscriber]

Let's just get the guts back together right and out of the equation because he already has too.

Yes.  Can't diagnose anything with it all in bits.

[byhsu]

Now the reason I opened her up was because I was getting some shots about 2 inches either high or low, don't know now which is the actual zero of the gun, as is sometimes high and sometimes low, I have not really been able to tell which point of impact corresponds to the zero.

Pablo,

At what range was this +- 2" shift? Did you try more than one type of pellet to see if they are all similarly affected?  If at less than 25 yards, it may not be due to velocity variations from a bad seal:

If all the work you have done does not resolve your point of impacts shifting, I wonder if it is possible that the barrel lockup is varying, and affecting the angle of the barrel relative to the action when shut?  +- 2" sounds like a lot of variation, unless the pellet velocity is extremely unstable.  And odd, for shifting between two positions: 

The barrel latch may not move freely.  If it sticks a little it may cause the barrel to lock up in two distinct positions.  You might want to see if the speed and force with which you close the breech correlates with the shifting point of impact.  If might be that a little moly grease on the latch related sliding surfaces fixes that, should the latch be sticky or suspected of being sticky.  Of course, you could take the latch apart and lightly polish the tunnel is slides in.  I would start with just checking and lubing it...

Usually, the first thing I would check if there is unstable point of impact would be the stock screws.  Then the sight attachment screws.  After you reassemble the action to the stock, the stock screws should be good.  If the instability returns, first check the stock screws.  Don't forget the sight screws...

I may be in the minority, but if with the original piston seal, the piston could hang vertically when you blocked the transfer port, that was sealing pretty well.  Especially if one considers that the direction is backwards, and that sealing can only improve on the forward stroke as the parachute lip seal opens.  That said, A tighter seal might have more wear life before it starts to leak.  But by your description, you did not have a leaking piston seal.

While you have your airgun open, you might consider installing a slightly snug fitting spring guide.  That has a huge effect on reducing spring twang.  With just the right fit, that can increase pellet velocity a little, rather than reducing it.

very good points to consider, specially regarding the barrel locking. a few times i laid the rifle on the shooting bag and it popped open without me noticing, since I was looking thru the scope. 

I did make a plastic piston sleeve from a plastic bottle.  Spring fits real tight, but i've sanded it down so it just fits snug now.  I will probably sand it a little bit more, so when the spring is compressed it goes in smoothly into the piston.

[Bayman]

Now the reason I opened her up was because I was getting some shots about 2 inches either high or low, don't know now which is the actual zero of the gun, as is sometimes high and sometimes low, I have not really been able to tell which point of impact corresponds to the zero.

Pablo,

At what range was this +- 2" shift? Did you try more than one type of pellet to see if they are all similarly affected?  If at less than 25 yards, it may not be due to velocity variations from a bad seal:

If all the work you have done does not resolve your point of impacts shifting, I wonder if it is possible that the barrel lockup is varying, and affecting the angle of the barrel relative to the action when shut?  +- 2" sounds like a lot of variation, unless the pellet velocity is extremely unstable.  And odd, for shifting between two positions: 

The barrel latch may not move freely.  If it sticks a little it may cause the barrel to lock up in two distinct positions.  You might want to see if the speed and force with which you close the breech correlates with the shifting point of impact.  If might be that a little moly grease on the latch related sliding surfaces fixes that, should the latch be sticky or suspected of being sticky.  Of course, you could take the latch apart and lightly polish the tunnel is slides in.  I would start with just checking and lubing it...

Usually, the first thing I would check if there is unstable point of impact would be the stock screws.  Then the sight attachment screws.  After you reassemble the action to the stock, the stock screws should be good.  If the instability returns, first check the stock screws.  Don't forget the sight screws...

I may be in the minority, but if with the original piston seal, the piston could hang vertically when you blocked the transfer port, that was sealing pretty well.  Especially if one considers that the direction is backwards, and that sealing can only improve on the forward stroke as the parachute lip seal opens.  That said, A tighter seal might have more wear life before it starts to leak.  But by your description, you did not have a leaking piston seal.

While you have your airgun open, you might consider installing a slightly snug fitting spring guide.  That has a huge effect on reducing spring twang.  With just the right fit, that can increase pellet velocity a little, rather than reducing it.

very good points to consider, specially regarding the barrel locking. a few times i laid the rifle on the shooting bag and it popped open without me noticing, since I was looking thru the scope. 

I did make a plastic piston sleeve from a plastic bottle.  Spring fits real tight, but i've sanded it down so it just fits snug now.  I will probably sand it a little bit more, so when the spring is compressed it goes in smoothly into the piston.

I'm out. Good luck to you.

[byhsu]

Put back the HW35E with a new Vortek Vac-seal 30mm piston seal,  built a piston sleeve with a plastic bottle and deburred all the sharp edges and put Moly to lube the internals.  Now the rifle is very consistent.  I zeroed it today and started target shooting at 20 yards, now I can pretty much hit what I aim at.  I think the Seal did the trick, but I also made sure all the screws were tight and the latch was polished and has a positive lock up everytime.  Also that liner does make the rifle shoot with NO twang at all. 

As for velocities, I've only tested it with Crosman 7.9 gr  avg. 767fps and JSB 8.44 gr 750 fps. Before tune it was 807 fps for 7.9gr and 790fps for 8.44gr.   Not too concerned with FPS as Accuracy is more important for me in this rifle, with 750 fps I think I'm good for my regular shooting range of 20 yards and I think even if I went 40 yards it would be OK.

Also it  doesn't seem to have as much recoil as before and the rifle is more accurate for sure.  I actually did my target shooting today with the 7.9gr, but i think with the JSB it would probably group better.

I am very happy with the rifle now.  Thank You everyone for your advice.

[Bayman]

If the new seal was tight going in it'll probably pick up a velocity as it conforms to the bore. Also a too tight a sleeve to spring fit will slow a gun down. See what happens. All that counts is the POI and velocity is consistent and you're happy.