Mystery of low power B26 explained... ??

Mystery of low power B26 explained... ??

Joined: April 2nd, 2005, 6:57 am

November 20th, 2010, 7:32 pm #1

In my previous post, I was struck by a comment from Mike regarding a possible problem with the compression tube that I had suspected as well, as he put it:

"I also notice a grease smudge down one side of the CC Tube when i disassemble, and always some grease on top of the seal, in both my B26 guns." (courtesy DMikeM)

Same with me. The persistent migration of moly paste from behind the piston seal to in front of the piston seal and all down the sides of the cc wall caused my to ponder: "What simple diagnostic could be used to verify whether the defect in the compression tube is radial or longitudinal, and where it is located?"

It hit me that I had a tool right at hand in the form of my rubber tipped air blower gun. It occurred to me that I could pressurize the barrel of the gun and listen for a "hiss" in the region of the compression cylinder. So I did it.

My finding:
By putting 100psi of air pressure into the barrel of the rifle while the barrel was in the closed position and the piston/spring were in the un-cocked state, I was able to hear a definite whistle in the cocking slot of the receiver. But more importantly, the piston did not budge. It was not moved backward by the resulting total pressure (approx. 140lbs ) against the piston crown. Considering that 140 pounds of pressure should have been enough move the piston several inches against the spring pressure in the relaxed position, then it seems reasonable to assume that the barrel is distorted enough to permit a significant volume of air to escape past the seal when the seal is resting against the bottom of the chamber.
Another observation: I have noticed that the seal itself rests against the transfer port in a way that partially blocks it, so it is possible that with the help of the spring preload, is partially blocking the passage of air into the cylinder.

Conclusion:
It may be possible to get good information about the longitudinal and radial location of the chamber wall distortion, namely: if the leak is persistent even though the piston is positioned in multiple locations along its course of travel, then it would indicate that the barrel distortion is due to a radial runout defect that is located down the length of the cylinder. On the other hand, if the leak disappears the moment the piston is moved away from the bottom of the cylinder, then it would seem to indicate that there is a local bulge or defect in the cylinder wall near the transfer port end.

If it is true that the piston seal itself is blocking the transfer port enough to prevent much air from entering the chamber in a reverse flow situation (pressurizing the barrel with by air blower), then is it possible that it is also blocking air that is trying to exit through the transfer port when the rifle is fired?

Or worse, is there a cascade effect? Namely, a defect in the cylinder wall near the transfer port allows a loss of critical high pressure air volume, which in turn allows the piston seal to get close enough to the transfer port to cause a flow restriction, which in turn forces an additional volume of air through the gap between the seal and cylinder wall.

I will perform a pressure/whistle test with the piston in multiple locations to test this theory and report back.

Tom
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Joined: May 1st, 2001, 8:16 pm

November 20th, 2010, 11:44 pm #2

Tom, can't you use telescoping "snap" gauges to check the cc? nt
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Joined: May 1st, 2001, 8:16 pm

November 20th, 2010, 11:50 pm #3

In my previous post, I was struck by a comment from Mike regarding a possible problem with the compression tube that I had suspected as well, as he put it:

"I also notice a grease smudge down one side of the CC Tube when i disassemble, and always some grease on top of the seal, in both my B26 guns." (courtesy DMikeM)

Same with me. The persistent migration of moly paste from behind the piston seal to in front of the piston seal and all down the sides of the cc wall caused my to ponder: "What simple diagnostic could be used to verify whether the defect in the compression tube is radial or longitudinal, and where it is located?"

It hit me that I had a tool right at hand in the form of my rubber tipped air blower gun. It occurred to me that I could pressurize the barrel of the gun and listen for a "hiss" in the region of the compression cylinder. So I did it.

My finding:
By putting 100psi of air pressure into the barrel of the rifle while the barrel was in the closed position and the piston/spring were in the un-cocked state, I was able to hear a definite whistle in the cocking slot of the receiver. But more importantly, the piston did not budge. It was not moved backward by the resulting total pressure (approx. 140lbs ) against the piston crown. Considering that 140 pounds of pressure should have been enough move the piston several inches against the spring pressure in the relaxed position, then it seems reasonable to assume that the barrel is distorted enough to permit a significant volume of air to escape past the seal when the seal is resting against the bottom of the chamber.
Another observation: I have noticed that the seal itself rests against the transfer port in a way that partially blocks it, so it is possible that with the help of the spring preload, is partially blocking the passage of air into the cylinder.

Conclusion:
It may be possible to get good information about the longitudinal and radial location of the chamber wall distortion, namely: if the leak is persistent even though the piston is positioned in multiple locations along its course of travel, then it would indicate that the barrel distortion is due to a radial runout defect that is located down the length of the cylinder. On the other hand, if the leak disappears the moment the piston is moved away from the bottom of the cylinder, then it would seem to indicate that there is a local bulge or defect in the cylinder wall near the transfer port end.

If it is true that the piston seal itself is blocking the transfer port enough to prevent much air from entering the chamber in a reverse flow situation (pressurizing the barrel with by air blower), then is it possible that it is also blocking air that is trying to exit through the transfer port when the rifle is fired?

Or worse, is there a cascade effect? Namely, a defect in the cylinder wall near the transfer port allows a loss of critical high pressure air volume, which in turn allows the piston seal to get close enough to the transfer port to cause a flow restriction, which in turn forces an additional volume of air through the gap between the seal and cylinder wall.

I will perform a pressure/whistle test with the piston in multiple locations to test this theory and report back.

Tom
Sorry, I was still thinking B30; please ignore -nt
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Joined: April 2nd, 2005, 6:57 am

November 21st, 2010, 12:15 am #4

but they do not have the reach to "swipe" them in the bore of the B26 near the compression chamber.

Oh well...

Tom
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Joined: March 8th, 2007, 2:43 am

November 21st, 2010, 1:19 am #5

In my previous post, I was struck by a comment from Mike regarding a possible problem with the compression tube that I had suspected as well, as he put it:

"I also notice a grease smudge down one side of the CC Tube when i disassemble, and always some grease on top of the seal, in both my B26 guns." (courtesy DMikeM)

Same with me. The persistent migration of moly paste from behind the piston seal to in front of the piston seal and all down the sides of the cc wall caused my to ponder: "What simple diagnostic could be used to verify whether the defect in the compression tube is radial or longitudinal, and where it is located?"

It hit me that I had a tool right at hand in the form of my rubber tipped air blower gun. It occurred to me that I could pressurize the barrel of the gun and listen for a "hiss" in the region of the compression cylinder. So I did it.

My finding:
By putting 100psi of air pressure into the barrel of the rifle while the barrel was in the closed position and the piston/spring were in the un-cocked state, I was able to hear a definite whistle in the cocking slot of the receiver. But more importantly, the piston did not budge. It was not moved backward by the resulting total pressure (approx. 140lbs ) against the piston crown. Considering that 140 pounds of pressure should have been enough move the piston several inches against the spring pressure in the relaxed position, then it seems reasonable to assume that the barrel is distorted enough to permit a significant volume of air to escape past the seal when the seal is resting against the bottom of the chamber.
Another observation: I have noticed that the seal itself rests against the transfer port in a way that partially blocks it, so it is possible that with the help of the spring preload, is partially blocking the passage of air into the cylinder.

Conclusion:
It may be possible to get good information about the longitudinal and radial location of the chamber wall distortion, namely: if the leak is persistent even though the piston is positioned in multiple locations along its course of travel, then it would indicate that the barrel distortion is due to a radial runout defect that is located down the length of the cylinder. On the other hand, if the leak disappears the moment the piston is moved away from the bottom of the cylinder, then it would seem to indicate that there is a local bulge or defect in the cylinder wall near the transfer port end.

If it is true that the piston seal itself is blocking the transfer port enough to prevent much air from entering the chamber in a reverse flow situation (pressurizing the barrel with by air blower), then is it possible that it is also blocking air that is trying to exit through the transfer port when the rifle is fired?

Or worse, is there a cascade effect? Namely, a defect in the cylinder wall near the transfer port allows a loss of critical high pressure air volume, which in turn allows the piston seal to get close enough to the transfer port to cause a flow restriction, which in turn forces an additional volume of air through the gap between the seal and cylinder wall.

I will perform a pressure/whistle test with the piston in multiple locations to test this theory and report back.

Tom
during a shot; in order to (try to) expell all the compressed air. Every one I ever took out has had an impression of the end of the comp tube including the transfer port inlet.
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Joined: March 14th, 2008, 10:55 pm

November 21st, 2010, 1:28 am #6

but they do not have the reach to "swipe" them in the bore of the B26 near the compression chamber.

Oh well...

Tom
See if they will check it with some
Sunnen Bore gages. They do not
require as much "sweep" as a telescoping
one.
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Joined: April 2nd, 2005, 6:57 am

November 21st, 2010, 5:32 am #7

It looks as if there is a defect right at the bottom of the transfer port.

Tom
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Joined: April 2nd, 2005, 6:57 am

November 21st, 2010, 6:03 am #8

In my previous post, I was struck by a comment from Mike regarding a possible problem with the compression tube that I had suspected as well, as he put it:

"I also notice a grease smudge down one side of the CC Tube when i disassemble, and always some grease on top of the seal, in both my B26 guns." (courtesy DMikeM)

Same with me. The persistent migration of moly paste from behind the piston seal to in front of the piston seal and all down the sides of the cc wall caused my to ponder: "What simple diagnostic could be used to verify whether the defect in the compression tube is radial or longitudinal, and where it is located?"

It hit me that I had a tool right at hand in the form of my rubber tipped air blower gun. It occurred to me that I could pressurize the barrel of the gun and listen for a "hiss" in the region of the compression cylinder. So I did it.

My finding:
By putting 100psi of air pressure into the barrel of the rifle while the barrel was in the closed position and the piston/spring were in the un-cocked state, I was able to hear a definite whistle in the cocking slot of the receiver. But more importantly, the piston did not budge. It was not moved backward by the resulting total pressure (approx. 140lbs ) against the piston crown. Considering that 140 pounds of pressure should have been enough move the piston several inches against the spring pressure in the relaxed position, then it seems reasonable to assume that the barrel is distorted enough to permit a significant volume of air to escape past the seal when the seal is resting against the bottom of the chamber.
Another observation: I have noticed that the seal itself rests against the transfer port in a way that partially blocks it, so it is possible that with the help of the spring preload, is partially blocking the passage of air into the cylinder.

Conclusion:
It may be possible to get good information about the longitudinal and radial location of the chamber wall distortion, namely: if the leak is persistent even though the piston is positioned in multiple locations along its course of travel, then it would indicate that the barrel distortion is due to a radial runout defect that is located down the length of the cylinder. On the other hand, if the leak disappears the moment the piston is moved away from the bottom of the cylinder, then it would seem to indicate that there is a local bulge or defect in the cylinder wall near the transfer port end.

If it is true that the piston seal itself is blocking the transfer port enough to prevent much air from entering the chamber in a reverse flow situation (pressurizing the barrel with by air blower), then is it possible that it is also blocking air that is trying to exit through the transfer port when the rifle is fired?

Or worse, is there a cascade effect? Namely, a defect in the cylinder wall near the transfer port allows a loss of critical high pressure air volume, which in turn allows the piston seal to get close enough to the transfer port to cause a flow restriction, which in turn forces an additional volume of air through the gap between the seal and cylinder wall.

I will perform a pressure/whistle test with the piston in multiple locations to test this theory and report back.

Tom
I tested the piston seal at multiple locations by pressurizing the tube and here is what I found:

First of all, the pressure I was putting in the barrel was 90psi and after I rechecked my math it turns out that the total pressure on the piston is actually 70lbs - which is not enough to move the piston against the spring preload. So I was wrong about that point.

In order to test the different locations, I removed the trigger and spring and used one of the trigger locating pins to act as a backstop for a piece of 1/2" pvc pipe that I cut to different lengths to hold the piston in position.

The seal did whistle faintly when the piston was positioned against the bottom of the cylinder, but it did not whistle in any other spot.

I made the mistake of pressurizing the cylinder with the piston all the way down and no backstop - and the piston shot right out the end of the receiver, through all the tools on my bench and across the shop floor, without damage... lucky me )

It looks as if my cylinder may have a small distortion or gouge at the very bottom that causes a slight audible leak, but I think that the flexible parachute lip of the piston seal is able to conform to this defect, especially with the high pressures that occur during the shot cycle.

Conclusion: I do not think that my compression cyl. is sufficiently distorted or gouged to cause power loss. So I am back to looking for a 26mm receiver.

Tom






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Joined: April 2nd, 2005, 6:57 am

November 21st, 2010, 6:06 am #9

during a shot; in order to (try to) expell all the compressed air. Every one I ever took out has had an impression of the end of the comp tube including the transfer port inlet.
This is my first break barrel so I this information is very helpful to me.

Tom
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