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PostPosted: Wed Jan 22, 2020 6:07 pm 
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Anyone ever tried to shave a few thou from the head of a Crosman/B19 break barrel rifle piston? I know it means having to make a new seal and seal retaining method, I already did that. I haven't tested the piston yet, I guess I'll have to elongate the piston's cocking slot too. Was just wondering if somebody tried that, and what were the results :?:


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PostPosted: Wed Jan 22, 2020 6:46 pm 
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Interested in your results, however shorter piston means less volume of the compression chamber & stroke, does it not?
How to make up for the lost stroke length?

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PostPosted: Wed Jan 22, 2020 7:09 pm 
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Daryl wrote:
Interested in your results, however shorter piston means less volume of the compression chamber & stroke, does it not?
How to make up for the lost stroke length?


Actually not :) . The sear notch in the piston stays at the same place, but the face of the piston is closer to the rear of the chamber, thus INCREASING stroke and chamber volume. That is why I mentionned having to perhaps elongate the piston cocking slot, because the piston will travel further forward.

As for now, it's for an under-500 fps rifle. I want to make it the most efficient as possible, for accuracy and smoothness of shot cycle. I'd like to have the lowest cocking effort possible, while still retaining decent power. I am using a piston with no hole and a very weak (0.099'' wire) spring. I'll also have to make a new spring guide since the spring's ID is bigger than the OEM Benjamin spring. Good opportunity to build a tune kit!

Cheers! :drinkers:


Last edited by airmec on Wed Jan 22, 2020 8:07 pm, edited 1 time in total.

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PostPosted: Wed Jan 22, 2020 7:32 pm 
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About the smoothing shooting B18/19 rifles were the original Benjamin classic & Crosman Quest/Phantom under 500 fps. They used a very long piston and a detuned spring - but the shot cycle was very mild and cocking effort was minimal. I can't see shortening the original short piston improving the shot cycle or "efficiency". You may gain some fps over what you got going on though.

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PostPosted: Wed Jan 22, 2020 8:48 pm 
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Dukemeister wrote:
About the smoothing shooting B18/19 rifles were the original Benjamin classic & Crosman Quest/Phantom under 500 fps. They used a very long piston and a detuned spring - but the shot cycle was very mild and cocking effort was minimal. I can't see shortening the original short piston improving the shot cycle or "efficiency". You may gain some fps over what you got going on though.


Interesting you say that, this is for my Benjamin Classic .22 :). I was tempted at first to use a ''long'' under-500 fps Trail NP piston, but the spring would need to be stiffer to compensate for the shorter stroke, to get the same power. The spring I'll use came from an under-500 .177 Optimus, which had no hole in the piston. I'd like to see if I can get good power for .22 cal with this spring. I'll chrony the gun with the original spring guide, top hat, ''weak'' spring and short piston with original seal, and then with the shorter piston with Teflon seal only, and then with the tune kit.

Other ''efficiency mods'' will include:

-polishing the TP, no enlarging - it's already 0.128'', with lightly radiused inside edge
-honing the comp. cylinder's walls within 0.001'' of variation in diameter, and polishing to a mirror finish for the Teflon seal. That would be nice to also be able to polish the front chamber wall.
-Tune kit made with low-friction materials (bronxe, teflon, delrin)
-barrel lapping - done. Wonder if lapping with Tungsten disulphide powder would help velocity?
-shimmed breach seal, and mating surface polished
-barrel block locking wedges (alive jam) polished and tungsten paste lubed - done. It insures a good pressure on the breach seal, and in conjunction with bronze pivot washers, helps a lot with accuracy.

:drinkers:


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PostPosted: Sat Jan 25, 2020 1:13 pm 
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You could button the piston, I used glue on delrin buttons from airrifleheadquarters in my Diana280

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PostPosted: Sat Jan 25, 2020 9:05 pm 
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GarthThomas wrote:
You could button the piston, I used glue on delrin buttons from airrifleheadquarters in my Diana280


Thanks for the info. I was thinking about riveting some bronze inserts :?:


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PostPosted: Sat Jan 25, 2020 10:05 pm 
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airmec wrote:
GarthThomas wrote:
You could button the piston, I used glue on delrin buttons from airrifleheadquarters in my Diana280


Thanks for the info. I was thinking about riveting some bronze inserts :?:


Riveting.... :shock: :|

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PostPosted: Sun Jan 26, 2020 1:20 pm 
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Ace wrote:
airmec wrote:
GarthThomas wrote:
You could button the piston, I used glue on delrin buttons from airrifleheadquarters in my Diana280


Thanks for the info. I was thinking about riveting some bronze inserts :?:


Riveting.... :shock: :|


Yup, riveted bronze buttons :wink: . I still have to flat spot and drill the piston, but here's a preview:


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finished_button.JPG [ 84.21 KiB | Viewed 240 times ]
set_of_4_buttons.JPG
set_of_4_buttons.JPG [ 72.11 KiB | Viewed 240 times ]
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PostPosted: Sun Jan 26, 2020 10:50 pm 
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Ive heard of braising some buttons on

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PostPosted: Sun Jan 26, 2020 10:59 pm 
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interested to see how this turns out... 8)
even though brass is a little softer then steel
your still going to have metal to metal contact
that's why they use delrin...

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PostPosted: Sun Jan 26, 2020 11:24 pm 
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Ace wrote:
interested to see how this turns out... 8)
even though brass is a little softer then steel
your still going to have metal to metal contact
that's why they use delrin...


That's a good point, but I don't use brass, I use industrial grade bronze alloy designed for very high loads and wear, the kind used for bushings for example. Very hard to machine, be it with razor-sharp HSS 5% Co or sharp carbide inserts for SS. It's self-lubricating by itself, and with WS2 lightweight paste, it's almost as slippery as Teflon. Also I mirror polished the compression tube walls for the Teflon seal, I guess the buttons will like riding on that smooth surface 8) . I must also say that it's an experiment in itself, I want to see what it'll do :lol:


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PostPosted: Mon Jan 27, 2020 6:21 am 
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I’ve had many B18/19 based guns apart for spring replacements and the like. From what I recall there is very little room to stroke the crosman guns . I believe less than 1mm before the seal is past the cocking slot. If there is indeed some room there by shortening the piston the swept volume will increase. Don’t forget to open the slot for the cocking arm by the same distance. Please post more pics , maybe of the piston in the tube at full stroke. Some measurements would be good too.

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PostPosted: Mon Jan 27, 2020 6:23 pm 
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Mirror finish means ALL the lube gets wiped away by the seal. Imo the cylinder works best with a cross hatch finish. Anyways keep us posted on results. Also make changes one at a time or you will be endlessly frustrated if something does not work.

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PostPosted: Mon Jan 27, 2020 9:17 pm 
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Hobbyman2007 wrote:
I’ve had many B18/19 based guns apart for spring replacements and the like. From what I recall there is very little room to stroke the crosman guns . I believe less than 1mm before the seal is past the cocking slot. If there is indeed some room there by shortening the piston the swept volume will increase. Don’t forget to open the slot for the cocking arm by the same distance. Please post more pics , maybe of the piston in the tube at full stroke. Some measurements would be good too.


I have been able to extract 0.100'' (2.5 mm) more stroke with the mods I made. :)

In the first picture you see on top the Benji tube with shorter piston, bottom a Crosman Optimus with the same exact action, stock configuration. There's about, by coincidence, 0.100'' of space between the end of the cocking arm and end of slot, which I lost by shortening the piston. I actually ground the piston cocking slot and cocking arm ''shoe'', 0.050'' each.

Second picture is the angle at which the barrel breaks when at ''full cock''.

Third picture, you see the front of the cocking slot in the tube. With the piston cocked the seal is still at least 1/4'' from reaching the slot.

Today I flat spotted and drilled the piston for the bronze inserts, just need to install and machine them to size, and I'll be able to test the rifle with that piston at last!


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Piston_position.JPG
Piston_position.JPG [ 50.15 KiB | Viewed 177 times ]
cocked_barrel_angle.JPG
cocked_barrel_angle.JPG [ 82.09 KiB | Viewed 177 times ]
cocked_short_piston.JPG
cocked_short_piston.JPG [ 30.54 KiB | Viewed 177 times ]
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