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PostPosted: Sat Apr 04, 2020 11:11 am 
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Joined: Mon Mar 30, 2020 5:50 am
Posts: 18
airmec wrote:
Dukemeister wrote:
I have to laugh. I guess I had one just like it - honestly I don't remember it at all. I do remember plugging small bleed holes, but not the bigger blind hole. Anyhow, the bigger hole won't increase power, it will provide some dead air volume that could prevent piston slam. By plugging the small hole it will bring up the power. When I saw your picture I thought the blind hole was a through hole.


Sorry Duke, I don't agree with that; more ''dead'' air space in front of the piston will provide a lower compression ratio, thus increasing piston slam because of a lower pressure air cushion. This is why chevota sands his seals, to up the compression ratio; simply put, it works sorta like a car's engine: with higher compression you get more power.

Jetronic wrote:
Plugging the big hole will be faster but can reduce accuracy with heavier pellets


Sorry again, it's the opposite with heavier pellets. They will tame the shot cycle better, travel slower and be better stabilized, increasing accuracy. Very often heavy pellets are recommended in magnum springers for that very reason.


But this isnt a magnum yet with the weak spring piston. Otherwise i agree but still possible to go heavier than optimum. Around 10 grain seems like a sweet spot for these b18 clones on full power in 0.177.


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PostPosted: Sat Apr 04, 2020 1:10 pm 
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Joined: Thu Apr 02, 2020 4:06 pm
Posts: 16
Location: North shore Montréal
yes

bordurat wrote:
ulricbb wrote:
i dont have pictures of the modified piston....i threaded the hole and insert a screw, with the head about 1/32 deeper the the piston surface, i will add jb weld tomorow....

i also ordered the gas spring from autralia, customairseals 48 all in on ebay, because everyone in canada si back order....

look at my 2 shot, front and back(10 yards). both with gamo Rocket Pellets 9.6g on a range hood stainless steel:

top: with only the piston plug.
bottom: piston plug and shorten



but still with low power ram right?


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PostPosted: Sat Apr 04, 2020 1:49 pm 
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Posts: 1556
Location: Eastern Townships
Jetronic wrote:
But this isnt a magnum yet with the weak spring piston. Otherwise i agree but still possible to go heavier than optimum. Around 10 grain seems like a sweet spot for these b18 clones on full power in 0.177.


I know it's not a magnum rifle, i mentionned that to illustrate the fact. Yes it's possible to go heavier than necessary, but I don't see the point in using heavier than 10 grains pellets in sub-500 fps airguns, which is what I define as a ''heavy'' pellet. I've always heard of increased accuracy when using 10 grains-ish pellets, even in sub-500 airguns.

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PostPosted: Sat Apr 04, 2020 3:19 pm 
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Joined: Fri Apr 03, 2020 7:08 pm
Posts: 6
chrony result of the stock remington express hunter 495 fps rated (canada detuned)

with crosman 7.4g destroyer 440 fps average (this batch fitted tighter in the barrel than the two other tested)

with crosman 7.4g wadcutter 455 fps average

with crosman 7.4g pointed 490 fps average (hit 509 once)


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PostPosted: Sun Apr 05, 2020 9:33 am 
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Posts: 7
Hi Folks… new guy here, and a new owner of the ‘Princess Auto special’ … :lol: .

I have already done the trigger bearing mod and it made big difference in the trigger pull and ‘creep’.
I’m also planning to up the power with a replacement BT9M22-00-5 gas ram when I have some extra cash, and I’m looking forward to following your comments and results.

After looking at ulricbb’s piston pics, I believe the large blind hole was simply a way to facilitate the small ‘detuning’ vent hole previously seen in the centre of the piston face on coil spring rifles.
On a gas ram rifle the centre hole would be blocked by the ram, so it seems Crosman simply found a way to do the same thing, but vent the air out the side of the piston.

Questions…
What is the purpose of shortening the piston?
Is it necessary when installing the full power BT9M22-00-5 gas ram?

Ulricbb… thanks for starting the thread and please keep us posted on your mod progress and results.

Bordurat… sounds like you are going to try the new gas ram without plugging the hole. Please keep us posted on your results as well.

Thanks again folks.


Last edited by RADS on Sun Apr 05, 2020 10:19 am, edited 1 time in total.

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PostPosted: Sun Apr 05, 2020 10:06 am 
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Joined: Thu Apr 02, 2020 4:06 pm
Posts: 16
Location: North shore Montréal
What is the purpose of shortening the piston?
the purpose of shortening the piston is adding movement when you cock it, resulting in more air in the chamber...
more air volume=more pressure(compression ratio)=more fps

https://en.wikipedia.org/wiki/Stroke_ratio

Is it necessary when installing the full power BT9M22-00-5 gas ram?
no

my bearing is still in the mail! i cant wait to try it

RADS wrote:
Hi Folks… new guy here, and a new owner of the ‘Princess Auto special’ … :lol: .

I have already done the trigger bearing mod and it made big difference in the trigger pull and ‘creep’.
I’m also planning to up the power with a replacement BT9M22-00-5 gas ram when I have some extra cash, and I’m looking forward to following your comments and results.

After looking at ulricbb’s piston pics, I believe the large blind hole was simply a way to facilitate the small ‘detuning’ vent hole previously seen in the centre of the piston face on coil spring rifles.
On a gas ram rifle the centre hole would blocked by the ram, so it seems Crosman simply found a way to do the same thing, but vent the air out the side of the piston.

Questions…
What is the purpose of shortening the piston?
Is it necessary when installing the full power BT9M22-00-5 gas ram?

Ulricbb… thanks for starting the thread and please keep us posted on your mod progress and results.

Bordurat… sounds like you are going to try the new gas ram without plugging the hole. Please keep us posted on your results as well.

Thanks again folks.


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PostPosted: Sun Apr 05, 2020 11:08 am 
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Joined: Mon Mar 30, 2020 5:50 am
Posts: 18
The most effective way to increase power from a springer is to increase the piston swept volume by increasing the stroke length.

I'm going to try to reduce the stroke on my phantom to bring it to 12 ftlb with 10 grain pellets.


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PostPosted: Sun Apr 05, 2020 11:31 am 
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Joined: Sun Apr 05, 2020 7:58 am
Posts: 7
Thanks for the info guys... keep it coming. :D


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PostPosted: Sun Apr 05, 2020 2:33 pm 
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Joined: Sun Apr 05, 2020 7:58 am
Posts: 7
BTW... along with the trigger bearing mod, I also replaced the 'way too short' trigger adjustment screw and replaced it with a longer one which allows you to fine tune the exact amount of trigger 'travel' or 'creep' to your liking.

At least, that's what worked on mine.


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PostPosted: Sun Apr 05, 2020 3:54 pm 
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Joined: Thu Apr 02, 2020 4:06 pm
Posts: 16
Location: North shore Montréal
with that longer screw, you can ajust the travel before or after the release of the piston?

because on my riffle, the travel, before the release of the piston is soooooo long!!!!!
i did play with the screw, but i can hardly see difference between maximum and minimum adjustment....

RADS wrote:
BTW... along with the trigger bearing mod, I also replaced the 'way too short' trigger adjustment screw and replaced it with a longer one which allows you to fine tune the exact amount of trigger 'travel' or 'creep' to your liking.

At least, that's what worked on mine.


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PostPosted: Sun Apr 05, 2020 5:09 pm 
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Joined: Sun Apr 05, 2020 7:58 am
Posts: 7
The original (factory) adjustment screw was 3mm x 5mm, and was to short to have 'ANY' effect on the trigger whatsoever.
Don't know why it was even there in the first place.
The longer screw was a 3mm x 10mm, I scrounged from my Radio Controlled Aircraft hobby bits and pieces.

So, the bearing mod...
1. Eliminated the fake 'first stage' of the 2 stage trigger.
2. Lightened the trigger pull somewhat by eliminating the spring which provided the aforementioned fake first stage.
3. Shortened the trigger pull somewhat.
4. Provided a smoother feel to the trigger (of course that is somewhat subjective :wink: )

And the Longer Adjustment Screw then allows you to fine tune the length of trigger travel before it breaks and the piston releases.

But please note, the need for the screw may be dependent upon the size of the bearing you use for the mod, and on the individual gun.
On some of the Youtube vids I watched, after the bearing mod and with the screw completely removed there was almost no trigger travel (ie: a hair trigger).

I used a bearing I had on hand from my RC hobby, which didn't list the actual size on the package.
But I measured it and believe it is a 8mm o.d. x 5mm i.d. x2mm w. bearing.

Hope that's a little more clear.


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PostPosted: Sun Apr 05, 2020 5:45 pm 
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Joined: Thu Apr 02, 2020 4:06 pm
Posts: 16
Location: North shore Montréal
great info thx

i purchaced this bearing, size 5x8x2.5mm,
https://www.ebay.ca/itm/RC-Bearings-5x8 ... 2749.l2649

I will try the trigger after bearing install and spring removed
i will see after if it need a longer screw...

thx again

RADS wrote:
The original (factory) adjustment screw was 3mm x 5mm, and was to short to have 'ANY' effect on the trigger whatsoever.
Don't know why it was even there in the first place.
The longer screw was a 3mm x 10mm, I scrounged from my Radio Controlled Aircraft hobby bits and pieces.

So, the bearing mod...
1. Eliminated the fake 'first stage' of the 2 stage trigger.
2. Lightened the trigger pull somewhat by eliminating the spring which provided the aforementioned fake first stage.
3. Shortened the trigger pull somewhat.
4. Provided a smoother feel to the trigger (of course that is somewhat subjective :wink: )

And the Longer Adjustment Screw then allows you to fine tune the length of trigger travel before it breaks and the piston releases.

But please note, the need for the screw may be dependent upon the size of the bearing you use for the mod, and on the individual gun.
On some of the Youtube vids I watched, after the bearing mod and with the screw completely removed there was almost no trigger travel (ie: a hair trigger).

I used a bearing I had on hand from my RC hobby, which didn't list the actual size on the package.
But I measured it and believe it is a 8mm o.d. x 5mm i.d. x2mm w. bearing.

Hope that's a little more clear.


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PostPosted: Mon Apr 06, 2020 4:57 pm 
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Joined: Mon Apr 06, 2020 4:39 pm
Posts: 2
Hello all new to the forum, but not to air guns although I've never opened many up to upgrade them, just finished plugging the small bleeder hole...made a huge difference in accuracy and up'd the power by depth of penetration into half inch plywood...thanks to you folks. I of course also purchased the p. auto rem.hunter express, first nitro piston air rifle, I also rebuild nitrogen charged shocks the rebuildable kind that you find on snowmobiles, bikes, quads, sxs etc.. so I have nitrogen tank gauges and all the bits and pieces to facilitate shock rebuilding. So my question is has anyone heard of modifying the nitrogen cartridge itself, increasing the nitrogen charge in it, I have done similar to "non rebuildable shocks" by adding a small self sealing port to allow for discharging and recharging the nitrogen. The though is increase the speed and ftlbs of the piston which in turn should affect the end speed/power?

Thanks


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PostPosted: Mon Apr 06, 2020 6:16 pm 
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Joined: Sun Apr 05, 2020 7:58 am
Posts: 7
Welcome AirPNB

Can you explain how you plugged the "small bleeder hole"?

As far as moddiing the gas spring, I don't enough about them to answer your question... but it sounds kind of intriguing.

Thx.


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PostPosted: Mon Apr 06, 2020 6:51 pm 
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Joined: Mon Apr 06, 2020 4:39 pm
Posts: 2
RADS wrote:
Welcome AirPNB

Can you explain how you plugged the "small bleeder hole"?

As far as moddiing the gas spring, I don't enough about them to answer your question... but it sounds kind of intriguing.

Thx.


I opened up the gun removed the nitro piston gas ram, took out the piston and plugged the small bleeder hole in side with jb weld, it's all in the previous posts in this thread.

Cheers


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