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PostPosted: Thu Apr 02, 2020 4:33 pm 
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Joined: Thu Apr 02, 2020 4:06 pm
Posts: 16
Location: North shore Montréal
hello.

i just ordered a (crosman) remington express hunter (nitro piston) .177 495fps from princess auto, only 99$, selling 160-220$ other places....

i am looking to mod it to have more power, i have a PAL....

i read and watch a lot of informations before posting and get my rifle into pieces...lol

i wasnt expecting finding no spring inside...

and a big hole in the piston (dia. 5/16 about 3/8 deep)
the gaz ram is BT5M77-00-2 170mm long

can i plug the hole and/or add a spring to have more power?

thx.
sorry for my bad english!


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PostPosted: Thu Apr 02, 2020 4:44 pm 
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Location: Kingston, ON
ulricbb wrote:
hello.

Hello and welcome to the forum, lots of friendly ppl here.
ulricbb wrote:
i wasnt expecting finding no spring inside...

there is a spring, but its a gas spring (nitrogen filled gas piston, =nitro pistpn or gas ram, which is what the NP in the name means.
ulricbb wrote:
and a big hole in the piston (dia. 5/16 about 3/8 deep)
the gaz ram is BT5M77-00-2 170mm long

can i plug the hole and/or add a spring to have more power?

Yes, if you plug that hole you will increase the power. That's one big hole however. Suggest you tap it and fill with a big enough threaded filler rod along with permanent (red) Loktite.

No you can't add a spring, you could use a different NP or convert it to wire spring, but that would require enough parts to make it uneconomical.

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PostPosted: Thu Apr 02, 2020 5:05 pm 
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Joined: Thu Apr 02, 2020 4:06 pm
Posts: 16
Location: North shore Montréal
thx for your reply

i thought that it was always a gas ram + spring combination...

i already tap (3/8-16) and plug the end of the hole, easy job for a machinist :wink:

but i didnt have time to try it yet, raining crappy day here! :(

what fps should i expect?

thx


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PostPosted: Thu Apr 02, 2020 5:15 pm 
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Location: Kingston, ON
There us something very strange with the piston hole and the spring combo you have. How was the power before you pulled it apart?
The gas ram you listed is a low power ram, and I have never seen a hole that big - in fact I can't imagine it would develop any power at all with a hole that big. Anyhow, only way to tell is to try it. Full power will be 750-800 fps I'd think. With low powered ram and plugged hole, maybe 600s. Just guessing.

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Air guns? I have a few; but then again, too few to mention.


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PostPosted: Thu Apr 02, 2020 5:26 pm 
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Location: North shore Montréal
hard to know the original power...

at 15 yards, crosman wadcuter 7.9 grains, the pellets gets flush (all inside) drywall.


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PostPosted: Thu Apr 02, 2020 6:10 pm 
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Location: st catharines ontario canada
Not very strong at all, at 15yds that pellet should go though

As Duke mentioned "there is something strange" about that hole size, I would just order a FP gas ram and piston kit

Steve

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PostPosted: Thu Apr 02, 2020 7:43 pm 
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Location: Edmonton
Honestly, I'd just take it back and start with something a little better. (Love Princess Auto for that feature :prayer: ).


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PostPosted: Thu Apr 02, 2020 7:57 pm 
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Edmonton<500 wrote:
Honestly, I'd just take it back and start with something a little better. (Love Princess Auto for that feature :prayer: ).

I think that's what happened to the one he bought, someone tinkered then and returned it.

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PostPosted: Thu Apr 02, 2020 8:35 pm 
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Location: North shore Montréal
i found out some people had same hole in their piston.... https://www.gatewaytoairguns.org/GTA/in ... ic=38179.0

actualy, would it made it more powerfull to plug only the small hole, and keep the big one?

more space (volume), air in chamber=more power. right?

or the most important is smaller the space(volume) is, in chamber after the piston close on the breach port?


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PostPosted: Thu Apr 02, 2020 8:48 pm 
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Location: Kingston, ON
ulricbb wrote:
i found out some people had same hole in their piston.... https://www.gatewaytoairguns.org/GTA/in ... ic=38179.0

actualy, would it made it more powerfull to plug only the small hole, and keep the big one?

more space (volume), air in chamber=more power. right?

or the most important is smaller the space(volume) is, in chamber after the piston close on the breach port?

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.

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PostPosted: Thu Apr 02, 2020 9:01 pm 
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Location: North shore Montréal
haha, no the big hole is not thru the piston.

in my first picture, you can see a small hole in the side of the piston, that hole connect to another small hole in the bottom of the big hole, (both small hole are at 90 degrees.

alright, now i want to cut that piston, does someone know how much i have to remove, and where a have to drill the new hole, for the catch mecanisim...


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PostPosted: Fri Apr 03, 2020 4:41 am 
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Location: Eastern Townships
ulricbb wrote:
haha, no the big hole is not thru the piston.

in my first picture, you can see a small hole in the side of the piston, that hole connect to another small hole in the bottom of the big hole, (both small hole are at 90 degrees.

alright, now i want to cut that piston, does someone know how much i have to remove, and where a have to drill the new hole, for the catch mecanisim...


I measured the standard, ''short'' piston at 5.820'' (+/- 0.010''), from the end of the skirt to the face under the seal. You will need to cut a new sear notch, but you don't need to touch the cocking slot. Position of the sear notch is critical for engaging the sear, so cut the notch the same exact distance from the end of the piston's skirt that it is now. Be aware that the piston is surface hardened, so better to use carbide tooling. Ideally, you'd want to anneal the piston's skirt, and then re-harden and temper it so the sear notch will last without deformations (safety first!). Also you may want to order the full-power gas ram, part number BT9M22-00-5A. That should take your rifle in the 900s.


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PostPosted: Fri Apr 03, 2020 6:48 am 
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Joined: Mon Mar 30, 2020 5:50 am
Posts: 18
Also, keeping the big hole lowers the compression ratio. If you intend to shoot heavy pellets this might reduce piston bounce (on a buffer of compressed air) but with lighter pellets you run a higher risk of slamming the piston into the end of the compression chamber. With a longer stroke and a stronger gas spring you increase the above risk


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PostPosted: Fri Apr 03, 2020 9:09 am 
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Joined: Thu Apr 02, 2020 4:06 pm
Posts: 16
Location: North shore Montréal
thx for the informationm that help alot....

my piston is 170mm long, so about 22mm longer then yours

i measured my gas ram (with ruler):
body lenght 150mm
body OD 17-18mm
shaft lenght (stroke) 110mm (107mm real stroke (installed) because 3mm preload)
shaft OD 8mm
compression pression 33kg, measure on my body scale...

i also measured the stroke of the action of the piston, movement uncocked to cocked. (catch hole in piston to catch position in action) 73mm
so the maximum stroke of the gas spring is 107mm(installed) - 73mm= 34mm stroke left in gas spring!!!

34mm-22mm=12mm stroke left for the original short piston....


airmec wrote:
ulricbb wrote:
haha, no the big hole is not thru the piston.

in my first picture, you can see a small hole in the side of the piston, that hole connect to another small hole in the bottom of the big hole, (both small hole are at 90 degrees.

alright, now i want to cut that piston, does someone know how much i have to remove, and where a have to drill the new hole, for the catch mecanisim...


I measured the standard, ''short'' piston at 5.820'' (+/- 0.010''), from the end of the skirt to the face under the seal. You will need to cut a new sear notch, but you don't need to touch the cocking slot. Position of the sear notch is critical for engaging the sear, so cut the notch the same exact distance from the end of the piston's skirt that it is now. Be aware that the piston is surface hardened, so better to use carbide tooling. Ideally, you'd want to anneal the piston's skirt, and then re-harden and temper it so the sear notch will last without deformations (safety first!). Also you may want to order the full-power gas ram, part number BT9M22-00-5A. That should take your rifle in the 900s.


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PostPosted: Fri Apr 03, 2020 7:15 pm 
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Joined: Fri Apr 03, 2020 7:08 pm
Posts: 6
I ordered the same, same place. Pretty smooth shooter with nitro piston but not powerfull enough to spin my resetting target. I'll post some chrony readings of the stock version.

I ordered a piston at custom air seals (australia) 50 cad free shipping. Its the hp version equivalent. So at 150 i'll have a decent air rifle (I also have PAL)

I' am still wondering what to do with the piston, I'll check your updates


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