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PostPosted: Sun Nov 30, 2014 6:15 pm 
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Location: West of Ottawa
Here is a picture that shows the difference in the crosman nitro piston full power compared to the Canadian sub-500 fps version.
There are two changes, 1) the piston gas spring/strut is much stronger - you can see by comparing the thickness of the strut itself and 2) the piston sleeve is shorter on the PAL version so that the cocking action is longer. With the sub-500 fps version the spring cocks at about 90 degrees, with the PAL version it's more like 120 degrees.

In the pic the order is a bit screwy, the top spring is PAL and the bottom piston sleeve is PAL.
Sorry if my nomencature is not correct but I hope it's clear.


Attachments:
piston.jpg
piston.jpg [ 676.21 KiB | Viewed 1205 times ]

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PostPosted: Sun Nov 30, 2014 7:05 pm 
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Thanks for posting that, makes for an interesting comparison. That's a decent way to detune, unlike some of their other efforts.


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PostPosted: Sun Nov 30, 2014 7:52 pm 
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So are both pistons same diameter? They look different in the pic.

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PostPosted: Sun Nov 30, 2014 8:52 pm 
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I think they are. Pretty darn close if not exact. I no longer have access to the sub-500 version.

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PostPosted: Sun Nov 30, 2014 11:52 pm 
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Seems like an interesting way to go nitro piston, how do you like the mod?

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PostPosted: Mon Dec 01, 2014 9:41 am 
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I did not change a mechanical spring for a gas strut - I changed a gas strut for a stronger gas strut, so I can't give a comparison between mechanical and gas.

The upgrade went in easily enough and supplied substantially more power. But I have removed it and have gone back to the non-pal version.

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PostPosted: Mon Dec 01, 2014 12:55 pm 
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OK, my bad thinking you went from spring to gas. So why the return to non-pal?
Did not like the shooting cycle?

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PostPosted: Mon Dec 01, 2014 9:39 pm 
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I wonder if you could interchange just the strut and piston to come up with 4 possible results and end up with a 12ftlb set up out of them.That could make for a nicer shooting rifle.

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PostPosted: Tue Dec 02, 2014 4:55 am 
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Looks like the low powered 695fps spring combined with a short stroke piston. You could add a regular spring, but I doubt it would do much. Imo the added cocking effort would not be worth the gain. If you got a normal piston, or shortened that one to normal length, then you'd have closer to 15ftlbs with no additional cocking effort, just a little longer cocking stroke like your other gun. If it were me I'd make the piston a but shorter than a normal one for even more gains. You can't go too far, maybe 4mm, but if you're cutting already I'd do it. My regular springs have a hole in the butt end, my low powered spring's butt is sealed. The ones with the holes have a much higher failure rate which I always assumed was from the seal, by my last one leaked from that hole... I'm thinking about threading a bolt into that hole with epoxy to reduce future failures. It should take a 4mm bolt if you want to try it.


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PostPosted: Tue Dec 02, 2014 8:27 pm 
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GarthThomas wrote:
I wonder if you could interchange just the strut and piston to come up with 4 possible results and end up with a 12ftlb set up out of them.That could make for a nicer shooting rifle.

I'm sure you could. Not sure how the Canadian spring would work with the longer cocking American piston.

Sorry, but I couldn't parse Chevota's post at all.

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PostPosted: Tue Dec 02, 2014 9:26 pm 
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From your photo it looks like the pistons are the same length so I imagine the stroke is the same , so with the Canadian strut and the american piston there would be more air available for the cycle and it should add a little zip. If you had a chrony then it would be interesting to check out and if that was a .177 then somewhere in the middle would be real nice.

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PostPosted: Wed Dec 03, 2014 8:31 am 
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Sorry, my bad. It makes sense to me because I know the parts, plus the nomenclature. The spring is the nitro/gas ram. The piston is the part with the seal attached. A lot of people call the nitro spring a piston which gets confusing... So judging by looking at them the spring with the fatter 10mm chrome shaft is the stronger one by ~35%,which you can check by trying to compress them. I "assume" the spring with the smaller 8mm black shaft is the one used in the US on the "695fps Titan". That 695 model uses the normal (for USA) short piston.
The longer piston means you'll have a shorter stroke and thus less air to compress and less air volume and pressure to push the pellet. So with the weaker spring and the long piston they make 500fps. With the strong spring and short piston it would be 800fps (not really but that's the claim). Does that make more sense?
So the stronger spring and short piston has the most stroke and strength to make the most power. The lighter spring with the longer piston would mean the least power, but I think you know that already.
As for mixing parts, which I mentioned since Garth asked, I think putting the stronger spring with the long piston would have little gain, maybe 550fps, but your cocking effort would be high. Putting the weaker spring with the shorter piston should give you what the 695 Titan does, which is ~695fps, but much easier cocking. These fps #'s "should" all be based on a standard 14.3gr .22cal Crosman pellet. Fps #'s will vary with the condition of the gun, so typically less than advertised.
If you cut the long piston down to say 4mm shorter than the short one, and combined it with the stronger spring, you could net even more power, maybe even hit the 800fps mark. That mod can be tricky tho, but I mentioned it in case anyone had the urge.
To make it more confusing they have another spring that has the small 8mm shaft but chrome, and it is even stronger. It's also a bit shorter, but it will fit is shimmed correctly.


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PostPosted: Wed Dec 03, 2014 11:12 am 
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That was clear. Thanks.

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PostPosted: Thu Feb 19, 2015 5:13 pm 
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Location: Ottawa
DougP wrote:
The upgrade went in easily enough and supplied substantially more power. But I have removed it and have gone back to the non-pal version.


Hi Doug, I was just reviewing an older post I made about tuning a Benjamin Legacy NP. You replied and attached a link to a pic of both Pal and NonPal gas struts and piston tubes. I then noticed that you said you went back to Non Pal. I was wondering why?

I'm assuming the full power kit you bought was from Scopes and Ammo, was there a problem? What did you do with the "Kit"?

Jim


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