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PostPosted: Mon Jan 09, 2017 12:37 pm 
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Joined: Mon Aug 24, 2015 4:56 pm
Posts: 462
Location: Tecumseh, Ontario in South Western Ontario
My goal on this project is to build a 70+ fpe .25 cal tack driver. Why would I choose an expensive and finely tuned 40fpe rifle to modify? Well quite simply it has everything I want in a rifle except for that extra 30fpe.
Before I opened it up I cleaned the barrel and took these shot strings. Each string started with a full fill of 200 bar and ended at about 135 bar and the air tube holds 231cc.

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The first thing I noticed and I don't know if it's common but the heavier the pellet the later it peaked in velocity. Also the lines were relatively smooth and I credit that to Air Arms use of a 'Pot'. It's a 4 by 1/2” brass chamber that leads to the air inlet valve and has a small hole at the other end to take in air from the main air tube. It works, I believe, by limiting the volume of air per shot. Clever for a high consistent shot count but it reduces the potential power so I'm removing it and will later install a pressure regulator.

No matter what else I do to the gun, the first thing will be opening and equalizing air flow through all the ports to near bore size. There is a blow up diagram at this site http://cdn.pyramydair.com/site/manuals/ ... manual.pdf. I couldn't find a schematic showing the power adjuster.
This crude and so not to scale drawing shows the layout of the main parts.

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Bore ID is 0.25”, bore area is 0.0491sq. in.. Pellet probe OD is 0.1190” and I plan to reduce that to 0.0787” (2mm) for a probe area of 0.0049 sq. in.. If we subtract the probe area from the bore area we get an area for air flow of 0.0442 sq. in. or the equivalent diameter of 0.2372” or 15/64” a common drill bit size. Therefore my goal will be to open all ports to a diameter of no less than 15/64” or it's equivalent area which is 95% of bore. The valve seat ID is 0.1320” or an area of 0.0765 sq. in. and the valve stem has an OD of 0.1551” or an area of 0.0189sq. in. Subtracting the stem area from the seat area we have 0.0576sq. in. which is larger than my goal of 0.0442 sq. in. so I'm OK there.
The other ports have an ID of 0.0158” so they will be drilled out to 15/64”.

The barrel port will need extra attention because if I were to drill that large a hole through it the front of the pellet would partially fall into it and be deformed before it gets fully chambered. So I'll drill the barrel port most of the way through but not into the chamber. Then I'll use a dremal bit no wider than the original port hole to elongate the port hole on the chamber side so the resulting air flow should match the other ports and the pellet will pass by unscathed.

The power adjusting port will be a challenge since the shaft's diameter is less than my target port size. I can't just drill through because the resulting two halves would go fling out on the first shot. Also I can't replace it with a larger diameter shaft because there is no room in the valve body.

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I plan to turn down an aluminum rod to fit snugly into the adjuster shaft hole. It would be held in place by a screw on either side of the not yet drilled port hole. Also I'd use epoxy for extra strength and sealing although I'm not sure if epoxy is the best choice there. Then I could drill through the valve body and aluminum rod to form the port. I never would have used the power adjuster anyway.

When all the porting is done I'll reassemble and take another shot string and figure out what needs to be done next. If anyone has suggestions or comments I'd love to hear them.

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PostPosted: Mon Jan 09, 2017 5:17 pm 
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Joined: Sun May 07, 2006 5:42 pm
Posts: 649
Location: Ontario
Excellent job!

I did the same thing with my S410 FAC S/L .22 cal as well and achieved 61 ft/lb: http://www.airgunforum.ca/forums/topic29959.html

I enjoy tinkering but lately been content with factory settings. I have one more project I want to pursue but not sure when I'll have the time.

Keep us posted on your numbers.

Cheers,

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Air Arms S510 Carbine FAC .22cal
Air Arms S510 Carbine FAC .117cal


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PostPosted: Thu Jan 12, 2017 7:52 am 
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Joined: Mon Aug 24, 2015 4:56 pm
Posts: 462
Location: Tecumseh, Ontario in South Western Ontario
PLAN B.

After going over the numbers and measurements I found that porting to 15/64" would have been too aggressive. The tolerances in the valve body and the barrel were tight and too risky so I went with the next smallest drill bit in the set 7/32" or 0.2188" which is nearly 80% of bore. Not bad when you consider the original ports were less than 40% of bore. This meant the power adjuster could stay in place but barely. I drilled it out until there was very little metal left on the sides. This meant that even at it's lowest power setting it would only slightly reduce velocity but that may come in handy when fine tuning for accuracy. I also used a file to expand the hole by elongating it until it's area was equal to that of a 7/23" diameter hole.

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I put it together and since it previously peaked at 140 bar I only filled to 150 bar. I'm only interested in max velocity so I can later set a regulator to that pressure.
Jsb King 25.4gr shot at 975fps.
H&N Baracuda 31gr. peaked at 900fps.
The Jsb King Heavy 34gr maxed out at 858fps.

That's about 56fpe up from 40fpe. from just porting and cutting the pot open.

Then I went back and looked at the inlet valve spring who's preload was not adjustable to a lower tension (my pot was not treaded like the older ones).
To lower the tension I drilled the spring stop in the pot back about 2mm which is all it can be drilled. Because the spring was long and not particularly stiff I didn't think it would make that much of a difference but I had to start somewhere. Now the pellets shot like this;
The Jsb King 25.4gr pellets peaked at 1000fps.
The H&N Baracuda 31gr peaked at 930fps.
And the Jsb King Heavy 34gr pellets made it to 890fps.

Not bad, that's 60fpe. Now I don't know whether to put a weaker valve spring in or go on to shimming the hammer spring for greater preload. I'll probably do both since I'd like to find another 10fpe.

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PostPosted: Fri Jan 13, 2017 10:21 am 
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Joined: Mon Aug 24, 2015 4:56 pm
Posts: 462
Location: Tecumseh, Ontario in South Western Ontario
I thought I'd install a weaker valve spring and increase the hammer spring preload today but thought the better of making two changes at once.
So I decided to pursue the valve spring tension a little further. I installed a weaker spring and it actually decreased the peak velocity by 15fps.:?: I reinstalled the original valve spring and renotched the pot (really just a valve spring retainer at this point) further back so there would be an additional 2mm less preload. The Jsb 34gr pellets now peaked at 920fps a 30fps increase for 64fpe up 4.
I will leave well enough alone there and move on to other power upgrades. I know decreasing the valve spring preload is similar to increasing the hammer spring preload however the valve is also held closed by the HPA. I'm going to try increasing the hammer spring preload gradually and see what happens.
Does anyone know if increasing the area for air flow through the valve will increase power if it's already greater than the transfer port area?

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PostPosted: Fri Jan 13, 2017 10:51 am 
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Joined: Sun Dec 22, 2013 12:25 pm
Posts: 624
Location: New Mexico, USA
topic65614.html?hilit=air%20arms%20polish&start=105


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PostPosted: Sun Jan 15, 2017 9:54 am 
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Joined: Mon Aug 24, 2015 4:56 pm
Posts: 462
Location: Tecumseh, Ontario in South Western Ontario
Yesterday I tried adding a washer on the main spring guide to increase the hammer preload and soon discovered that the thin 0.07" washer was enough to cause coil bind. Wow, that's precise intentional engineering to prevent tampering, a big issue in England. That must be why they also no longer thread the pot onto the valve body to make it more difficult to adjust the valve spring for extra power. Well there's always a way and I came up with the idea of adding an extra spring in-between the main spring and the hammer guide. I found a spring that fit well and it was even counter wound to the main spring. They must of thought of that too because there is no space in-between the hammer spring guide and the rear spring guide when cocked. Since the rear guide was only a couple of mm wide that meant the entire 0.4" - needed to fit the new compressed spring - would have to be cut of the hammer spring guide - nearly all of it. That would have removed a lot of weight off the hammer and may of made it less stable on the guide. Gladly I came up with another idea. I noticed that when cocking manually with the receiver off, that the hammer moved forward quite a way before the sear engaged it. It actually moved forward 0.49" and the tab is only 0.11" wide. That left 0.38" of lost main spring preload.
You can see in this picture the long toggle sear and the large gap in-between it's cocking tab and the one that locks the hammer back.

Image

The solution was to increase the width of the hammer tab to take up the slack.

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The piece of metal I added to the hammer is temporally glued on with medium lock tight. When I find out if it works and determine it's final length I'll harden and epoxy it on.
I'm soon off to the Detroit auto show so the testing will have to wait until tomorrow.

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PostPosted: Mon Jan 16, 2017 2:58 pm 
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Joined: Mon Aug 24, 2015 4:56 pm
Posts: 462
Location: Tecumseh, Ontario in South Western Ontario
I had a chance to test the gun with the extra 0.3" hammer spring preload today. Before I get into that I just want to mention the sound level with the shrouded barrel. It's true that originally it really was quiet compared to the non-shrouded rifles I own. That started to change as the power began to rise and at 60+fpe it was definitely earplug time.
So I filled the gun to 150 bar knowing it peaks in the 130s. Of the 10 shots I took with the 34gr pellets, 4 were over 950fps and one reached 976! That's 72fpe and plenty for me.
I have as Altaros regulator on it's way and I chose that brand because no vent hole is necessary. Not that I mind drilling a hole in the air tube but I want to experiment with different plenum lengths/volumes. Drilling a hole in the air tube would mean the plenum length couldn't be adjusted. Reducing the plenum volume is also an efficient way to reduce velocity if necessary for accuracy.
I'm happy the way it turned out and relieved. Still being an amateur at this I was nervous about ruining this expensive beauty.
I plan to use the gun for shooting at 100 yards and all I need now is a warm day to get to the range.

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