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PostPosted: Wed Dec 11, 2013 5:33 am 
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Has anyone disproved the results of an FTP conversion or any of the faults associated with the original Crosman piston & valve design? Most members I know achieved positive results with them. I have Crosman pumpers both with and without FTPs, but did not record the before and after chrony results (or any results yet between the guns). My chrony is awaiting spare parts from England (yes, I shot my Combro chrony :x ).

I took apart a 2289 today and wanted to revisit the topic out of curiosity. I have an alternate theory regarding the original Crosman design and it's purpose.

More to come...

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PostPosted: Wed Dec 11, 2013 8:52 pm 
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I took some crony readings when I did mine and the results were positive. I might still have them written down some where if you want the numbers


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PostPosted: Wed Dec 11, 2013 9:27 pm 
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I am anxious for your theory. I am sure you will enlighten us all.

I converted almost all of my 13xx and 2289 to flat top. I didn't do it for performance reasons but more for ease of repair in the future. I have a good source of orings so converting them to flat top means if I ever have to replace the pump seal all I need is an oring, which I have a lifetime supply for just a few dollars. I also had pistons made for my B5-10's to use the same oring. It was a B5-10 needing a pump seal that started the whole conversion process for me. Not being able to easily find a replacement I came up with a solution.

I have not done a lot of testing before and after with the flat top. I have been on a springer kick lately, can't get enough of them. From the little bit of testing I did do, I don't think I noticed a huge difference in performance, not as much as I was expecting anyways. It may be I have to tinker with with piston length a bit, but that is a project for another day.


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PostPosted: Thu Dec 12, 2013 2:14 am 
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Regarding Crosman's pump cup and valve design, the common and prevailing thought is that they did it on purpose to create head space (extra air present in the gaps between the piston and valve, which would not go in the valve). Since that last bit of compression is crucial to the overall pressurization of the valve, it is thought that they did this in purpose so that the gun could not be over pumped to dangerous levels.

While that may be true in one regard, there may be another purpose to the cone shaped valve and pump cup design. I believe that the main purpose of the cone design was to achieve a pump cup with a skirt shape. Similar to diabolo airgun pellets, the skirt of the rubber pump cup flares out against the tube when compressed during the pump stroke, ensuring an excellent seal against the tube wall. Even as the outer edge of the rubber wears out, the flaring action of the pumping process will ensure a tight seal (as long as the rubber maintains its elasticity). The valve followed the mating surface of the pump cup as a way to minimize headspace (or it was intended).

In the actual Crosman valve, the cone tip protrudes past the limit of pump cup’s inner surface. Since the pump cup is rubber, it deforms to accommodate the extra tip. This can be evidenced in the indent created on the stock pump cup (see Picture 1).

It may be that Crosman made a mistake regarding the dimensions of their valve cone. Due to this, the intake port of the front cone is already sealed before the end of the pump stroke (see Figure 1). Figure 2 shows the theoretical overlap in the dimensions at the end of the pump stroke. In reality, it will look like Figure 3 at the end of the pump stroke. Since the intake port is already blocked, the remaining air (from Figure 1) is trapped as the rubber pump cup warps to fit as best as possible to the front valve cone. This is what likely creates the dreaded headspace, leading to pump inefficiency. It is my belief that Crosman made an error when manufacturing the valve cone in the sense that the tip was too long. Their original intent was to have the front valve cone shaped as a perfect negative mold of the inner surface of the rubber pump cup.

If this is true, then the solution of the file down the tip of the front cone until it mates perfectly with the inside surface of the pump cup. I did this with mine and verified that the contours indeed matched. The tip of the cone will have the same diameter as the inner flat circle of the pump cup (see Figure 4).


Attachments:
Crosman piston interaction.png
Crosman piston interaction.png [ 359.55 KiB | Viewed 1126 times ]

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Last edited by TriggerHappy416 on Thu Dec 12, 2013 2:38 am, edited 2 times in total.
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PostPosted: Thu Dec 12, 2013 2:28 am 
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If this is true, then the pumping efficiency and thus FPS (same number of pumps from the same gun) should theoretically be very similar between using the stock piston & filed cone front valve and the FTP and flat front valve since they would have eliminated most of the head space.

I have not verified this through the use of a chronograph yet. My Combro chrony was broken for more than a year now and I was waiting for replacement parts (that were out of stock for the longest time). I decided to investigate this when I was chatting with Ace. He mentioned that (while using a flat front valve) the stock piston cup seal filled with epoxy was much better at compressing air than the o-ring on the FTP. I imagined that this had to do the stock seal (which still flared out against the tube wall) offering a superior seal compared the o-ring on the FTP.

What do you guys think and has anyone tried this before?

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PostPosted: Thu Dec 12, 2013 10:22 am 
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that would make a somewhat valid point TH if it weren't for couple things.

one: when the piston is pressed against the valve face/cone, due to the soft rubber there is no air space left in the tip, the tip is actually pressed very tight into the cup seal, hence the lock up.

two: the head space you want to eliminate is in the actual cone itself not the tip of the cone, but the actual space from tip to check valve face.

three: if your your idea about them designing the valve that way to reduce FPS then they would not make the tip like this,
below is a pic of an unmodified stock valve. :wink:

Image
Image

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Last edited by Ace on Thu Dec 12, 2013 10:56 am, edited 2 times in total.

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PostPosted: Thu Dec 12, 2013 10:38 am 
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TriggerHappy416 wrote:
If this is true, then the pumping efficiency and thus FPS (same number of pumps from the same gun) should theoretically be very similar between using the stock piston & filed cone front valve and the FTP and flat front valve since they would have eliminated most of the head space.



Shaving the cone off the tip of the valve gives you that space as extra volume, the same could be said for the flat piston head, essentially you are gaining all the area that is wasted by that material.


My 1377 shot JSB 10.3 at 470FPS ~ 10 pumps stock...With FTP and a little tweaking it shoots the same pellets at 495FPS ~ 6 pumps


Quote:
Has anyone disproved the results of an FTP conversion

I feel quite confident saying this is not possible, I mean, anything is possible with enough time, money and skill, but generally speaking; No chance if the FTP is setup properly

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PostPosted: Thu Dec 12, 2013 1:10 pm 
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There are lots of examples of guys stuffing Crosman pistons with metal rods to stiffen them, and shimming behind the pump cup to decrease the headspace.... Careful attention to fitting the shape of the cup to the cone so that they mate perfectly will decrease the headspace at low pressures, and if you then preload the cup so that it is compressed just slightly, and using a stuffed piston, you can get virtually the same results as a flat-topped piston.... at low pump numbers.... However, as the pressure builds, the pump cup will flex and compress slightly and create more headspace....

That is the primary advantage to using a Flat-Topped Piston, the reduction in head space at ALL pump pressures.... It is that ability that allows pressure to still be gained well beyond the 15 or so pumps where a 13XX starts to retain air with stock springs and an FTP.... which incidently is about 1500 psi.... Using a properly made and adjusted FTP eliminates over half the headspace inside the inlet hole in the valve (by eliminating length), virtually all of the headspace between the piston and valve, but does add a bit between the O-ring and the end of the piston in the annular clearance between the piston OD and the tube ID.... This can be kept to a minimum with a close fitting piston and O-ring groove, and having the O-ring within about 0.020" of the end of the piston, leaving only enough material to contain the O-ring.... Don't drop the piston, though, or you will bend that very fragile lip....

I've made pistons from Delrin and aluminum, and while the Delrin works quite well, when used in a long stroke pump like a 140/1400 you can cause so much heat to melt the end of it and char it (yes, I've done that too).... It may be fine for a 13XX pump, especially at below 25 pumps, but for the longer pumps with higher compression ratios, I would use aluminum.... Also, the forces on the piston rod can be so great that making the fork section out of aluminum will prevent it from compressing around the pin hole like plastic can.... That it must be adustable for length, and adjusted properly, goes without saying....

Bob

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PostPosted: Thu Dec 12, 2013 1:55 pm 
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Ace wrote:
that would make a somewhat valid point TH if it weren't for couple things.

one: when the piston is pressed against the valve face/cone, due to the soft rubber there is no air space left in the tip, the tip is actually pressed very tight into the cup seal, hence the lock up.

Ace, thanks for showing me the picture of the valve cone. It seems like there are two versions. Mines had a much pointier extended tip. Because of this, if you look at my Figure 1, you would see that the the intake port is already blocked by the pump cup before the rest of the air can be pumped in. No matter how much the rubber deforms, air is likely still trapped since it cannot go in the valve.

two: the head space you want to eliminate is in the actual cone itself not the tip of the cone, but the actual space from tip to check valve face.

The headspace refers to the gap or air left between the piston and front valve, regardless for their shape. For example, if we can eliminate all the space between the pump cup and front valve cone (as in Figure 4), then we would have eliminated headspace. Getting rid of the entire cone is not necessary.

I have done FTP conversions before and have guns have have flat front valves with and FTP installed so I understand what you are trying to convey, but that is not my point in the above post.


three: if your your idea about them designing the valve that way to reduce FPS then they would not make the tip like this,
below is a pic of an unmodified stock valve. :wink:

Image
Image

Again, thanks for this picture. My front valve cone was definitely not like this, leading me to conclude that Crosman made 2 versions. This one, and a cone with a longer/pointier tip. I did a quick google search and it seems someone else has observed this before.

http://www.network54.com/Forum/275684/t ... pers+(Pics)

The first picture here corresponds to what I was trying to depict in Figure 1 and the second picture is exactly what I did with my valve by filing the tip down so it matches perfectly to the valve cup (see Figure 4).



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PostPosted: Thu Dec 12, 2013 2:02 pm 
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jgoodz420 wrote:
TriggerHappy416 wrote:
If this is true, then the pumping efficiency and thus FPS (same number of pumps from the same gun) should theoretically be very similar between using the stock piston & filed cone front valve and the FTP and flat front valve since they would have eliminated most of the head space.



Shaving the cone off the tip of the valve gives you that space as extra volume, the same could be said for the flat piston head, essentially you are gaining all the area that is wasted by that material.


My 1377 shot JSB 10.3 at 470FPS ~ 10 pumps stock...With FTP and a little tweaking it shoots the same pellets at 495FPS ~ 6 pumps


Quote:
Has anyone disproved the results of an FTP conversion

I feel quite confident saying this is not possible, I mean, anything is possible with enough time, money and skill, but generally speaking; No chance if the FTP is setup properly


JG, thanks for sharing your results with us. That is indeed a significant improvement over the stock one. I am thinking that your 1377 may have had the same pointier/extended cone shape that I had, leading to extreme inefficiency before the FTP conversion.

I will confirm how my "mod" performed when my chrony parts arrive (though I will not know how it did before :( )

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PostPosted: Thu Dec 12, 2013 2:07 pm 
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This video confirms that there are indeed 2 versions of the front valve cone (I had the extended cone). The flatter one appears to be American, so the pointier version could be an additional method to detune Canadian pumpers.

http://www.youtube.com/watch?v=Qjf23V1MIEA
(start at 4:30. Ignore the commentary. I think he might have it backwards in terms of benefits of the flatter cone)

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PostPosted: Thu Dec 12, 2013 2:16 pm 
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rsterne wrote:
There are lots of examples of guys stuffing Crosman pistons with metal rods to stiffen them, and shimming behind the pump cup to decrease the headspace.... Careful attention to fitting the shape of the cup to the cone so that they mate perfectly will decrease the headspace at low pressures, and if you then preload the cup so that it is compressed just slightly, and using a stuffed piston, you can get virtually the same results as a flat-topped piston.... at low pump numbers.... However, as the pressure builds, the pump cup will flex and compress slightly and create more headspace....

That is the primary advantage to using a Flat-Topped Piston, the reduction in head space at ALL pump pressures.... It is that ability that allows pressure to still be gained well beyond the 15 or so pumps where a 13XX starts to retain air with stock springs and an FTP.... which incidently is about 1500 psi.... Using a properly made and adjusted FTP eliminates over half the headspace inside the inlet hole in the valve (by eliminating length), virtually all of the headspace between the piston and valve, but does add a bit between the O-ring and the end of the piston in the annular clearance between the piston OD and the tube ID.... This can be kept to a minimum with a close fitting piston and O-ring groove, and having the O-ring within about 0.020" of the end of the piston, leaving only enough material to contain the O-ring.... Don't drop the piston, though, or you will bend that very fragile lip....

I've made pistons from Delrin and aluminum, and while the Delrin works quite well, when used in a long stroke pump like a 140/1400 you can cause so much heat to melt the end of it and char it (yes, I've done that too).... It may be fine for a 13XX pump, especially at below 25 pumps, but for the longer pumps with higher compression ratios, I would use aluminum.... Also, the forces on the piston rod can be so great that making the fork section out of aluminum will prevent it from compressing around the pin hole like plastic can.... That it must be adustable for length, and adjusted properly, goes without saying....

Bob


Thanks for the insight Bob. I can definitely understand how the plastic piston might begin to flex at higher pressures past a high pump count. I don't usually pump my multi-pumps past 10 so that's good. 5 pumps is what I usually go for since the marginal benefits (additional pressure per pump) of the additional pumps beyond that decreases significantly on most multi-pump pneumatics.

I already have an FTP kit on standby incase I choose to move forward with the conversion. I do prefer the stock piston though if the FPS differential at the same # of pumps would be minimal after my "mod". I understand the benefits of the FTP and just wanted to share some thoughts with you guys regarding the original valve cone being too long and causing potential inefficiencies. I now know that this problem is only apparent in the Canadian version.

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PostPosted: Thu Dec 12, 2013 2:53 pm 
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TriggerHappy416 wrote:
This video confirms that there are indeed 2 versions of the front valve cone (I had the extended cone). The flatter one appears to be American, so the pointier version could be an additional method to detune Canadian pumpers.

http://www.youtube.com/watch?v=Qjf23V1MIEA
(start at 4:30. Ignore the commentary. I think he might have it backwards in terms of benefits of the flatter cone)


lol...TH according to the vid cutting or filling the little tip decreases the FPS :| and no i think he's got it right you have to remember that the piston is compressed in the tube, the skirt doesn't flare out as you would think when you have it out of the tube and in your hand doing a visual as your saying. With it in the tube your going to leave a little pocket of air in the tip. hope that makes sense.

Like Bob said you need to eliminate head space inside the inlet hole in the valve.

If you like I'll bring my chrony over and you can see, but if you filed the tip already you wont know a before and after. :|
There is only one real way to find out and its a chrony test :wink:

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PostPosted: Thu Dec 12, 2013 3:08 pm 
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Ace wrote:
TriggerHappy416 wrote:
This video confirms that there are indeed 2 versions of the front valve cone (I had the extended cone). The flatter one appears to be American, so the pointier version could be an additional method to detune Canadian pumpers.

http://www.youtube.com/watch?v=Qjf23V1MIEA
(start at 4:30. Ignore the commentary. I think he might have it backwards in terms of benefits of the flatter cone)


lol...TH according to the vid cutting or filling the little tip decreases the FPS :| and no i think he's got it right you have to remember that the piston is compressed in the tube, the skirt doesn't flare out as you would think when you have it out of the tube and in your hand doing a visual as your saying. With it in the tube your going to leave a little pocket of air in the tip. hope that makes sense.

Ace, I disagree with the opinions of that on the youtube video. I suspect the opposite is true (though I can't prove it yet). Have you seen the link I posted on the green forum? the flatter cone is a perfect fit for the rubber cup, there is not pocket of air in the tip. Also, I was implying that the pump cup skirt flares out during the pumping process because the compression of the air against the rubber forces the edge to compress tightly against the tube wall (not when it's not moving).

Like Bob said you need to eliminate head space inside the inlet hole in the valve.

I agree with that, but that extra volume is insignificant IMO. With an FTP, you also have some deadspace between the o-ring and the "lip" of the piston.


If you like I'll bring my chrony over and you can see, but if you filed the tip already you wont know a before and after. :|
There is only one real way to find out and its a chrony test :wink:

That would be awesome buddy 8)


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PostPosted: Thu Dec 12, 2013 3:26 pm 
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I'm probably not explain it well, never good at doing so on a keyboard :lol: but when we meet up I'll explain to you what I'm trying to say :wink:

Its not about the actual fit because you'll never be able to actually see inside the cap seal to tell if its perfectly fit with the valve head or what not, kind of understand where I'm going with that, anyways we will leave it to the chrony test :D

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