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PostPosted: Sat Dec 13, 2014 7:00 pm 
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My 953 has had some down-time recently with a broken indexing lever (topic65415), so seeing as I had it apart I thought I'd take the opportunity to do something I've been thinking about for a while - replacing the valve poppet and stem. There are two reasons I wanted to do this. Firstly, I think the original poppet wasn't closing properly at the beginning of the pump stroke, so a variable amount of air escapes until the poppet forms a seal. This is the only reason I can think of for double pumping increasing the fps in these guns (since there isn't a check valve to retain air). The second thing was to increase fps. The only downside to replacing the barrel with an LW was that I lost about 50 fps, and I wasn't able to recover all of that even with an adjustable piston. As a result my 953 was running about 420 fps with Crosman wadcutters or Crosman hollowpoints. I suspect the drop in fps is caused by the barrel being much tighter than the orginal. I've noticed that fps with the LW barrel is determined more by pellet size than by pellet weight.

Here's the original poppet and stem (top) and my replacement below:
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The stem has been reduced in the area between the poppet and transfer port to improve flow - part of the attempt to increase fps. The other difference is that I've doubled the thickness of the poppet. The increased thickness of the wider section increases the pre-load on the poppet spring and helps to ensure that the valve closes as soon as possible during the pump stroke. The longer narrow section provides more support for the spring, and the overall increase in volume of the poppet reduces the volume available for air inside the valve and so increases compression ratio and pressure. The increased pressure is where I was hoping to pick up most of the extra fps.

The original poppet was surprisingly soft, it feels quite similar to o-ring rubber, which left me a bit uncertain about what material to use for the replacement. I have delrin, nylon and ptfe. The ptfe felt the softest, and the self-lubricating properties seemed like a good idea. The deciding factor was that I haven't used any before and I wanted to see what it was like in the lathe, so ptfe it is. (It machines very nicely, much like delrin.)

When I first put the rifle back together the new poppet wouldn't hold air. I thought I was going to have to take it apart again and find some way of spinning the poppet in the valve to lap it in, except with the design of the 953 valve body/frame I couldn't think how I was going to be able to do that. Fortunately, after 10 or so sharp pumps it started to hold air and once seated it hasn't leaked since.

A quick run through the chrony indicates success on increasing the fps, although there is a bit more variation than I'd like to see:

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Smaller pellets like the Daisy wadcutters run a little faster, averaging 496 fps with less spread. I'll let the gun settle in a bit and keep an eye on the fps. If necessary I can back off the adjustable piston (another mod) a little to keep it under 500.

Gratuitous shot of the rifle for no good reason :)

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Happy Shooting!
Jim


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PostPosted: Sat Dec 13, 2014 9:34 pm 
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Location: Hamilton
Wow great work, you should create an upgrade kit :)

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PostPosted: Sun Dec 14, 2014 6:43 am 
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mike87gt wrote:
Wow great work, you should create an upgrade kit :)


Ah, you should see how long it takes me to make stuff - I'd be no good at turning out kits. In some ways it's probably not really that great an idea. The Daisy x53 series rifles aren't very strong, and anything you do that increases pressure adds to the stress on the parts. In this one you can clearly see the flex in the frame through the scope as the POA shifts by about 3/4" when the pressure is released. In contrast, my 1377 based rifle doesn't move at all during the shot. Crosman's "bolt everything to a steel tube" design may look crude, but it is very effective.

Jim


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PostPosted: Sun Dec 14, 2014 7:34 am 
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Location: Eastern Ontario
Nice work (and rifle), Jim... 8)

I'm guessing (and that's all it is - a guess at best) that your shot-shot variation could possibly be coming from the soft PTFE poppet "sticking" to the valve once under pressure? I know the 953 doesn't generate huge psi like a PCP, but it would be interesting to see the poppet after a few hundred shots to see if the PTFE is distorting at all.

EverHopeful wrote:
Gratuitous shot of the rifle for no good reason :)


When a gun looks as good as that (not to mention the quality of pic itself) that's a good enough reason in my books! :D

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Todd


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PostPosted: Sun Dec 14, 2014 8:08 am 
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Hi Todd, that's exactly what I was wondering. Seems like a distinct possibility doesn't it. I'll definitely be taking a close look at the poppet next time I have the gun apart.


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PostPosted: Mon Dec 15, 2014 1:47 am 
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Location: Winnipeg, Canada
That should shoot quite nice!
Probably groups like a house afire....

I'm using a delrin stem (poppet) in my hpa 20-78. It seems to be holding up fairly well, and I'm getting good shot to shot consistency. There was a minor velocity loss at the low pressure side, compared to the factory stem- 40 fps or so @1400 to 1600 psi. Trade off their was well worth it though- I ended up with good strings on the high pressure side that the stem was designed for.
Might be worth investigating...

Regards,

Doc Sharptail

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