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PostPosted: Wed Jan 18, 2017 1:00 am 
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Joined: Sat Sep 03, 2005 9:25 am
Posts: 3432
Location: Ontario, Canada
P17 Adjustable Piston

The German made Weihrauch HW40 is one of my favourite air pistol designs. It’s a light-weight pistol with a nice adjustable trigger, scope rail, and comfortable finger groove handle. Accuracy is very good and velocity is quite acceptable for a single-stroker design. I bought my HW40 back in early 1999 as a Beeman P3 label. I also have the Chinese P17. The same Chinese copy was also marketed over 10 years ago as the Marksman 2004 and Beeman 2006.

Shooting buddy and forum member “ricksplace” recently came up with the plan to make an adjustable piston for his P17. He machined up an excellent example and gave it to me to try out. The piston rod is deeply threaded into the piston body and locks into position with a knurled ring. The design works perfectly so I ran it through some tests to satisfy my curiosity.

My first adjustment gave approximately 240fps with 7.9gr Crosman HPs. A velocity string was surprisingly consistent at 241, 245, 243, 240, 239. I dialed down the piston as short as possible and got one shot at 229fps. The next pellet stuck in the barrel with a phffffft sound. I pushed it down the barrel a quarter inch with a small rod and it shot out fine. I cranked up the power to 250fps and got two shots followed by another stuck pellet. I tried seating pellets but it did not help.... 249, 251, 253, 251, 251, stuck pellet. Velocity needs to be up around 300fps to shoot without any concern with Crosman pellets sticking.

The 8.44gr JSB Exact has a thin skirt, compared to the Crosmans, so I turned down the power again and tested a string. Thin skirted pellets like the JSBs seem to shoot fine with the lower pressures. A firm finger push to seat the JSBs resulted in no stuck pellets. The 10 shot string went as follows.

227, 228, 230, 229, 228, 230, 228, 233, 230, 229 --- Ave = 229 fps

The piston is completely adjustable and can be dialed from a low of 230fps to over 400fps with Crosman HPs. I was not comfortable testing much over 400fps so I stopped there. In my opinion, the stress on the frame isn’t worth a few more fps.

Here are the results with 7.9gr Crosman HPs for each complete turn of the piston rod (from bottomed out).

1 – 240fps
2 – 254
3 – 273
4 – 301
5 – 330
6 – 373
6 ½ - 400

Here is a test sample with 7.9gr CPHPs and 6 ½ turns... 403, 400, 403, 402, 401 fps.

The difference in adjustment length from bottomed out to 6 ½ turns is 0.325”. Adjusting this third of an inch brings average velocity up 170fps (230 to 400fps). Velocity can be adjusted to anywhere in between these numbers.

What good is an adjustable piston?
> You can adjust the P17 so a small person can compress the air during cocking
> A low velocity might be preferred for short distance indoor shooting
> Maximize power
> Reduce noise with less power
> Experiment with accuracy at different velocities


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PostPosted: Wed Jan 18, 2017 1:11 am 
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Joined: Fri Sep 23, 2005 2:29 pm
Posts: 6209
Location: Okanagan,BC
NICE!

Is he going to market this mod - or how can one get a P17 modded like that?

8)


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PostPosted: Wed Jan 18, 2017 10:03 am 
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Joined: Mon May 26, 2014 1:08 am
Posts: 480
Location: Thunder Bay
First thing: Thanks for a great review, Todd. Excellent, as usual!
Todd also replaced that tiny o-ring breach seal and even managed to get a couple of wraps of dental floss behind it.

I really don't want to produce or market this thing. It was an idea I had based on my left hand getting sore after 50 shots. I will draw up plans and post them on this thread for anyone who wants to make one. If someone wants to use the design and produce and market it to make a few bucks, you have my blessing. I mentioned in another thread, that the knurled locking ring really isn't needed. It took me as long to make the fancy ring as it did to make the piston! A witness mark on the piston would show any movement every time it's cocked. During internet-based research I did prior to making it, I found folks used the adjustable piston to boost power. The result was often bent hinge pins and cracked frames, with very little gains. It seems the P17 is pretty much maxed out in standard trim.

It really amazes me what Airgunners can make without machine tools. I originally made this as a drop in exchange with the stock piston assembly. Now that we know it works, and can produce full and reduced power, the stock piston can be cannibalized. The piston with the o-ring groove is the only part that must be made on a lathe. I have pretty average skills with hand tools, so if I can make one without using my lathe, anybody can. Another project...

Stay tuned,

Rick

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12 springers and a couple of pumpers.
2 lathes and lots of scrap metal.

I'm not multitasking. I'm doing something else until I remember what I was doing.


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PostPosted: Wed Jan 18, 2017 11:09 am 
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Joined: Mon Feb 11, 2013 12:46 am
Posts: 4186
I think you did a great job rick curious though did you get the idea from here...
http://anotherairgunblog.blogspot.ca/20 ... n-p17.html

while its nice to have an adjustable piston not all are handy or brave enough to try it
my tape mod gave good results and was much easier to achieve...
topic58676.html

if you have the skill and machining tools you can also make the P17 a multi stroke...
http://www.network54.com/Forum/275684/t ... e+finished

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PostPosted: Wed Jan 18, 2017 5:47 pm 
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Posts: 480
Location: Thunder Bay
Thanks, Ace.
I think I saw that first site in my travels. There were a lot of great ideas. I didn't create anything new. I took the idea of an adjustable stroke and used it to reduce the compression ratio to reduce cocking force. The quieter report was a bonus. It seems that around 280 fps ensures reliability with the quieter report. Todd and I shot this afternoon. Accuracy at the reduced velocity was around 3/4" at 10 meters. Maybe Todd can add some more observations in another post.

The last site you mentioned I found early on. While I have an RPAL, I don't want a restricted air pistol. One site I found had a similar idea, but was pushing over 700 fps with a different check valve, a 9" barrel, and four pumps. However, the piston would pull completely out of the tube when pumping since the larger valve took up more space in the tube, decreasing the compression stroke.

The only difficult part was figuring out dimensions eg. depth of thread in piston, length of piston, length of strut, length of thread, in order for the gun to produce from min to max in one assembly. When you look at an open P17, there is no room for a stock piston to move further out. The piston must be shortened by the amount of the adjustment plus the thickness of the locking ring, and still keep the piston o-ring in the tube at full cock. There isn't a lot of room in there. The fact I got it right the first time is pure BS luck. Maybe I should go buy some lottery tickets.

Rick.

_________________
12 springers and a couple of pumpers.
2 lathes and lots of scrap metal.

I'm not multitasking. I'm doing something else until I remember what I was doing.


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PostPosted: Wed Jan 18, 2017 9:38 pm 
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Location: Montreal
How cool is that!
And for the price of the pistol I would just get another one so I can keep on stock and a modded one.

I have carpal tunnel syndrome and the hit my hand gets on the closing stroke is hard on my hands too.

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You can’t buy a single airgun for everything. Golfers use many different clubs, airgunners need more than one gun


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PostPosted: Thu Jan 19, 2017 5:16 am 
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Joined: Mon May 26, 2014 1:08 am
Posts: 480
Location: Thunder Bay
DocGadget wrote:
How cool is that!
And for the price of the pistol I would just get another one so I can keep on stock and a modded one.

I have carpal tunnel syndrome and the hit my hand gets on the closing stroke is hard on my hands too.

Thanks Doc.
That's exactly what I did. Darn things are only $35 US at Pyramyd Air.
Next unit will have some notches on the back of the piston to make turning it easier while it's in the gun.
An added bonus is that it's quiet enough at the lowest setting to shoot in an apartment.
The pellets are going so slow, they don't deform on their way through the paper/cardboard of the target. An old towel inside a cardboard box will stop the pellets. I actually shot a few of them again and they shot fine!

Rick.

_________________
12 springers and a couple of pumpers.
2 lathes and lots of scrap metal.

I'm not multitasking. I'm doing something else until I remember what I was doing.


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PostPosted: Fri Jan 20, 2017 11:10 am 
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Joined: Sat Sep 03, 2005 9:25 am
Posts: 3432
Location: Ontario, Canada
I shot a few pellets last night with the P17 detuned to 295fps with 8.44gr JSB Exact. My eyes are not what they once were but with a 4X scope I managed a 3/4" group and a group that had 4 in a ragged hole and one slight flyer that put the group a hair over 1/2". Cocking effort was very comfortable. Accuracy doesn't seem to suffer when reducing velocity but the target paper doesn't tear as nice.


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