Canadian Airgun Forum

We all know about the common problem with the P17 often having a slight burr on the air intake hole. The burr quickly slices a chunk out of the piston seal and velocity decreases and becomes less consistent. My 2010 example had this issue so I fixed it over 6 years ago. Some very fine sandpaper was used to remove the burr.

With the recent P17 talk I started shooting mine again. However the velocity was a little bit low and inconsistent with 8.4gr JSB Exact. I was averaging around 375fps rather than 390fps. I pulled out the old seal and found a nick. I polished up the intake hole with 400, 600, and 1500 grit paper. The new seal quickly developed an even bigger nick! It seems that polishing the hole can leave a very sharp edge on the perimeter. The snug seal must slightly squish into the tiny hole when it slips over when cocking the pistol. The seal then gets sliced. Not good.

I used a couple of small Dremel burr bits to put a bevel on the hole. Hand twisting a few turns each did the trick. Then I gave the top a polish with the mild sandpaper. New piston seal, the old breech seal was changed, dental floss shimmed, 30W-ND oil lube, and velocity checked. The P17 now shoots a bit faster than my old 1999 HW40/P3. Consistent too. I'll be very happy if the P17 maintains these velocity numbers.

8.4gr JSB Exact - 396, 393, 396, 396, 395 fps
7.0gr RWS Hobby - 422, 423, 422, 424, 424 fps

Here's a pic of the two damaged pump seals. The front seal was in the pistol for quite a while. The back seal was shot about 10 times. I'm not saying that everyone will experience the sharp edge from removing an intake hole burr... but it seems to have happened on my P17. Rounding over the edge is not difficult to do if you have the gadget.

Good to know Todd. I recently bought two P17s from Pyramyd. Seems the manufacturer has dealt with the sharp intake hole problem. Mine is shooting the same velocities as yours with the new seals. I have about 300 shots through the stock gun. The second one is for a project. I'm almost finished the first prototype of a drop-in variable piston assembly that will allow adjusting velocity to find a sweet spot for accuracy and ease of cocking. Stay tuned... (I just realized what a nasty pun that is)

_________________
Theory is when you know how something works, but it doesn't.
Practice is when something works, but you don't know why.
In my shop, Theory and Practice are combined!
Nothing works and I don't know why.

Two lathes and too many projects

Good note about the O-ring squishing into the hole, which you can see when the assy is out of the gun. I've wondered how many people ran their finger over it and called it good w/o realizing the rubber squirts pretty far down into that hole. People also need to think about the rubber going in and out of the hole under force which no doubt stresses it so I'd also be sure the hole has a nice radius.
Since the hole is so small I rolled up 320 and 600 sandpaper into a toothpick sized roll and used that like a mini rat tail file to get in there. Like for yours I would do that if you only sanded the exterior edge of the hole. It may be fine but it would bug me if I wasn't sure the scratches from the bits were smooth.
Have you tinkered with it other than that? I have a partial making of a guide for the P17 if you want it. Better than nothing and might have something of value.

Ricksplace: looking fwd to the adj piston! I have a pix of one I found on the net where the shaft is threaded into the body, which by the looks of it seems factory... I tried to unscrew mine to see but stopped when it was going to cosmetically damage the alum. I guess I need to break down and use a vice and vice grips but I'd hate to do that only to find out it's really one solid piece. Do you know????

Chevota....look closer...factory piston is one piece.

Al

I'm pretty sure it's once piece too, but two things give the impression it "might" be threaded. One is the pix below looks like someone simply took one apart and took the pix. If someone went thru all that trouble to machine it I'd assume it would be all shiny new.
The other reason is on mine there's a seam or crack where the two pieces would meet so it gives the impression it's two pieces.
Probably just a fluke but it's hard to imagine how that seam/crack got there. Why would they bother with the extra machine work, but it sure makes me wonder...
It wasn't worth the risk of damage as mentioned so I just added .020" plastic to the face to get max pressure/volume so all's good.
I also buttoned it, which for those who know me probably isn't much of a surprise