Thursday, October 30, 2008

Derrick's P17 Pistol

Derrick had a problem with his P17 and text by our hard working Ohio correspondent.

I was going to shoot my identity confused Beeman 2006 and/or P17 and/or Marksman 2004 this afternoon. Sadly, the single-stroke part of the single-stroke pneumatic mechanism refused to cooperate and actually seal. From past experience, I knew that it was likely that the piston o-ring was either dry or had lost it's elasticity.
How many names has this gun been sold under by Beeman now? I pulled the piston and found that, surprise, it was the main o-ring. A replacement R-14 o-ring had the gun running again in about 5 minutes. What bothered me the most, was that I had replaced this ring less than 6 months ago. If the gun saw more regular use, the seal would probably last longer. This seems to be a common problem with this gun. Too bad, because it's otherwise a decent design for $50.

It didn't seem like a huge leap to imagine that in this application, the single o-ring seal is tenuous at best. Maybe adding a second piston o-ring as a sort of "back up" seal would fix the gun more permanently. I pulled the piston again and chucked it up into the Taig lathe in a 3-jaw chuck. Not good. The piece had miles of runout and was nowhere near straight enough to make an accurate cut. I switched to the 4-jaw chuck and got out a dial indicator. It took about 5 minutes of positioning each jaw individually while watching the indicator slowly settle down until I was satisfied. I only checked the runout at the location I wanted to make the groove.

Having to make the cut so far from the jaws, I was concerned about the piece deflecting. I removed the cross slide and installed a steady rest.

Checked the runout one last time.

Funny, it doesn't look blurry in real life. Matched the same 0.095" width groove. Didn't bother to measure the depth. I just used a comparison caliper and made it the same.

So here's the final. If this doesn't seal now, I think there's enough room for about 11 more o-rings...After cocking, the gun held for 45 minutes w/no pressure loss. Never did that before. I'll let you know next year how it's holding up. By the way, if you remove the "Beeman" sticker on the gun, underneath it says "Marksman". Unfortunately, they sanded the "Marksman" imprint down so the sticker would lay flat. So I glued the sticker back on. Too bad. I wanted to be a Marksman.


Anonymous said...

That's a good fix, however perhaps a better but mor laborious one would have been to install a parachute seal as found on most HW springers and the single stroke pneumatic Diana 100.
I've got the latter and it holds air for more than 2 weeks no problem. Mind you the piston seal for the Diana is expensive, but very easy to install on your piston IF the diameter is right.

jh said...

Thanks a lot about the P17 information. It's helped.

Anonymous said...

Thanks for this, just bought a P17 and after a few dozen shots to get first impressions when straight to my little TAIG and cut that second O-ring groove. Next time I'm out on errands I'll hunt down a couple of quality rings to fit. Cleaned out a fair bit of factory grit while in there, so well worth pulling the piston assembly and lubing properly.

KR said...

Thanks for the great information. I've had my P17 for 1 1/2 years. Put over 1000 rounds thru it with no problems. Started to have air leak problem now. Followed your instructions and found the small oring at the end of the valve cracked and shreaded. Trip to the hardware and $1.50 in orings later the gun shoots like new. This is the most accurate air pistol I have ever shot. Great trigger. Use mine alot when the Cicadas come out in late summer. Thanks again!

Gerard Samija said...

After some more poking around with my P17 (I posted as 'Anonymous' on August 16th, regarding machining a second O-ring groove in the piston) I've found another small modification which might help. The tiny air intake hole below the main compression cylinder has very crisp edges, effectively knife-sharp 90 degrees. Perfect for wearing, even cutting, the O-ring edge over time, or rather quickly as I found when first adding a second ring and discovering I needed to go just a hair deeper in rounding the base of the piston groove.

Since it's rather difficult to get a countersink into such a small cylinder, I used a 2mm diamond ball mill in a Dremel. Just a very gentle contact, a second of light polishing then check by eye and finger, then another light touch or two... Came out feeling a lot less sharp to the fingertip, and the next O-ring I put in didn't get a bite taken out of it. Probably lost about 0.25mm of compressive volume in the cylinder but that doesn't seem to be a big deal.

The added O-ring makes for a slightly more solid feel on cocking the pistol. I'm guessing it can only help with consistency in discharge pressure through various cock-to-fire intervals.

Gerard Samija said...

Oh yes, and on the subject of the anonymous posting of Hallowe'en 2008; if one were to install a piston seal in this pistol it'd be necessary to first carefully measure the location of the air intake hole in the outer cylinder. Then you'd have to trim the piston length down to accommodate that position and the seal edge location so as to allow air intake at the end of the stroke.

Another option would be to fill the air intake hole, probably by welding on a spot of steel or braze-filling it and polishing very smoothly inside, then re-drilling for air intake further back on the bottom of the cylinder.

Or better yet, do both. Shorten the piston slightly to allow the piston head and new seal to travel closer to the cylinder end (careful, as too short might allow the piston to deform the cylinder due to angular cocking force), then weld shut the intake hole and drill a new one slightly closer to the end. Get this right and it might win you a slightly increased fps rating and some added cocking force to boot.

Jerry Norbury said...

Firstly - terrific article and great description. It's rare to see this level of detail and I think you did it really well.

I bought a Beeman P17 just over a year ago and the first one failed after a day - brought it back and got it replaced. The second one has now started "leaking" during the pump; air leaks out from somewhere (you can hear it).

Do you have an idea what this is?

derrick38 said...


Go to our post index and read posts 178, 179 and 180. The problem is either the piston o-ring or the valve o-ring. Hopefully it's the piston o-ring as that's faster to repair. The posts will walk you right through.

Anonymous said...

can some one please help me reassembly my trigger system one of buddys droppeed it and the springs dont line up

Anonymous said...

This is actually a great gun which can be equally as good as the HW40 version. The various numbers are all the same chinese copy of the HW40.
The seal issue is owing to a small burr thrown up on the inside cylinder side of the tiny transfer port hole which sucks in the air.
It catches on the O ring and then leaks. Simply remove the piston and take off the burr with a sharp knife or deburring tool.
You will have no more trouble. Indeed the Chinese variant shoots slightly faster as the cylinder is 0.8mm longer in the length

Anonymous said...

I have successfully tuned for more power (23 fps) by boring a step out of the end of the piston which is 18mm in diameter by 3mm deep.
Then turning up a press fit bung in aluminium which is 3.5mm long.
This gave me a 0.5mm protudance of 18mm dia. (very like half thickness 5 pence piece stuck on the end of the piston.
Result....slightly heavier cocking and 23 fps from JSB Match or 424 fps in my gun.

Whiteleather said...

I am making a pitch to my gun club to expand its Veterans Shooting Program. We currently use only Daisy/Avanti 853 rifles. Brilliant little guns!

I would like to expand to pistols for the vets. Naturally I am a fan of the HW45....but too expensive. Someone suggested these P-17s. Since I do all the maintenance on the 853s, it would fall to me to maintain the P-17s as well. Am I going to be very frustrated with quality of construction and constant repairs?

Kris Whiteleather