Modified Piston for a Beeman P17

Feeling under the weather and didn't take pics as I worked this project along. Sorry everybody. Nick thought you'd all follow along just fine so I'll go with what I've got.

If you've read all the P17 posts here, you may remember that I modified a P17 piston with a second o-ring to fix the durability issues that the gun is known for.

At the time, I also wondered if there were any gains to be had by making the piston adjustable in length. Could the piston be adjusted to compress more air into the valve?

I'd already finished the project when I took these pics. Up top is a piece 1/2" diameter water hardening drill rod. I cut a 4.1" piece off and turned the ends flat on the Taig lathe. Next, I reduced the diameter of one end of the rod to 0.472"over 2.590". The 0.472" diameter is necessary to fit snugly in the channel on the P17's lower grip frame. Drilled a hole 3.2mm in diameter in the reduced end.

Cut 0.900" of threads on the other end 1/2" x 13 tpi. Drilled the cross pin hole, threaded for the M4 set screw and called it good. Absolute length measurement was not at all critical, but for the record, it's 4.052" OAL.

Here's a pic of the piston I started this project with.

Cut the head of the piston off the original one-piece affair. Made it 1.825" long. Length not critical. Drilled a hole part way through the piston (0.940" deep actually) and tapped a matching 1/2" x 13 female thread.

Did a couple secondary machining steps to the piston head. Spotted, then drilled 6 holes around the circumference of the piston. These holes are accessible at the bottom of the compression tube after the piston is installed. They allow a short tommy bar to be inserted into a hole and the piston head threaded up or down the steel rod changing the air volume. The grooves in the piston head are just for visual appeal. Though, in theory, the ones on the outer body may help retain lubrication. In theory...

Used a chemical etching pen and marked some o-ring info on the body for future reference. I believe that the o-rings are size #116's.

The original piston was 5.070" OAL. This one is adjustable from 5.025" to about 5.250". Way more than necessary.



These nylon ears necessitated the diameter reduction. I went for a snug fit.

Gun open for loading allows piston adjustment by using a short tommy bar. It's possible to thread the piston in so far that the gun won't close. But, at some point close to that, the piston will not retract far enough for the front o-ring to clear the air intake hole. I was hoping to show some gains of 20 or 30 feet per second. Sorry, it's not gonna happen. If I back the piston head OUT, the velocity decreases into the low 300 fps range--and the gun cocks like a dream. With the piston screwed in as far as possible based on the air intake hole, the gun is just breaking 400 fps with Beeman Silver Bear pellets. It IS possible to rap air into the valve by very sharply trying to close the top cover--then quickly re-open fully and close normally. (Watch your fingers) The velocity jumped past 450 fps on several shots--enough to say it works. Unfortunately, the amount of force required to close the top cover was clearly putting the materials and design to their limit. Looks like this simply is what it is--an inexpensive 400 fps gun.

Derrick's Beeman P17 Overhaul Finale

Back to the overhaul. Time to remove the compression chamber and the valve assembly. Using a 2mm allen wrench, loosen the setscrew bearing against the cross pin anchoring the chamber to the lower grip frame shown below.

It's unnecessary to remove the setscrew. About one full turn out is sufficient.

With the setscrew loose, drive the cross pin completely out with a drift.

The compression chamber lifts right out. At the bottom of the chamber there are 3 steel "fingers" held in slots by a small cross pin. Punch out the small pin and pull the "fingers" out of their slots. Note that the center "finger" is different at the base. The small tab at the bottom guides the sear through its travel pulling the hammer. It's not a bad idea to polish the bottom guiding edge of that center finger.

Here's the compression tube. The breech o-ring can be seen near the top, left side. The valve stem is sticking out on the top right. It's a pull open valve. The hammer pulls the stem downward releasing the trapped air through the transfer port at the breech.

A 13mm wrench removes the brass nut to access the valve.

Oops. Pulled the top o-ring off already. I'm ahead of myself. The o-ring fits on the end of the valve stem in the groove. It's a #006.

I took that #006 o-ring off so I could carefully hold that end of the valve in a padded vise. A 7mm wrench removes the black nut that the hammer pulls against. The stem is peened slightly so the nut won't come off in use. Removing the nut also removes the slight peen and allows for complete disassembly.

I polished the entire valve stem on the buffer and carefully cleaned off the buffing compound. I also found a higher quality replacement hnbr o-ring (the green colored o-ring) to seal the valve circumference. Note: this o-ring is the same size as the breech o-ring.

Buzzy's Slick Honey is a superb o-ring lube--especially for sliding fit parts that still need to air seal.

The threadlocker will replace the peening I removed.

Thought I'd also teflon tape the valve nut. You know...just in case.

The PTFE tape was too wide. I cut it in half lengthwise and gave it three full wraps.

Got all liberal with the Slick Honey. Seriously, this stuff is almost impossible to wipe off. It is incredibly thin and lasts dang near forever. It was first marketed years ago by a world renown suspension tuner, Arlo Englund, as a shock bushing grease. It was first called Englund Slick Honey. Be sure to at least lube the o-rings.

Here's the reassembled valve. I didn't have a hnbr o-ring small enough for the valve seat seal. WHAT? I know. It seems impossible to me, too.

She goes back in like this.

Holding the stem stationary while threading the brass nut back in will prevent damaging that #006 o-ring.

A dental pick lifts out the breech o-ring. If better sealing against the breech was required, this is where a small super thin shim would be placed, then the o-ring reseated.

Ah yes. My green 0-friend.

Did I mention that the black nut was installed with a thread locking agent? I don't want the nut to loosen at all in use, as the hammer pulls the black nut straight down to fire the gun.

Time to reassemble. Almost as an afterthought, I decided to quickly polish the main pivot pin for the upper assembly.

Polished out. This was the pin with the small e-clips from part one.
Please refer back to the earlier post's pictures for reassembly. Essentially it goes back together as follows: First, reinstall the barrel into the top cover. The compression tube was then pinned back in place in the lower grip frame. Don't install the piston yet.

Let's put this mess back together first.

This is actually very straightforward. About 11 individual parts, but only 3 assemblies. The hammer, sear and trigger group.

Here's the hammer assembly. I lightly greased all the parts with the Buzzy's Slick Honey, then assembled the parts onto the pivot pin.

Like this.

Then, put the hammer assembly onto the valve stem and rotate it forward on the stem and into place, pressing the pin into the hole. Notice, I did not attach the small coil spring onto the lever just yet. Then, I reinstalled the torsion spring into the bottom of the hammer assembly and set the center of the spring loop onto the post molded into the grip.

Don't seat the lower end of the torsion spring. Leave the tail end of the spring hanging out the rear of the grip as shown above. Now the sear assembly can be slid into location.

Here's the sear with it's small torsion spring oriented correctly. Not shown is the pivot pin. As an aside, I should mention that in my gun--and in buddy Jason's--the hammer and sear were about as hard as glass.

I've had best luck installing the pin just far enough to go through both the spring and the sear and then sliding the sear forward through the hammer assembly. Push the sear's pivot pin into it's hole. Don't tension the sear's torsion spring.
Now is a fine time to install the trigger and trigger axis pin. No pics necessary.

Using a small pair of needle-nosed pliers, reattach the small coil spring to the hole in the lever (part of the hammer assembly) and hook the bottom loop of the spring onto the post directly below. The coil spring is directly behind the trigger if you look at the above picture.

Here's another view of that coil spring. Finally, using the needle-nose pliers, tension both the sear and the hammer torsion springs by seating their ends into the grip frame. They bear directly on the inside plastic of the grip housing. Replace the right side grip panel--taking care to align all the pivot pins, and snug down the single grip bolt with a 2.5mm allen wrench.

Pin the top cover assembly (barrel housing) back together with the long cross pin, snapping the e-clip into place. Finally, it's time to install the piston. Is the piston's #116 o-ring in perfect condition? If not, replace it and lightly lube the o-ring so it forms a positive air seal. MOST of the P17 problems will be traced back to this single o-ring. Slide the piston into the compression tube, tap the pin back in and snug down the 2mm set screw.

With this setscrew, it's done.

Shooting the gun across the chrony with a wide variety of pellets gave the following numbers:

CROSMAN PREMIER WADCUTTER ---VERY CONSISTENT
376.9, 376.1, 377.3, 379.8, 380.8, 380.6, 379.5, 379.2, 380.5, 381.5

GAMO MATCH WADCUTTERS
374.5, 383.1, 378.2, 384.5, 381.0, 385.1, 381.8, 387.5, 386.1, 387.0

BEEMAN SILVER BEAR HOLLOWPOINT ---FAST!
399.6, 399.4, 401.1, 405.3, 402.5, 403.9, 406.8, 401.3, 406.2, 411.6

And just for fun some weird ones,
WEBLEY FLYING SCOT
388.2, 369.0, 388.6, 382.8, 376.5, 380.8, 393.1, 374.5, 378.7, 384.8

BSA PYLARM
366.5, 373.8, 380.3, 385.1, 381.2, 379.6, 369.6, 392.1, 383.7, 371.7

A strange fact: While Beeman has sold this gun now for several years under 3 different model names, it's never been listed on their website.

Updated note:
I've gotten many questions about P17's dumping their air on the closing stroke. Mine started doing this the other day and I immediately assumed it was the air valve not seating. A rebuild of the air valve showed nothing was wrong and the problem continued. Close examination revealed that the sear wasn't catching the hammer consistently. Sometimes it worked fine, then--whoosh. Frustrating. It appears that the problem was the v-shaped hammer spring binding inside the grip frame. I'd put the rear end of the spring into the left grip panel on reassembly and that's not the way to do it. The tail end of the spring must be seated into the RIGHT grip panel or the spring will bind.

Derrick's Short List for the Beeman P17

Part of me wants to run this segment for the P17 first, but I suppose it doesn't really matter.
Please keep in mind that the Beeman P17 has also been sold as the Beeman 2006 and the Marksman 2004. Currently, it goes by the Beeman P17 moniker. Beeman must still be moving through old packaging, as I just saw a gun marked "P17" in a Beeman 2006 clamshell package. They are all essentially the exact same gun and as far as I have been able to ascertain, all the parts are completely interchangeable.

So, if you aren't yet moved to fully disassemble your own P17, here are a few easy tasks that solve most of the gun's annoying little problems. Note: It is unnecessary to remove the entire barrel shroud assembly for the following modifications.

The rear sight screw has a wonderful tendency to back out and cause your point of impact to gradually climb higher and higher. Fun! The simple fix is a slightly longer M4 x .7mm bolt and a corresponding nylock nut.

Nut goes on the inside. I coated the threads of the bolt with a medium strength loctite and allowed it to dry before installation. Care should be exercised when using thread locking products around plastics--like most of this gun. Many plastics don't care for anaerobic thread locking compounds and will be permanently damaged if they come into contact. Be safe and let the compound dry completely on the bolt threads before assembly as a precaution.

Another common problem with the P17 is the tendency for the top cover's rearmost cross pin to wander out of location. That would be the long pin at the far right. A 4-40 tap and a couple 4-40 setscrews will hold it fast. The "latch that looks like a hammer" holds onto this pin when the top cover is closed.

Push the cross pin out with a drift and run the tap through the holes that appear to be made for this exact purpose. Seems like the manufacturer just knew that pin wouldn't stay in place by itself. And it doesn't!

I'm not even sure I needed a tap handle for this. (maybe not even the tap!) The plastic is that soft. While I had the pin out, I also took a couple seconds and polished it--paying attention only to the center latching area.

Started the tiny 4-40 setscrews with a .05" allen wrench, reinstalled the cross pin and GENTLY snugged the screws down.

It's imperative that the setscrews be flush or below flush with the bottom edge.

I did the same procedure with the bottom half of the frame, too. The "hammer that functions only as a latch" also sits on a cross pin. I did NOT remove this pin as I didn't want to fight the latch spring to reinstall. I did start with the tap and ran the setscrews home with the allen wrench. The plastic is actually so soft, I bet I could carefully run the setscrews in without even tapping the frame first. Again, the holes were already molded into the frame. Lucky! And again, the setscrews need to be flush or below.

This small strap, affixed with 2 phillips head screws, holds the barrel in place.

Removing the strap allows the barrel to be slid backward about 1/8" and then straight down and out.

I re-cut the crown on the Taig lathe. It's important to not shorten the muzzle more than a hundredth of an inch or so. This step can be omitted if you don't have access to a lathe.

Next, I polished the breech end of the barrel. This end gets slid across the breech o-ring at the end of the stroke every time the gun is cocked. A smooth surface here will minimize wear on that breech o-ring.

The goal here is simply a nice, smooth finish. Excess metal removal will make the breech effectively too short to reach the o-ring, causing some velocity loss.

This is all we're after.

While the barrel is still out, it's cleaning time. It's unlikely I'll ever remove the barrel again, so the gloves are off. I used J-B Non-Embedding bore cleaner and made a dozen passes. Cleaned it until the patches came clean then finished it off with a good polarizing oil to protect the rifling. Even if you don't want to give the barrel a top notch cleaning, remember to remove the buffing compound from the breech polishing by patching the barrel several times.

I'll get back to the complete overhaul next time.