Tuesday, June 16, 2009

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.


Anonymous said...

Hi, i found a much better method for myself recently. I merely turned up an exact replica of the barrel but slightly longer at the muzzle end. I did not want to spoil the look of the gun by extending out of the muzzle but ran it through the compensator void rigth to the muzzle, instead of stopping short of the compensator slots like design. It adds approx 22fps dependant on pellet (lowest of 14fps to 22fps highest) If you are concerned regarding losing the compensator feature dont, it does not work as far as i can tell with no noticeable effect on accuracy. No increased strain on mechanics too.

Anonymous said...

Actually the adjustable piston mod post has a flawed error. It does work but a mistake has been made here. If you increase the piston face of the P17 by 0.7mm or the HW40 by 0.8mm they will both hit 415fps with JSB Match. Make it slightly bigger or adjustable and you run the definate risk of bottoming out the piston before the top strap fully closes, bending the fulcrum pin at the muzzle end as you force it shut. The result of this a top housing which is then moved slightly forward on closing by the amount that the pin is bent. This obviously results in the barrel also moving foward and not sealing at the breech washer.
I suspect this is why the system felt under so much extra strain.
Its about getting it the right length.
A barrel run through the void to the maximum point of the muzzle will add 10 to 15 fps tested but the guns std barrel has extremely good rifling. I would be hesitant to change it unless similar quality barrel blank is used.

Anonymous said...

Thanks for the reply to my barrel suggestion. Im suprised so few are interested in the topic. For the record i machined my barrel from an old Webley Longbow barrel which i got for free from a visit to Webley before it closed. It had a damaged breech but once i had machined it to length and re-crowned it, the rifling was perfect. It added a minimum of 14fps and a high of 22fps from a group of pellets but accuracy completely unaffected.
Strangely, i found velocity did not get big gains when i iniitally tried it at 10 inches long. In otherwords the powerplant probably peaks at an optimum barrel length of around 9 inches or so, rather than increasing continually as the barrel length grows unlike CO2. I suspect power may even start to drop if the barrel goes much past 10 inches.

Anonymous said...

I suspect accuracy comes from the remarkable consistancy of SSP rather than barrel quality