by Tom Gaylord
Writing as B.B. Pelletier

Today’s report is a guest blog from reader Streetmusician. As far as I can remember he has never signed into the blog under that name, but that is his handle. He tells us how he got more velocity out of his Beeman P17.

If you’d like to write a guest post for this blog, please email me at [email protected].

This report is an important part of the Beeman P17 series, so I am linking it to the other reports we have done on the pistol. The first two parts tell you how to reseal the gun, so if today’s report becomes a project you want to do, you now have the rest of it.

Part 1
Part 2
Part 3
Part 4
Part 5

Over to you, Streetmusician.

Beeman P17 valve modification

By Streetmusician

This report covers:

What others have tried
The original valve
The eureka moment!
The test
Assemble the valve
Assemble the pistol
Did it work?
For DIYers

Just recently I had occasion to renew all the seals on my Beeman P17 pistol, I have done some basic modifications to mine over the years, and, to get more power, I tried the method of putting a shim on the end of the piston.

What others have tried

I have seen blogs where guys have epoxied a gossamer-thin plastic shim on the end of the piston to gain 10-20 thousandths of an inch in length. There isn’t very much room in there. The space ahead of the piston when it’s fully forward measures 0.70 mm (0.0276-inches).

Filling that space increases the pistol’s compression. Anything much more than 20 thousandths and the piston bottoms out on the end of the chamber, preventing the gun from being fully cocked. The top strap will not fully close to lock and thus be in the firing position. In other words, the piston bottoms out before the gun closes.

In my experience shims break away after a brief period of time. And, if you use an “O” ring, it becomes shredded. Also, compressed air and grease are present in this tiny space, so anything you add does not seem to last very long.

To get a little extra power from the gun, the air needs to be compressed a little more. Anything that displaces the volume of air in the piston’s firing chamber, which is inside the firing valve, will do this. So — what if I shim inside the valve instead? It’s not going to be in the way of anything, like things put into the piston chamber are. I’ll show you.

The original valve

Looking at the original valve I wondered what I could use. It has to fit inside the pistol and not impede the operation of the valve.

P17 valve
The original Beeman P17 valve assembly.

Looking at this valve, I started thinking out of the box. Being an engineer in a past life, I had all sorts of bits and pieces lying around in the bottom of my toolboxes. I came across one — of all things a universal thermocouple for a gas boiler!

P17 thermocoupler
This thermocouple is universal to fit many gas boilers.

The eureka moment!

Hmm, I had a bit of a wonder if…. Ah HA! — a  eureka moment! Now, what does a thing that senses a gas boiler flame have to do with a Beeman P17 air pistol? I just wanted the brass sleeve that comes with the kit. Just out of interest, Universal thermocouples are £ 3.60p to buy. [Ed. I looked them up for us here in the States and a Honywell universal thermocouple for the home is $7.89.]

P17 brass sleeve
Universal thermocouple brass sleeve.

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The test

Now to put my theory to the test. Taking the valve apart (which I had to do anyway to replace the seals), I slid the brass sleeve over the spring. Plenty of clearance there. Next I took just the brass sleeve and pushed it into the valve port in the gun. Once again it was a loose clearance fit. Hmmm — “Made to measure,” as my dear departed father used to say.

Now, the original brass sleeve I salvaged from the thermocouple is too long, so I took the original pistol valve and compressed the spring until it fully bottomed all the spring coils. I reasoned that the valve stem cannot open any more than this. It’s at its physical limit. The space from the top of the original “O” ring brass spacer to the underside of the top flange of the valve stem measured 9.0 mm. To leave a little clearance I cut and filed the brass sleeve from the thermocouple to a length of 8.5 mm. I’m sure with closer attention to detail and with the right engineering tools this compression spacer could be made even better, but I’m just doing it as a quick fix.

P17 valve with brass sleeve
There is the modified brass sleeve, next to the valve stem and spring in the disassembled P17 valve assembly.

Assemble the valve

Now, it is a straightforward process of assembling the P17 valve, with the new brass spacer in place.

P17 valve with spacer
Here is the P17 valve assembled, with the new spacer in place.

What is interesting to note, although it may not be too clear in the picture above (my apologies), is that the additional sleeve just rests against the top of the small brass “O” ring spacer and does not interfere with the spring’s function. Also, the length of the additional sleeve does not interfere with the valve fully opening when the trigger is pulled. And the clearance space between the inside of the valve transfer port and the outside of the brass spacer allows compressed air to flow directly to the tip of the valve when it opens at firing. If anything, the compressed airflow is smoother.

P17 valve comparison
This shows a comparison of the P17 valve before (top) and after modification.

Assemble the pistol

Now it’s time to assemble the pistol. The new modified valve goes in exactly where the original valve came out.

P17 assembly
The modified valve fits back in the valve chamber exactly how the original valve came out.

Did it work?

After putting the pistol back together, it was time to put my theory to the test. This pistol had been shooting at about 400 f.p.s. in original trim. Now, with the same pellet, it’s averaging about 460 f.p.s. The compressed air force in the pistol is higher. Also, the swept airflow seems to be far better — leaving the valve more efficiently when the gun is fired.

The cocking effort does increase in the last inch or two of compression. So, be prepared for that change.

For DIYers

What if the same thermocouples I used are not available? Well, the ID of the sleeve I used measures 6.35 mm (0.250-inches) and the OD measures 7.72 mm (0.303-inches). So, get a piece of tubing like that off the shelf. 

Enjoy! Streetmusician.