1750 PCP Target Pistol

[sholo]

Just starting the process of turning the wife's Co2 powered 2250B (now .177 cal) into a 1750 PCP target pistol. I was originally going to get a Hi-Pac (and still might if this idea doesn't pan out), but instead picked up a Disco valve, fill adapter and gauge from Eric.

I will get into the specifics in a later post, but for now I have pretty much everything figured out for the build except for one thing - gauge placement. I want a gauge on it to monitor tube pressure so I'll know when to refill BEFORE accuracy heads south, but not sure what route to take...

1) With such a small reservoir for air, using a Disco gauge block is out of the question.

2) I could take the male Foster off the adapter, install a street "T" and then re-install Foster as well as gauge (Foster straight out, gauge pointing down). This would be the easiest way, but I would like a cleaner look (i.e - not look like a plumbing job).

3) After looking at the fill adapter (or more precisely - the space between threads and o-rings) and gauge for awhile, I came up with this idea but wanted to ask you braniac's first to see if it is even feasible before proceeding:
m_Disco fill adapter&gauge 004.jpg

Would this be possible...or even safe?

Enough meat on adapter to thread and safely secure gauge with 2000 psi (max) fill?

I took a few measurements and found that if the outermost portion of threads on gauge were .030" from o-ring groove, then opposite side of threaded hole would just BARELY touch the first thread on adapter. With a 3/8" hole in tube to accept gauge, it would leave .53" of tube material between end of tube and start of hole. Acceptable or no?

Thanks for any insight you can offer, and as always, I'm open to suggestions!


Cheers,
Todd

[AirGunEric]

I'm thinking it might be safer to move the o-rings back to where you have the gauge sitting, and then thread where the o-rings are currently to hold the gauge (?)

[rsterne]

You have to have the gauge outboard of the O-rings as you have shown.... if it were inboard, the air would leak out the hole around the gauge.... Whether it was safe or not would depend on the thickness of the material in the fill fitting.... ie how many threads you can get.... You only need one O-ring to seal, so don't be afraid of destroying the outer O-ring groove.... I think it should work....

Bob

[SeanMP]

Yup that should work just fine as long as there is enough wall in the fill adapter for a full strength thread. As Bob says its only the first oring that's doing any work. The second one doesn't even see pressure until the first one fails.

[sholo]

Well, I guess I will be the guinea pig for this one!

Worst case scenario, I ruin a pair of underwear when gauge blows out and end up with a pooched fill adapter....I've had worse things happen


Todd

[rsterne]

When I'm making gauge adapters for aluminum for a 22XX, I use a 3/8" center hole in the 3/4" OD insert and that's plenty of threads for a 1/8" NPT gauge fitting, provided it comes through just flush with the inside of the hole.... If your fill fitting has larger than a 3/8" hole where you will be tapping it, measure it and post here and we'll give it another think....

Bob

[sholo]

Damn, missed your last post, Bob. Now I have it all together...

Well...it's not the prettiest job ever, but it works!

I was able to stay away from the o-ring groove AND not mess up the threads on fill adapter as well. It's a tight fit, but doable. Once I had hole drilled, I was a bit surprised at how much meat there actually was on fill adapter - 5 full threads (minimum):
m_1750 PCP 002.jpgm_1750 PCP 004.jpg

Sufficient enough? LOL...Hope so, seeing as it is assembled and pressurized sitting on my bench...

De-burred the rough edges on both tube and adapter, did some touch up bluing on tube once all the shrapnel was cleaned out/washed down, then lubed the o-rings and put them in place and reassembled gun. Pumped it up to 2000 psi and found that 2000 psi on Hill pump equals about 1850-1900 psi (+/-) on Disco gauge. To be on the safe side I will go by the gauge on pump as it registers the same as my glycerin-filled 3500 psi gauge I use to check regulator output pressures.

Still have some trimming to do on the front of the forearm to even up the gap some and smooth it out, but so far the gauge hasn't moved in 4hrs....hopefully it reads the same in the morning:
m_1750 PCP 006.jpg

I haven't bothered putting sights on yet (most likely a scope) until I'm done playing with TP's and hammer spring combo's in order to keep it sub-500 fps. Currently has a 14" barrel which IMO makes it a bit long for a pistol, but may end up getting a 10.1" LW to replace it. We'll see...
m_1750 PCP 009.jpgm_1750 PCP 007.jpg

If it hasn't lost pressure by morning, I will start playing around and tuning it.


Todd

[rsterne]

Very nice job!.... How much air volume do you think you have?.... It seems pretty efficient, a lot of shots from a small volume.... I do have a measurement on a 2250 tube with a Disco valve and fill fitting (nothing extra or drilled out) and I have 24 cc.... Adding 2.5 cc for the valve volume give a total of 1.62 CI.... If that is the case for your gun, and you dropped 1350 psi (93 bar) you used a total of 1.62 x 93= 151 CI of air.... Using 500 fps average (4.2 FPE ) for 60 shots that is 252 FPE total, which works out to 252/151 = 1.67 FPE/CI... Like I said very efficient.... Using that 0.050" Challenger transfer port was genius....

PS, you can add a tiny amount of volume by drilling the front of the Disco valve out to 3/8" ID almost down to the spring seat.... You can also drill the end where the O-rings are to 1/2" OD stopping at least 1/16" short of the shoulder where the groove for the trigger screw is.... If you want even a bit more volume, cut a couple of threads off the front of the valve inside.... Every little bit of volume you gain will mean either more shots or a narrower velocity spread over the same number of shots....

Bob

[rsterne]

Lots of thread, no problem there....

Bob

[rsterne]

Amazing what efficiency you can get with minimum dwell and a tiny transfer port.... I've hit 1.93 FPE/CI so far, and I'm sure over 2.0 is in reach....

Bob