Shoehorning a Cothran Powerhouse valve into a pp800 .25

[Long_Gun_Dallas]

Even at 30fpe, my 25 caliber pp800 feels under-powered. I sent it off to Don Cothran to see if he'd fit a valve to in February, and just got it back today. He did not get a chance to really look at it for a large number of reasons I won't get into (personal matters for him). And I had it shipped back with a new hpa 2240 powerhouse valve not installed. I knew the tube diameter was VERY close to that of a 2240. The valve does slip into the tube and is very snug, but no binding. I need a slightly smaller oring. And a laundry list of other doable modifications that I'll be executing by hand. This should be interesting.

 

[SqrlHntr]

Following. Im not familiar with that pistol. Where are the valve retention screws? Curios how its held in place.

 

[Long_Gun_Dallas]

Originally it has 2. One for the trigger guard, and another right behind it that just secures it to the tube. The new valve will have 3. One for the trigger guard, and 2 on the left and right side of the tube, so new holes will have to be made.

 

[ChrisS]

Looks a lot like all the Dianna Co2/HPA pistols. Also very similar to the 2240/50/60/Discovery stuff. Should be relatively straight forward. Drilling the anchor bolts will be the hardest part, But, take your time, measure twice and you'll do fine.
Which valve did Don send you? The .180 or .250?
Look up any number of the million 2240 PCP conversion threads out there and you'll be on target.
Should be quite potent when you get it done, for a shot or two. May want to tether it. Looks like fun.
Chris

 

[Long_Gun_Dallas]

Yep, my drill bits are not up to the task, lol. I got one hole in there, working it up to the right size and position slowly and patiently. Also shortened the valve, but have not shortened the valve pin yet. Looks to be a .18 Which is fine. I have to do a lot of work to the port, and I am restricted to .20", anyhow (max I could widen factory port to)

As far as shot count goes... I'll honestly be happy with 2 shots, haha.

 

[Long_Gun_Dallas]

Alright, so I waited till my carbide drill bit set showed up. The steel is very hard to work with otherwise. I did get one hole for the side pinning screws made and centered, but not nearly large enough. Got the valve body cut back to the proper length for the valve body stops stamped into the tube. Valve pin still needs to be shortened about 5-6mm. Made and deburred the 2 pin holes on each side. The bottom screw hole is very very close to the position it should be. I'll tap it for the factory thread pitch. Luckily the hole is not too large to do this. Elongated the bottom screw hole in the tube maybe 2-3mm. So the tube is going to mount to the grip/trigger guard every so slightly forward from normal, but should not be an issue. I might 3d print an end cap to take up the extra space inside the wood grip. The transfer port obviously needs work. I ordered a square 10mm boring bit. The hole in the tube for the tranfer port is 10mm. So I ordered some 10mm OD by 6mm ID ptfe tube on the slow boat from china. I'll use that instead of the original TP as it'll be far easier to work with, and I can open the ports more. (was restricted to 5.1mm prior)

 

[SteveV]

I would make the hole in the tube just slightly larger than the head size of the retention screws. And I'm sure you will before it's over, just thought I'd mention it anyway -

 

[Long_Gun_Dallas]

Hm, yep, that makes sense. I was looking down into the valve and looked like the screw holes had a lot of thread left over, like the screws weren't long enough... (dead space in the valve port, which you don't want). I'll give it a go. Thanks!

 

[Long_Gun_Dallas]

Well, she is together. My battery pack for my chrono light bars is missing, so I ordered what I believe will be a functional hard wired adapter... I tried getting a reading in sunlight, but just couldn't get it. I'm guessing somewhere around or over 850fps. From 180 bar, one shot drops it to 140 bar. Very loud, and substantial kick for an airgun. I need to tear it down again and take photos of the work I did. I'm very bad about not doing this in the process as I'm so focused on the work at hand.

End work needed was:

Trim rear of valve body to seat onto the stamped tube notches for original valve at correct(ish) location.

Trim rear of valve stem to allow more hammer and spring travel but still allow for full valve lift.

Increase depth of valve body to tube oring seat as tube is slightly smaller ID than what valve is intended for.

Slightly elongate toward the rear... bottom bolt hole in tube for original trigger guard to valve body retaining bolt.

Bore side holes in tube for new retaining bolts.

Bore new valve body port to 10mm just short of the original transfer port seat to accommodate 10x6mm ptfe transfer port. (Tube is bored 10mm here, as is the breech for original brass transfer port)

FFashion transfer port

Smooth port transitions

Gun has previous port work that I'll go into later that is still in use.

 

[Long_Gun_Dallas]

So, I did get a few numbers. 38fpe with short barrel (pictured) and 54fpe with 15" barrel. This was only at 180bar. Need to fill my scba tank, and tune.

 

[bluemoon_wulf]

Guess you are much more patient than I am.
Since I wasn't impressed with the accuracy of mine, I gave up getting past the 20fpe and sold it.
I am much more happy with my evanix ar6. Course, still can wish for more power....

 

[Long_Gun_Dallas]

Well, I tried benjamin pellets, hades, and kings, and the kings are lights out in this for whatever reason. The other two, not so much. Extremely impressed with the accuracy, and the kicker is that I don't get any drift with the kings with even large power fluctuation. Only drop or rise depending on power output. Which is very easy to predict or compensate for. Most any airgun I have sees some type of drift if power level changes much.

This used to be a ~8 shot gun from 24-30fpe on either side of the bellcurve. It will now only be a 1 or 2 shot gun. Maybe even 3, but power will likely be from 42-33 ish fpe for 3 shots with short barrel.

If I could not have gotten any accuracy from it, it would be of no use to me. But the 15" barrel behaves the same as the 9" accuracy-wise. Though I have not tried anything other than the kings or AA equivelant (both do extremely well)

Almost 1000fps with this is pretty nuts, but even with a huma mod40 4.5 it's quite loud. The 2.5 config below is VERY loud.

 

[Long_Gun_Dallas]

I opened up the ports today on this gun a smidge more. Very close to 6mm now instead of 5.1mm. I'll try and charge up my fill tank within the next day and get some chrony numbers at higher pressures. I need to re-work the port on the long barrel. But that's a project for another day.

Here's some photos of some of the work I did.

Just keep in mind all of this was done by hand. With pretty pasic tools like dremel, power drill, angle grinder, calipers, ect.

Couple notes of interest... The rear of the valve body is pictured first. You can see it was shortened, as well as the valve stem with it. Valve is still capable of full lift. It was bored 10mm to accept the new transfer port I made from 10x6mm ptfe tube, you can see a tiny remnant of the original transfer port seat. Port orientation was a bit forward of where it is now, as seen in some photos above as well.

The next photo is the forward portion of the valve body and the oring seat that seals it to the air tube. I had to cut the oring seat just a little bit deeper since my tube ID is slightly smaller than the 22xx tube this was intended for.

Next is the 9" barrel, and opened port. You'll see the port went thru one of the original barrel oring grooves. I had to cut another oring groove by hand rear of the original.

Last is the ptfe transfer port I fashioned.

 

[Long_Gun_Dallas]

180 bar with 5.1mm ports

 

[Long_Gun_Dallas]

881 fps now, 44fpe with 9" barrel and 6mm ports. Just a hair over 200 bar. I put more in my fill tank, but I think the cold temp now caused pressure drop.

 

[Long_Gun_Dallas]

48.25fpe @230 bar with 9" barrel, 925fps, jsb kings. Used 55bar for one shot