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Jaxom · Senior Member

After seeing all the wonderful results Bob has done with his 2260 pcp, carbine and now his .25 cal discover. I'm very close to wanting to jump in and start doing some mods myself.

But before I do, I would like to get some clarifications on a few points before I start. On that note too, if anyone has pictures of the modifications, that would be greatly appericated by myself, as I'm sure others would like to see your handy work as well.

I went back over and re-read many of bob's posts on his projects. And here are some of my concerns/questions.....

Valve port is the port where the compressed air exits the valve. From re-reading all the posts the optimal size would be .166"?

The barrel port, I assume is where the compressed air enters the barrel, which in turn forces the pellet down the barrel. Optimal size here is .172"?

Transfer port, what does it do and where is it located? I downloaded the exploded parts diagram and all I can see that's between the valve port and barrel port are parts #23 "sleeve" and #24 "breech gasket". If I can find it, the optimal size would be again .166"?

At first I didn't understand doing this modification. But after looking at the parts diagram schematic, now I do. Widening the hole that passes through the pressure gauge block to 5/16" allows more air to flow to the valve. By "off set", you need to remove material from the opposite side where the gauge is tapped in. Has anyone tried going larger? Or is 5/16" about the limit here?

Before I get to internal valve modifications. Do I need any special tools to open the valve?

Once I do have the valve open. I'm a bit confused here so bare with me. In bob's thread "22xx pcp" here's and his comments....

"While I was at it, I modified the Disco valve.... I removed 0.1" from the male threads inside the valve, and bored the rear part of the valve to 9/16" ID (which is the thread OD) from the rear of the threads all the way to the exhaust port.... I also angled the exhaust port 20 degrees and streamlined the corners and slimmed the valve stem a bit.... I drilled the front spring seat to 3/8" ID and the inside hole to 17/64" (both 1/64" larger than stock).... Then I used a tapered reamer to enlarge the forward section of the valve, tapering it all the way to the front of the spring.... Lastly, I tapered the front with a countersink.... The result is similar to the "Boss" valve.... "

Just by reading this, I have to assume a lathe was involved. I just can't see how some of this work was done any other way. Or perhaps I'm just dumb and inexperinced? If I cannot bore out the inside of the valve, is there any advantage to reaming out the forward face of the valve or drilling out the spring seat and inside hole as per this comment?

To do this I would need a "tapered reamer" and a "counter sink". What sizes would I ask for when I go to tool store?

Since it seems that I would need a lathe to do all that's talked about here, I can see by widening out the front of the valve, widening the gage block as well as opening up the the ports, I should gain some fpe here and there, at least enough to make it work going through all the effort.

It's either that or find someone sells a "boss discovery" valve for those of us that are tool/experience challenged. Which I haven't seen anywhere...yet. <hint>

I hope this all this wasn't too confusing. I just want my discovery to be all it can be and release some of the hidden power that Bob has shown us all is in there.

Jax

VENI VIDI VICI · Member

http://m.mcmaster.com/

every drill, reamer and countersink I've ever needed I got from here, order before 3 you can have it next day.

rsterne · Moderator

I'll try and answer some of your questions:

Barrel Port 0.172" (11/64") maximum.... I use 0.169"....

Transfer Port (the "sleeve") 0.166" maximum (larger and there is no wall thickness left and it will crush)....

Valve exhaust port.... I mill in on about a 20* angle with a 5/32" mill and then round the corners and finish off with a 0.161" drill....

The gauge mount in the Disco I open up to 5/16" ID with the hole offset to the opposite side to the gauge (otherwise you can't mount the gauge).... There is no point going larger IMO since the valve still feeds through a 1/4" hole.... I mill this offset hole because you can't just open it up with a drill as it is off center....

The valve simply unscrews.... If need be, use a couple of short pieces of split tubing to protect it and pliers....

To bore the inside of the valve you will need a lathe and a LOT of care.... I am now shortening the threads on the front half of the valve by 0.2" to 0.3" from the shoulder and boring the inside of the valve from that point to the valve seat to a diameter of 0.60"....

The front of the valve I drill with a series of holes (in a lathe) to approximate the taper of a tapered reamer (minus a bit) and then clean it up by hand until the hole is the regular 1/4" ID where the spring seats.... I then use a 3/4" countersink to just bevel the end....

You should still get some gain from opening up the front end of the valve and the gauge block even if you can't bore out the valve.... I just can't tell you how much....

You can get all these valve modifications by purchasing a Boss Max-Flow valve from B&A but you will have to drill and tap the outside of the valve to mount it properly (ala Disco) for safety with HPA....

All of these modifications will cost you in terms of the number of shots available, of course.... you don't get something for nothing....

Bob

Jaxom · Senior Member

Thanks for the reply bob. Also, thanks for being so patient with those of us that don't have as much experience in these matters as you do.

The more I thought about it, the more I realized there was just no way to do all that work without needing a lathe. Wanting to get as much out of my discovery as I posibly could, I then wrote a couple different companies that offer modification servies describing just what I wanted done. So far I haven't heard back from any as to whether or not they're willing to do it or what the costs might be.

That means all I would need to do on my end is doing the port work on the barrel and the trasfer/sleeve part. I think I can handle that much on my drill press. Just take my time and go slowly.

I fully do realize that I will by going full board on this I will limit my shot counts. But then again, that's why I knew this project would include an adjustable hammer spring guide. That way I can "dial in" what power setting I'll be using for the conditions or settings I'm operating the rifle at.

rsterne · Moderator

One other thing I haven't been able to put my finger on yet.... As you improve the "flow" of the valve the peak of the bell curve moves to a lower PSI.... It seems that the first part of the curve (up to the peak) becomes somewhat more "ragged" ie more shot to shot variation instead of a smooth curve.... Increasing the volume inside the valve and available to the valve (through opening up the front and the gauge block) seems to make this shot to shot variation even worse.... I think what is happening is that even a tiny variation in how the valve lifts off the seat in terms of duration and/or lift is magnified by the potential increase in air available to flow through the valve....

When viewed in the grand scheme of things, this in not important.... it's just annoying.... It also can change with a tiny adjustment of the hammer spring tension.... or by using a different hammer spring and preload even though the velocity is the same.... That's the frustrating part that I haven't figured out.... how all the varilables tie together to create (or minimize) this problem....

Another observation is that "dialing down" the power to get more shots can also increase the shot to shot variation.... Decreasing the size of the transfer port, on the other hand, seems to smooth out the shot string.... I'm sure that is the reason for the three separate adjustments on the MRod.... hammer preload.... hammer travel.... and transfer port size (the "velocity" adjustment).... Add to that the fill pressure and what pressure you shoot down to.... and there are almost an endless number of combinations depending on what you are trying to achieve....

Bob

Jaxom · Senior Member

Just so you know...

What I'm hoping to achieve with this project. We all know that the Discovery is a pretty good rifle right out of the box. But what I'm hope to do is use this for hunting small game like rabbits and squirrels. What I'm hoping to do is increase the range without any loss in accuracy, and still have enough fpe to take the game humanely.

I can also see myself actually having 3 discovery rifles. One in .177, .22 and .25. The .177 would get a mild tune with just a swap in springs and adjustable hammer spring guide. This I could use for dove, pigeion, crow. The .25cal would be for perhaps larger game like racoons or possum. I'd also like to make the .25cal a "tactical" rifle with synthetic stock and such just for yucks and being different.

rsterne · Moderator

Jaxom · Senior Member

Well, like I said, so I can reach out further. Why did you modify yours?

rsterne · Moderator

Mostly to learn what makes a PCP tick.... It's now a .25 cal.... 11 shots at 45 FPE.... ~900 fps with 25 gr. pellets.... Mr. Green

Bob

Jaxom · Senior Member

Bob,

You have no idea how jealous I am of your ablities. I haven't the training to do the wonder things you have done. If it wasn't for your extensive and detailed accounts on what you've done with your ag's I doubt I would be doing this project now. Lest I forget to mention other's too, like joe, eric and many many others. It is through you and all those others that we, that don't have the ablity or perhaps the time to tinker with our ags that we too learn how and what makes our ags tick.

Having said all that. There is another question about modifying a discovery I would like to pose. As has been repeatedly, the Discovery is a good rifle out of the box. Through efforts I've already mentioned above, we're all familar what can be done to max out the power capabilites of a Discovery. But what about making one more accurate?

Let's say for a moment you wanted to have two discovery's. One modified to max power, and a second that's even more acccurate then when it came out of the box? What would you change?

rsterne · Moderator

I haven't played around much with the accuracy.... The original .22 barrel on my Disco was never great for accuracy.... I don't know if it was the barrel or what, 'cause I've had great luck with standard 24" Crosman barrels.... but for some reason that one was never a tack driver.... I've experienced some sensitivity with the tension on the barrel band screw on various other 13XX and 22XX projects, but always got them to shoot great in the end....

I was so busy trying different variations of valve mods, hammer springs, fill pressures, etc. during the development I kept putting off worrying about the accuracy until I decided what the final version would be.... When my darling wife decided to get me .25 cal barrel for it for my last birthday I never did get around to sorting it out.... The .25 barrel basically doesn't care much what I feed it.... it's just plain deadly.... Mr. Green

The last two 22XX PCPs I have worked on both have floated barrels.... only supported by the Crosman steel breech.... They are the least fussy on pellets I have ever had.... and also have virtually no POI change when the pressure varies, which the Disco does suffer from.... I have read that the air tube on a PCP can change shape / stretch as the pressure changes and that with a tight barrel band it can change the POI.... I can believe that may well be true....

To answer your question, therefore.... If you have any accuracy problems with a Disco, I would try free floating the barrel.... or at least try loosening off the barrel band setscrew and making sure the muzzle brake is not touching the main tube....

As far as POI changes due to velocity fluctuations.... I don't think that is an issue.... If your shots are all within plus or minus 2% of the average velocity, and you are shooting at 50 yards or less, the variation is less than a pellet diameter.... for a MV of 850, BC of 0.032, and a total range from 833 to 867 fps the variation in POI at 50 yards is 0.17".... I don't know about you, but I can't shoot that well.... Rolling Eyes

Bob