New Old Project - .25 Disco Revisited

[rsterne]

I've learned a lot in the 3 years since I built my .25 cal Disco, in particular from bouncing ideas off of Lloyd.... With recent developments, I decided to revist that rifle and see what improvments I can make.... When I finished the gun before, I had it set up for 11 shots at 45 FPE starting from a 2100 psi fill, although the gun saw a high of 58 FPE during development, right at 2000 psi with Baracudas.... The specs were as follows:

.25 cal LW barrel & bolt from Mountain Air, Disco tube, stock hammer and breech, poly transfer port (0.166"), the hammer spring was 1.75" long by 0.045" wire, running in a homemade RVA.... The valve was bored out to 0.60" ID, the threads shortened to 0.30", the throat drilled to 0.228" and the stem thinned to 0.115" (0.197" equivalent).... The front of the valve was opened up and tapered, and the gauge port was milled off-center to 5/16" to eliminate the original 1/8" hole which restricted the valve from drawing freely on the air reservoir during the shot....

Compared to some of the guns I have done since, that is actually pretty tame.... My .25 cal Millenium Pumper I just finished had porting that was 0.204" or the equivalent as a minimum right through, which is 50% more area, plus it has a heavier hammer with a longer stroke, although the valve is otherwise quite similar.... A couple of years ago, I made a two-piece valve for a guy, but never followed up on it.... Here is what it looked like....



I did use the concept successfully on my Hayabusa, however.... It seemed appropriate to bring that back into the fold for this round of mods to the .25 Disco.... I also wanted to experiment with the new hammer I came up with while working on the Millenium Pumper.... It looks like this....



The hammer is threaded 3/8"-24 NF inside and carries an adjuster which can move back and forth from flush to the end to recessed about 1/4".... Recessing the striking surface increases the stroke (travel) of the hammer before it hits the valve stem.... which in turn increases its energy and momentum.... There is a spring guide / cocking indicator threaded and glued into the adjuster, and a slot in the end provides the means to turn the adjuster.... The threaded hole in the top is for a "locker" to add resistance so that it doesn't self adjust.... The hammer is the stock 1.300" length, but the cocking pin is moved back 0.060" to allow a longer stroke before it hits anything....



Anyway, that is the starting point, and a hint at what is to come.... This should be an interesting project....

Bob

[rsterne]

Here is the progress so far on the new parts for this project.... I decided to make a jig for drilling the valve body.... One of the hardest parts for me is getting the port and the three screw holes perfectly aligned and exactly at 90* to each other, as I don't have an index head for my lathe/milling machine.... What I did was I took a piece of 1 x 1.5" 6061-T6 aluminum and milled off one side to make a 1" square with a bump remaining for a 3/8" setscrew.... I drilled it 3/4" to accept round stock, or a Disco valve (a whisker under 3/4").... By rotating it 90* in my milling attachment I can now drill all the holes in the proper orientation.... I still have to indicate off the top of the round bar each time, but at least the results are accurate....



The valve is designed around an MRod poppet instead of the Disco one.... The head is smaller, and the stem is only 1/8" instead of 5/32" (no thinning required).... I machined a 40* angle (to the stem) on the sealing surface, and the seat in the valve is 45*, so it seals right at the throat, which at the moment is 0.250".... The exhaust port in the valve is 0.219" diameter and angled 20* for better flow.... The mounting screws are low profile 10-32 SHCSs with a 180,000 psi tensile rating, plently for use at 3000 psi....



A stock Disco valve is on the left.... You can see how restricted the flow is past the poppet, although the one in my .25 cal Disco is quite a bit better, having been bored to 0.60" ID.... My new valve is in the middle, and you can see the huge improvement in flow into the throat and around the head of the poppet.... The ID of the valve is the full OD of the valve insert, with the tube acting as the valve wall.... On the right is a PRod gauge port, showing the MUCH larger bore (11/32").... You have to use a 1/8" NPT male to female extension for the gauge instead of the original gauge mount which sealed to the Disco gauge port with an O-ring....

In the bottom row are a stock valve front end, and next to it, my new spring seat and air bypass, manufactured from it.... I added another O-ring groove aft of the trigger mounting screw (where the wide groove is), drilled it out 11/32" from the front, and then milled in four 3/16" holes at a 30* angle to pass air around the outside of the (shortened) spring seat.... The fitting on the right was the original version, which took me an afternoon to make.... It looks pretty, but there is a significant design flaw.... Can anybody see how I wasted an afternoon's work?....



The final photo is how it all goes together.... The upper item is a stock Disco valve, unscrewed until the spring is at zero preload.... Also shown is a stock Disco gauge mount, about the right distance from the valve (~1/16" gap between).... The new parts are directly below it, positioned appropriately for comparison.... The PRod gauge block is a bit longer, and the valve seat is sitting right up against it.... The back of the two valves are very close to the same position, meaning that the valve spring will have about the same preload.... I can adjust the preload by drilling the spring seat slightly deeper if required.... I will prevent the gauge port from sliding forward from the spring pressure (and interfering with the stock) by a simple 8-32 setscrew in the side of the tube.... The back of the new valve has a shoulder machined 0.250" forward to accept a washer of Lloyd's energy absorbing elastomer, and to allow additional hammer travel.... I left the center of the valve full length to act as a stem guide.... Since the photo was taken, I shortened the valve stem so that it projects 0.25" past the guide portion of the valve body, which is more than I need to allow for the lift....

So there are the major parts for the new valve.... I may yet drill the throat of the valve out a bit, as at the moment the total throat area is barely equal to the 0.219" port size I am planning on using.... It would probably be prudent to wait until after initial testing to do that, to see if there is any benefit....

Bob

[rsterne]

I tethered the gun directly to my SCUBA tank today (no regulator), filled it to 2000 psi and bled the line and shot a few strings.... Then I did the same with the tank valve shut off but the line not bled, so that it's volume becomes part of the reservoir.... However, by doing that I can use the (larger, more accurate) gauge on the fill clamp to monitor the pressure during the shot string.... It appears than an 8 shot string gets stretched to 9 shots with the added volume of the hose, fill clamp, and fittings.... so that means the total volume is 9.26 CI instead of 8.23 (the reservoir by itself is 135 cc).... Here are the results of the 9 shot string, broken into three segments of 3 shots each....

Shots 1-3: average velocity 998 fps, 180 psi used.... 169 FPE from 115 CI of air.... 1.47 FPE/CI
Shots 4-6: average velocity 1022 fps, 230 psi used.... 177 FPE from 147 CI of air.... 1.20 FPE/CI
Shots 7-9: average velocity 998 fps, 270 psi used.... 169 FPE from 172 CI of air.... 0.99 FPE/CI
Shots 1-9: average velocity 1006 fps, 680 psi used.... 514 FPE from 434 CI of air.... 1.18 FPE/CI average

Using the gun tethered to a 2000 psi regulator, I can therefore expect to shoot at ~56 FPE at an efficiency of about 1.5 FPE/CI with the 25.4 gr JSB Kings.... That is very good efficiency with a relatively light bullet at those power levels.... so I'm very pleased.... I'm looking forward to trying it with heavier bullets....

Bob

[SeanMP]

Hmmmm

I was afraid of that. The strongest spring I have here is a QB Spring. The .049 size.

I assumed that I was going to need a stronger spring but as you saw Trakar is a bust. The only thing they had was the light spring I wanted for a bolt assist spring.

So I'm shopping around for a spring supplier...any suggestions?

[rsterne]

I did a lot of measuring today, working on the hammer stroke.... Here are the pertinent details.... First of all, the hammer stroke for a stock Disco is only 0.58".... The lift of the valve is limited by the valve spring going coil bound, which occurs at 0.25" of lift.... The valve stem sticks out 0.32" from the back of the valve, however, so it can be shortened by 0.070" and still allow the valve full lift.... That increases the stroke to 0.65".... That is how I had my .25 Disco set up previously, and there is really no down side to doing that easy mod....

To understand some of the other limitations in the Disco design, it is useful to look at what the maximum possible travel of the hammer is, ignoring the valve stem.... The cocking pin limits the travel significantly.... It can hit the front of the cocking slot in the tube, and the back of the bolt (when closed), at about 0.8" of total travel.... Note, that occurs when the valve is open 0.22", eg. if you dry fire the gun, or use a mondo hammer spring wound tight at low air pressure.... If those are lengthened, then it will hit the front of the cocking slot in the steel breech.... It is easy to shorten the bolt, and grind the slot in the tube a bit longer, so let's assume those are done.... Now the hammer can move 0.90" at which time the face of the hammer will hit the back of the valve body, drive the valve to coil bind (at 0.25" lift), and the cocking pin will hit the front of the breech slot.... basically all at the same time.... If you are using a stock hammer, that is about all you can get, 0.65" of stroke, 0.25" of lift (which you will never need), and 0.90" total travel.... This makes grinding the slot in the tube about 0.10" longer and shortening the back of the bolt a pretty decent mod if you are using a heavy hammer spring.... One word of caution.... If you shorten the back of the bolt too far, it may not push the cocking pin back far enough to cock the gun.... About 0.10" is roughly the maximum, but it seems to vary a bit, gun to gun.... Another advantage is that this gives increased room to load the pellet, particularly useful if you have extended the bolt probe....

So I'm going for broke here, how can I get more hammer stroke?.... Well, there is enough material to machine off 0.25" from the back of the valve body.... The new valves don't have an O-ring, so this can be done easily.... I would recommend leaving a 1/4" diameter stub in the center to keep the valve guide full length.... OK, so that doesn't do anything, right?.... Well, how about if you could drill a hole 0.25" deep in the front of the hammer?.... Yeah, I know, it's hardened, bummer.... OK, how about making a new hammer, that I can do, I wanted to make it adjustable anyway.... So let's duplicate the original hammer except put a 3/8"-24 NF threaded hole in the front.... Now I can set the striker face anywhere I want relative to the front of the hammer..... I can adjust the lift, or the point at which the striker hits the valve.... Now when the outer rim of the hammer face hits the (new) back of the valve, it will have moved another .25", for a total of 1.15".... Darn, the cocking pin can't move that far without hitting the breech slot.... OK, I wanted to use a bumper anyway, and it's 1/8" thick, but I'm not sure if it will collapse a bit, so let's call it 0.10".... Now the hammer hits the bumper after travelling 1.05"....

OK, next idea.... How about moving the cocking pin back on our new hammer?.... There is room to move it 1/16" without making the hammer any longer.... In addition, when the hammer is sitting in the cocked position (against the sear), there is a pretty fair gap at the back, over 1/16".... It turns out that you can move the cocking pin back 0.10" and make the hammer about 0.040" longer and it still works.... Now the cocking pin can travel 1.00" without hitting anything.... Oops, I forgot, we shortened the bolt, and now the gun won't cock.... No biggy, I can cure that by machining the slot for the bolt handle in the breech further back, lengthening it 0.10" to compensate.... BTW, even more loading room....

So where would we be at this stage?.... The hammer can move 1.05" before it hits the bumper on the back of the valve, and 1.00" before the pin hits the end of the cocking slot in the breech.... However, the valve stem sticks out 0.4" from the back of the bumper, and we need to hit it to open it.... Assuming for a minute the hammer face is sitting against the bumper.... If the striker is recessed 0.40" (the maximum), the valve doesn't open at all.... However, if the striker is recessed only 0.15", by the time the hammer face hits the bumper, the valve will have opened 0.25" and the valve spring is in coil bind.... We know it never needs to go that far, so let's make the minimum distance from the striker to the hammer face a 0.20" inset.... The valve can still open 0.20" before the hammer hits the bumper (likely still more than we need).... With the striker adjusted this way, the operational stroke of the hammer is 0.85", and we've gained 0.20" over the 0.65" we had earlier (with the striker flush).... That's a 47% increase in hammer stroke over stock, and 31% more than I had with just the stem shortened.... When the striker hits the valve stem, the cocking pin (further back, remember) is still 0.15" from impacting the front of the slot in the breech.... That's probably enough lift, but I'm going to make sure by moving the front ends of the breech and tube cocking slots forward another 0.10", shortening the bolt more, and machining the slot for the bolt handle back further as well.... In other words, I'm going to make sure that the only thing that can hit is the outer rim of the hammer face on the bumper on the back of the valve.... Since I can adjust the striker face between 0.20" and 0.40" recessed into that face, I will have the following results....

Face recessed 0.20".... Stroke 0.85".... Maximum lift 0.20"
Face recessed 0.30".... Stroke 0.95".... Maximum lift 0.10"
Face recessed 0.40".... Stroke 1.05".... Maximum lift zero

Now you can see why Lloyd chose 0.90" as being the longest practical stroke he could get in a modded Disco.... he can still get about 0.15" lift, which should be all you really need.... In the next post, I'll show you the parts....

Bob

[rsterne]

I was looking over my data on this gun today, and noticed I hadn't correleated the performance against the pressure.... Here is a graph showing what happens with two pellet weights....



You can certainly see the big boost in FPE by using heavier pellets.... Also of note is the flattening of the velocity curve once Mach 1 is exceeded (with the light pellets).... These graphs are based on the highest velocities obtained with each pellet at each pressure....

Bob

[rsterne]

Two QB springs end to end on a guide, and a much longer hammer travel?.... You may be able to find something at a hardware store, automotive supply (Lordco?), etc.... Go up in diameter, wire size, and length?.... My DAQ uses a 4.5" long spring, 1/2" OD, made of 0.063" wire, and about 1 5/8" travel (I don't know the hammer weight)....

Bob

[rsterne]

To continue.... Here are the photos of the new parts....







Let me mention the valve first.... I haven't even assembled the gun yet, and I'm already on the Mark 3 version of the valve front end.... The parts I built yesterday would have worked fine, but there were three O-rings in between the gauge and the trigger screw that weren't needed.... I changed the design (yet again) and came up with what I think is an elegant and simple solution.... The spring seat on the valve poppet on my new valve ends up in the same place as it did in a Disco valve, meaning that by using the same valve spring, the front seat needs to be in the same place as it used to be.... The new valve front end is drilled through 3/8" for most of it's length, leaving lots of volume, a large air passage, and enough wall thickness to carry the gauge mount.... At the back of that hole, I milled four 3/16" holes on a 30* angle, which amounts to the same area.... The air from the reservoir passes through these holes around the OUTSIDE of the valve spring, into the (full diameter) chamber between the two halves of the valve.... The passages into the valve are nearly 3 times as much area as the port leading out, so the valve certainly won't starve for air.... The valve spring sits on the head of an 8-32 screw that I filed down slightly, and if I need to shim the valve spring tension I can add a washer under the screw head.... The clearance slot for the trigger mounting screw has been reduced to a shallow hole milled with a 5/32" mill.... The screw sitting in that hole will locate the front half from moving forward due to the force of the valve spring, and keep the gauge centered in the hole in the stock....

Now for the new hammer.... It is 0.040" longer than a Disco hammer, and the cocking pin is 0.10" further from the front.... The front half is drilled and tapped 3/8"-24 NF for the adjuster, which is made from a piece of bolt, and it is 0.40" long.... The rear half of the hammer is drilled 11/32" to clear a QB hammer spring.... The adjuster acts as the front spring seat, so that when you change the hammer stroke you don't change the preload.... and it carries an integral spring guide which doubles as a cocking indicator, and will be slotted to allow adjustment from the back with a screwdriver.... The slot milled on the top of the hammer is to allow clearance for the rearward position of the 4-48 breech screw used on the Disco, and is longer to compensate for the material removed from the back of the valve which will allow the hammer to move further forward.... The 8-32 hole in the side of the hammer is for a setscrew and plastic plug to act as a "brake" so that the adjuster can't self-adjust.... The RVA is longer to accomodate the longer QB hammer spring, and the adjusting screw is drilled through for the spring guide.... The guide will be shortened so that it is only visible when the gun is cocked.... A longer guide could also be used to carry a rear cocking knob if necessary.... The hammer/adjuster/guide assembly weighs 70gr, about 23% more than a stock Disco hammer....

These parts are now ready to be installed, but I have some things that need to be done to the gun to ready it first.... It needs a larger transfer port hole in the tube and breech, some milling done on the breech slots and tube cocking slot, and some work done on the barrel port and chamber.... Those are the next things to address....

Bob

[SeanMP]

Nice looking performance graph. I seem to remember that Lloyd was bustin his butt to break the 100fpe mark just a month or two ago....

And there you go blowing through it one attempt

[rsterne]

Sean.... I dug around looking at hammer springs today, and I also rebuilt the hammer on my Hayabusa to get a full 1.0" of travel.... I'm using a QB spring, and at 1900 psi, the last 4 turns of preload do nothing to the velocity.... I'm guessing it might be close to working properly at 3000 psi.... My .25 cal Disco would when the valve throat was 0.250" (the Hayabusa is the same), but didn't have enough hammer strike at 3000 psi with the QB spring with the throat drilled to 0.266".... The Hayabusa has a heavier hammer and more stroke, so it just might do it with a QB spring....

Anyway, I have two Trakar springs that are 0.360" OD (fit a 3/8" hole) x 2.5" long.... one is 0.051" wire, and acts identically to the QB (0.330" OD, 0.047" wire) rate is 17.9 lb./in..... The other is 0.059" wire, and it's a BEAST, 32.2 lb./in (#C0914-150-0635).... That spring has a whisker over a 1" maximum travel to coil bind.... I just noticed you can buy it in a 3".... I also have a 0.375" OD Trakar, 2.25" long, 0.062" wire (#C0953-157-0572) with a rate of 40.7 lb.in., about the same force to collapse to CB as the 0.059" x 0.360" one (but less total travel)....

I also have some 0.50" OD springs, 2.5" long, and they have more travel, but a lower spring rate.... The 0.062" wire (#C1270-160-0635) has a rate of 21 lb.in. but nearly 1.5" of travel, so at CB it has about the same force as the 0.059" x 0.360" as well.... Trakar also make 0.42" OD (fit a 7/16" hole) x 2.5" long x 0.063" wire (28.5 lb./in.) and 0.48" OD (fit a 1/2" hole) x 2.5 - 3.0" long and 0.063" - 0.072", many of which should be OK as well.... Hope that helps....

Bob