.30 cal Disco Double

[Sqrl Klr]

I have never seen any "strong screws" that are 8-32 thread with a 5/16" head to upgrade the Disco valve mounting.... 10-32, which require redrilling and tapping the valve....

Doh that's right Bob. Completely forgot about that.

You could ask Norm if he could do that work for you rd. Lloyd is too busy right now so I don't know if he would be able to.

[Rdsail]

I will eventually get it done. My disco has been down for 3 week now. I need to do some shooting and make sure the latest valve changes did any good. I opened the throat a little more, increase the size of the valve port, reduced the OD of the valve stem to allow for more flow and made a long probe bolt. The transfer port is still stock size. Right now I'm trying to make the disco a little more efficient between 25 and 30 fpe. I'm getting 15 shots at 28 fpe from 2100 to 1500 psi.

I will give the mcmaster gauge (0-4000) a try.

[rsterne]

I would be surprised if you gain any advantage by opening out the throat and slimming the stem, or opening the valve exhaust port, if you are using a stock 0.140" transfer port.... The barrel port (0.134") is the smallest restriction, followed by the transfer port, and everything else is already bigger on a stock Disco....

Bob

[Rdsail]

Bob, I'm not necessarily looking to get more power out of the disco. I'm looking for better air flow which should improve efficiency. I'm keeping the T.port and the barrel port stock because I'm using it so it can regulate itself. It gives me a very flat curve at least form 2100 psi to 1500 psi.

So far every adjustment, I gain a few FPS (10 to 25) on the same about of air. I know at some point this will stop. I'm not power hungry but looking for consistency in the shot string. I think I have reached that point but I figure I would try. Worse comes to worse I put the stock stem back on and the opening of the through should not have affected anything. 

If I decide to go for more power the first thing I have to do is open the barrel port.

Daniel

[rsterne]

ahhhhhhhh.... I know where you're coming from.... I often do the same thing, but in a different order.... I build the gun to be an efficient powerhouse, and then fit a restricted transfer port to drop the power, flatten the curve, and increase the shot count.... Yes, it works very well....

Bob

[Rdsail]

I'm new to the AG world so I'm learning as I go with lots to be learned. I do as much research and ask before I make changes. On thing I wish I had are better tools. I only have a drill with high quality drill bits,  a dremel tool and so machining files and dies that use to belong to my wife's grandfather. This makes it hard to make mods to the valve or build parts. I do have patience which helps.

My disco is shooting at 820-835 with 18.1 jsb's. I make shot faster but it seam that is the sweat spot with the current tport configuration.

I find your posts extremely helpful and the work you are doing in outstanding. I wish I had the skills to do the same.

Note: I talked to Norm and I will have to get the valve rethreaded for the larger screws. I think I'm going to hold on all that until I do more testing on the current set up. I also have an assortment of hammer springs to test out.

Daniel

[Sqrl Klr]

I personally recommend not using a .047" spring and just top out with a .041" spring since the largest one contributed to cocking issues and part problems in my disco. Several shipments across the country to fix the reoccurring problems add up in cost quick. I have well over $1k in my disco partly because of it such as a replacement breech, bolt handle and cocking screw from all the pressure of cocking with that heavy spring. There shouldn't be any damage developing from using the .041" spring if you're not rough with the cocking.

[rsterne]

I use a 0.040" wire spring in Discos, although I have on occasion gone to 0.042".... If I need more than that, I increase the hammer travel and weight and go to the hammer spring from a QB78, which is 0.047" wire but it is 1/2" longer and so the spring rate isn't that much higher.... You get the extra hammer energy and momentum from the extra travel and weight instead of a mondo, hard to cock spring.... That's what I'm using in this .30 cal build....

Bob

[Rdsail]

I have the heave sprint from discosrus which by my measurements it is about .039 wire. I have that one in two lengths. The regular length and a shorter version. I have found that the shorter length gives me more consistent number or I should say my standard deviation is smaller.

The next spring I'm going to try is .042. That is what I have have. I have other springs heavier than that by I discovered the same thing that sqlr kir mentioned. I was trying to use slightly heavier spring to reduce the preload. I don't know if that would help or not.

[Rdsail]

Update on the screws: Lloyd does have the valve Screws size 8-32. I'm getting some to test out. So I might be filling to 2500 psi after all

[Sqrl Klr]

Lloyd confirmed his 8-32 screws are stronger than the ones in that size that you currently have rd?

[Rdsail]

This is what he wrote to me:

Hi Daniel,
Yes I do. If you go to buy them, they go by the name of: Low profile socket head cap screws, high strength alloy steel. The direct replacement size is 8-32 x 3/16 long.  The problem is that the shortest I can find them in is 1/4" long, so I have to grind off 1/16".  I will send you some for a few dollars if you can grind them off your self.
You live in Richmond, right?  Maybe you can find some 3/16" long ones at an industrial supplier, and if you can, would you let me know?
I've attached a pic of some of the screws installed in a valve.
Lloyd

I don't know if they are rated for 3000 psi. I'm only looking to go up to 2500 psi

[rsterne]

Daniel.... Would it be possible for you to post the pic Lloyd sent you?.... If you right click on it, select Properties, you can find the URL where the photo is.... then copy/paste it here....

At 3000 psi, with three 8-32 screws, anything higher than 120,000 psi tensile strength will have a 2:1 safety margin or better.... At 180,000 psi (which is common for high tensile SHCSs) the safety margin in shear would be 3:1 at 3000 psi.... As long as the head diameter is 5/16" to fit properly in the holes in the Disco tube, you should be good to go....

Bob

[Rdsail]

here is the picture from lloyd!

[56chevy]

I looked on loyds site but cant find the button head screws listed to buy???

[rsterne]

^X2....

I've both PM'd him and emailed.... He's building a house right now, so very busy....

Bob

[rsterne]

I got a response from Lloyd RE the high tensile 8-32 screws he has.... They do NOT have a 5/16" head, it is the standard 0.270" diameter available from several suppliers.... Because of the high tensile strength, they have plenty of shear strength, but the smaller head diameter, when used in a Disco tube, will move the valve back 0.020", causing a small misalignment of the transfer port.... If you are enlarging the port, and allow for that, in itself that is not an issue.... I do have a concern about the poor fit in the hole, however.... Instead of the entire rear half of the screw head spreading the load into the tube wall, the bearing area is drastically reduced.... in fact it starts out as a point contact.... I have seen a very slight "settling in" of the screw head into the rear side of the hole in the tube when using 10-32 screws (which do have a 5/16" head) at 3000 psi in a Disco tube.... This is not a safety issue, it just indicates that the load on the screws is so great that it is exceeding the local yield strength of the tube until the valve moves back a few thou and all three screws take up equal load.... This is predicted by the strength formulas because of the relatively thin wall (0.065") of the Disco tube.... My concern with using screws with smaller heads is that they may have a tendency to move back even further until the ability of the tubing wall to resist the force equals the load.... The screw would have to move back an additional 0.020" before the contact width was 0.250".... which is still 25% less than the width of the stock 5/16" screw head.... It wouldn't surprise me to see that happen at 3000 psi....

Is this a safety concern?.... I'm not sure.... If the deformation stops at that point, it's only an inconvenience, albeit one that has to be allowed for by moving the transfer port forward in the valve by the same amount so it still lines up with the hole in the tube and breech.... However, I would be a LOT happier if the head diameter was the full 5/16" like on the stock pan head screws, or on the 10-32 screws.... I have used those 8-32 low profile SHCSs in a redrilled 2260 tube, and they worked fine.... However, the holes in the tube were also 0.270", so they provided a full cradle for the head of the screw.... I have every confidence in using them in that way....

Bob

[Rdsail]

Bob,

I went ahead and ordered the screws anyway. I'm going to try to shim them with a saw blade piece. The metal is ver hard and just by change the  thickness is .022. I only need to shim the back of the screw which make it easy.

This will allow the bolt and shim to make full contact with the tube hole. I'm attaching a picture to illustrate.

[lloyd-ss]

I got a response from Lloyd RE the high tensile 8-32 screws he has.... They do NOT have a 5/16" head, it is the standard 0.270" diameter available from several suppliers.... Because of the high tensile strength, they have plenty of shear strength, but the smaller head diameter, when used in a Disco tube, will move the valve back 0.020", causing a small misalignment of the transfer port.... If you are enlarging the port, and allow for that, in itself that is not an issue.... I do have a concern about the poor fit in the hole, however.... Instead of the entire rear half of the screw head spreading the load into the tube wall, the bearing area is drastically reduced.... in fact it starts out as a point contact.... I have seen a very slight "settling in" of the screw head into the rear side of the hole in the tube when using 10-32 screws (which do have a 5/16" head) at 3000 psi in a Disco tube.... This is not a safety issue, it just indicates that the load on the screws is so great that it is exceeding the local yield strength of the tube until the valve moves back a few thou and all three screws take up equal load.... This is predicted by the strength formulas because of the relatively thin wall (0.065") of the Disco tube.... My concern with using screws with smaller heads is that they may have a tendency to move back even further until the ability of the tubing wall to resist the force equals the load.... The screw would have to move back an additional 0.020" before the contact width was 0.250".... which is still 25% less than the width of the stock 5/16" screw head.... It wouldn't surprise me to see that happen at 3000 psi....

Is this a safety concern?.... I'm not sure.... If the deformation stops at that point, it's only an inconvenience, albeit one that has to be allowed for by moving the transfer port forward in the valve by the same amount so it still lines up with the hole in the tube and breech.... However, I would be a LOT happier if the head diameter was the full 5/16" like on the stock pan head screws, or on the 10-32 screws.... I have used those 8-32 low profile SHCSs in a redrilled 2260 tube, and they worked fine.... However, the holes in the tube were also 0.270", so they provided a full cradle for the head of the screw.... I have every confidence in using them in that way....

Bob

Yes, a slight misunderstanding here.  I have used both the 8-32 (.270 head dia) and the 10-32 (.312 head dia) low profile socket head screws and of course the 10-32, with full contact will always be the preferred choice.  Using the 8-32 screws will indeed allow the valve to seat .020 farther back in the tube, with a corresponding misalignment of the transfer port, which for many applications is not an issue.

I do want to clarify, that with the smaller head size, when loading the valve with air pressure in the tank, a 2000 psi fill will deform the rear semi-circular portion of the hole in the tube less than .001".  Filled to 3,000 psi, the deformation will still be less than .002".  This is assuming the 70ksi compressive strength of the tube material, and not adding in any benefits from strain hardening.   

So you have the .020 shift for the head size, and an additional .001-.002 for the heads to seat into the tube material.  Actually, the manufacturing tolerances in the tube holes and the valve holes exceed that .002, so any distortion observed will be as much from the manufacturing tolerances as it is from the screws.

One very good indicator that I have found that the screws in a disco have been overloaded or have passed their useful life is when unscrewing them from the gun.  If any eccentricity can be felt..... tight-loose-tight-loose... as you unscrew the screw, you know it has been bent.

So that I am not misunderstood, when going to the high strength alloy steel Low Profile SHCS, tapping the valve out to 10-32 is the first choice.  Using the 8-32 high strength alloy steel Low profile SHCS is still better than stock, but it does present different circumstances that must be fully understood.
I hope that helps.
Lloyd

 

[rsterne]

I understand the concept.... but do you understand the forces involved?.... At 3000 psi, there is over 1300 lbs. force on the valve trying to push it backwards.... How fast do you think one of those shims might travel if it decided to fly out under the pressure of over 400 lbs. pushing on it?.... close to supersonic comes to mind!.... Worse yet, then you only have 2 screws taking the load.... and if another shim gets forced out, then only one.... Suddenly your 3:1 safety margin has disappeared because only one screw is now taking the 1300 lbs. of load.... Will it fail and shear off, leaving you with only two?.... Maybe, maybe not.... If it does, when the valve suddenly moves back 0.020" will another screw fail from the impact?....

I'm sure you get my point.... No way in he!! would I even consider doing that....

Bob