Tale of THREE BRods - .25, .30 and .357 Completed, Page 19 !

[aceflier]

Is this your first Mrod based build Bob? How are you liking the trigger? I wish all my rifles had that trigger!

[Buldawg76]

Its coming together very good Bob and you hit a nice power level on the first test fire. I am following closely since your build is almost the exactly how I want mine to turn out or close to the level you have now.

I have been out of town this weekend but have all the parts needed to start the build now in my possession. It turns out I likely will not need the plenum from the BT65 .30 cal since my plan is to use a ninja reg in the bottle versus one in the tube. It will be used in another build at a later date.

Mike

[rsterne]

This is my first MRod build, but I have used the trigger in many other builds before, and love it....

Mike, your 20" barrel will require a bit more pressure, as we discussed before.... and as you realize, my valve has 0.219" ports, not the 0.187" you were contemplating.... My next test will be with the Cothran valve with a 1/4" transfer port to match it.... My barrel port is large enough, the restriction should then be the bolt probe in the chamber, which should give me the equivalent of a 15/64" (0.234") passage....

Bob

[Buldawg76]

This is my first MRod build, but I have used the trigger in many other builds before, and love it....

Mike, your 20" barrel will require a bit more pressure, as we discussed before.... and as you realize, my valve has 0.219" ports, not the 0.187" you were contemplating.... My next test will be with the Cothran valve with a 1/4" transfer port to match it.... My barrel port is large enough, the restriction should then be the bolt probe in the chamber, which should give me the equivalent of a 15/64" (0.234") passage....

Bob

Bob
Yes I am aware of the port size you are using and just deciding if I am going to go that big on mine or not and have a reg set at 2200 Psi now for if I do go that big. I may try with the .187 port to see just what it gives first and go from there. I am not sure I am comfortable elongating the port in the MM barrel I have if by chance I mess up it could be ruined and I would be out cash and it took four months just to get the barrel made as well.

Mike

[rsterne]

You can get the FPE you want with a 3/16" port.... the question is at what pressure....

Bob

[Buldawg76]

You can get the FPE you want with a 3/16" port.... the question is at what pressure....

Bob

Exactly and I am going to find out. Then decide which route I want to go.

Mike

[rsterne]

The story of todays post is.... NEVER BE AFRAID TO LEARN SOMETHING NEW.... or at least open your eyes to existing problems.... 

I am always careful to make sure that the area between the front of the hammer and the back of the valve can't get pressurized, either from the hammer movement itself, or from an air leak through the valve stem during firing.... If that happens, you can get (in the worst case instance of a leaky valve stem) a machine-gun action where the pressure partially recocks the hammer and fires the gun again.... until the reservoir is empty (yes, I've had that happen).... Travis suggested that I flute my hammers to prevent this, but I double checked, and the sear slot in the bottom of the tube vents into the trigger, and out through the bottom.... However, I haven't been paying enough attention to the BACK of the hammer, where a vacuum can build up and slow the hammer strike.... The only gun I ever vented there, on a suggestion from Scott (Motorhead), was my B-51.... WHY I haven't been looking for this problem in other guns I don't know.... complacency, I guess.... Well, that just bit me.... in spades....

While I was triple checking the space in front of the hammer this morning, I looked at the area behind it.... Yes, there is a rather large cocking slot, with a matching one in the receiver.... but guess what.... There is nowhere for the receiver to vent, other than past the bolt at the ends, in an MRod receiver, with the rear bolt.... I had noticed some large ES values yesterday when I had the SSG backed off with a large gap, and the lightbulb finally went on.... With an SSG (or SSS or TSS), the spring quits pushing the hammer before it gets to the valve stem, and it coasts the last part.... What if there is a vacuum behind the hammer?.... Yup, that will start slowing the hammer down before it gets to the valve.... In addition, that vacuum is unlikely to be 100% consistent, shot to shot, which could increase the ES.... and the more gap you have between the spring and hammer, the worse both might be.... Time to find out....

I set the SSG for 1.5 turns of gap, and shot an 8-shot string.... The average velocity was 811 fps, but the ES was huge, with shots going from 790-834 fps.... definitely not good.... I stripped the back of the gun apart, and machined two slots in the bottom of the rear plug (SSG housing), at about 4:30 and 7:30, in the section that inserts into the tube.... The slots were 3/16" wide and 1/8" deep, and continued back 3/16" behind the rear of the tube.... That is the equivalent area of a 1/4" hole through the rear plug to prevent any vacuum forming behind the hammer on firing.... Here is what you see with the action out of the stock....



There happens to be a slot in my stock at the location of the vents.... If that isn't the case with a different stock, a different vent design may be necessary.... Anyway, I assembled the back of the gun again, adjusted the SSG gap to the same, 1.5 turns out, and fired a 34 gr. JSB through the Chrony.... Instead of 811 fps, it was 956 fps.... WOW !!!.... Obviously the vacuum behind the hammer was a reality, I had moved up onto the velocity plateau.... I went through the process of documenting the velocity vs gap with the vented rear cap.... On the graph below, the dotted red line is the new data, with the vent.... while the solid red line is yesterday's data, without the vent....



There are two important things to notice.... First, it takes a LOT more gap to get to the knee of the curve with the vents.... which means that without them, the vacuum behind the hammer was slowing it dramatically.... Secondly, the slope of the downslope part of the curve is a lot shallower.... It takes more gap to lose the same amount of velocity.... To drop from 944 fps down to 760 fps now takes a 3 turn increase in gap instead of two.... That makes sense, because the bigger the gap, in the unvented setup, the more the vacuum would reduce the hammer velocity.... There is absolutely NO doubt this gun needs the area behind the hammer vented.... 

Since I had dramatically increased the hammer strike at the same settings, I could now do two things.... I could reduce the preload on the spring, making the gun easier to cock.... I reduced the preload to my original 0.40" (which was not enough yesterday, with the vacuum), and shot the data shown by the purple line.... As you can see, the "knee" of the curve still requires 3 turns more gap, even though the preload on the spring is reduced from nearly 8 lbs. to just over 5.... Since the larger gap means the spring is not compressed as far (and starts at less preload as well), the maximum cocking force, when the gun is set just below the plateau, at 944 fps, has dropped from 19 lbs. down to just 15 lbs.... In fact, I could further reduce the preload if I wish, and that would allow me to decrease the gap, and the cocking force, even more....

These vents did one more thing for me.... They increased the available hammer strike to the point that I could now test the gun at 2900 psi.... I checked it with the 34 gr. JSB Heavies, and the results are shown as the black line on the graph above.... This was shot with the preload on the spring still at 0.40", and to get to the plateau I had to set a turn of negative gap (actual preload on the valve stem).... I didn't try it, but I am confident that by going back to the 0.60" preload I could operate at 2900 psi and hit the plateau with a small gap.... In any case, these results proved that this valve is capable of cranking out 90 FPE at 2900 psi with the 34 gr. Heavies.... I didn't have the proper sized 51 gr. BBTs for this barrel, the ones I had were sized to 0.250" (so they were likely leaking a bit in the bore).... but by cranking in some preload I managed to find the plateau for them as well.... I was able to hit 963 fps (105 FPE) at 2900 psi with those bullets.... I have ordered an NOE Mould for the 51 gr. BBTs in 0.253/0.255", and the sizing bushings for them.... so at some point in the future I can experiment with them....

In conclusion, I am extremely glad I vented the area behind the hammer to prevent the vacuum from slowing the hammer.... I am frankly shocked at how much hammer strike was lost because of that vacuum, and the horrible ES that it caused.... You can bet I'm going to have a long, hard look at all my Disco based guns.... There may be more consistency, and easier cocking, lurking amongst any of them.... 

Bob

[Dairyboy]

Outstanding find! No wonder why Travis has a vent hole on the TSS!!

[pllagunos]

Crazy what a vent can do... Bob I'm really not understanding why a vacuum affects the hammer strike that much, but would it affect a qb79 running a SSS?

[Machinist]

I have a question about the SSG.  You used aluminum on the front of the 4 1/2 inch rod.  Won't that get beat up  after a while?  I was thinking it had to be steel, and I even wondered if it needed to be hardened.

Surprising how much the hammer strike was affected by vaccume.  I have a regulated Disco that has more ES than I expected.  I wondered if galling of the tube, or just that steel on steel is creating excess friction that is also inconsistent.  I made a plastic wear band for a piston I made for a 1377 years ago. No metal to metal contact, no galling, no need for lubrication.

Nice that you hit 100FPE   

Steve

[oldpro]

 Outstanding good work my friend! Keep at it Bob your almost there now.

[rsterne]

After writing the previous thread, I changed all the parts over to the tube with the Cothran Powerhouse valve in it.... This is the best way to compare it to my modded MRod valve, by tethering it at the same two pressures, 1900 psi and 2900 psi, and using the same pellets.... Here are the results.... The horizontal axis on this graph is the same as the one above for my valve, and I used the same colours for the 34.2 gr. JSB Heavies at the two different pressures.... In both cases, the SSG had the same 0.40" preload, and "Zero" is the point where there is just barely a gap in the SSG setup.... The end of the valve stem in my valve is further forward than that of the Cothran valve, so my setup has 0.88" of maximum hammer stroke, whereas with the Cothran valve it is only 0.79".... but that suits the two valves, because mine needs more hammer energy to open.... This means that the SSG has to be adjusted to a different point (about 1.5 turns further out) with the Cothran valve to end up with zero gap.... but zero on both graphs has the same meaning, minimum gap on the SSG....



You can see at once the huge difference in the response of the Cothran valve to the gap setting.... Basically this valve is either "ON" or "NOT".... It exhibits the typical "cliff" which we have seen in other tests of the Cothran valve.... The most efficient point to operate the Powerhouse valve is just above the point where it no longer opens properly and consistently.... This may only be a matter of one flat (1/6 turn) on the SSG gap adjuster.... I didn't explore the exact location of that point yet, all I was after was the maximum velocity available at the two test pressures, and the range of SSG gap where the cliff started.... Relative to zero gap, at 2900 psi the cliff was at about 2 turns out, compared to requiring actual preload with my valve.... at 1900 psi, the cliff was at about 4 turns out, fairly similar to where the knee was with my valve.... As expected, with the larger ports, the Cothran valve was able to produce a bit more power than my valve was.... Here are the maximum velocity and energy achieved with the two valves at the two test pressures, with the pellet and bullet tested....



The port size on my valve is 0.219".... The Cothran valve has a 0.257" exhaust port, I used a 0.250" transfer port, and the barrel port is equivalent in area, but there is a 3/32" diameter probe in the chamber.... This reduces the effective area to that of a 0.234" port.... so it doesn't hurt my valve, but probably handicaps the performance of the Cothran valve slightly.... However, the only way around that would be to just a probeless bolt with a flat face, that retracts flush with the back of the barrel port in the firing position.... This would require milling a "J-slot" in the MRod receiver for the bolt lockup and cocking pin to retract into.... Since I don't plan on using anything heavier than my 51 gr. BBT, and I can already push that to nearly 1000 fps, I simply don't need to bother making that modification to the receiver.... However, if you wanted to use an even heavier bullet, as you might if you fitted a .257 cal barrel, you would be well advised to take the trouble to use a probeless bolt arrangement to achieve full barrel size porting with the Cothran valve....

Bob

[rsterne]

I don't think the aluminum nose on the SSG will get beat up, but I guess time will tell.... There is no impact on firing, only when the hammer is thrown back by the closing of the valve.... and I made the shapes to match, so the area is quite large.... Motorhead actually put an O-ring in the front of his, not a bad idea.... I tried a vent on my recently built Regulated Disco Double this afternoon, and if only made about a 20 fps difference.... so obviously there were nearly enough leaks to get rid of the vacuum.... For those of you that may ask if "Gun XXX" needs a vent, I suggest you simply take a look.... I'm not about to pull apart every PCP I have just to tell you.... If I do find any that have a problem, you can be sure I will post about it in the PCP Gate....

Bob

[grand-galop]

May i asked???  Do you think with your recent discovery and analisis that, any gun highly mod would BENEFIT of fluting the hammer???
It`s not to criticized or find a winner because at the end, we are all winner finding new path in power using in those guns...
I think even a rolled out of factory discovery would benefit hammer fluting..

[rsterne]

Fluting the hammer will guarantee that it can't pressurize the air in front, or leave a vacuum behind.... regardless of whether the tube is vented or not.... As far as the friction is concerned, it should be a wash.... If the coefficient of friction (materials and/or lubrication, and smoothness) is the same, it doesn't matter whether you have a small area or large.... Back to Physics 101, guys.... more area equals less pressure per unit area.... less contact area increases the pressure per unit area.... and the friction force should stay the same either way.... Now if the hammer cocks on firing, and puts more pressure on the area between the flutes, the friction could go up or down.... I don't think there is any benefit.... but it looks KEWL, and will insure no "air lock".... so no reason why not.... One flute down each side should do no harm, IMO.... or just machine the sides off flat, like the PRod/Challenger hammers.... on the left in the photo.... Ignore the O-rings, they were part of a bstaley experiment....



Bob

[grand-galop]

Thanks for the precision..  I will not touch my QB`s for sure but want to give a try to my 2240...  I`m working on PIPING the valve directly on the gaz block for no leaks or fear of pressurizing the tube like before... I will show chony number as soon as i perform the change over..
Many thanks.

[Motorhead]

  Yea that air we move around when stationary parts get thrown by springs etc ... is one many have not paid attention too.
It is after all part of physics that cause and effect are always in flux altering set perceptions by other effects present escaping ones attention.

Kudos Bob .... fine tuning further while traveling down that bottomless rabbit hole we're all in 

[oldpro]

 Not sure I agree with this statement Bob In this situation anyway. "If the coefficient of friction (materials and/or lubrication, and smoothness) is the same, it doesn't matter whether you have a small area or large.... Back to Physics 101, guys.... more area equals less pressure per unit area.... less contact area increases the pressure per unit area.... and the friction force should stay the same either way..." This is only true if all forces are equal on all sides but we have gravity and spring torque here also. I flute the top and sides where Gravity Isnt affecting the hammer like it does on the bottom so having less surface area in areas of lower drag does reduce drag overall I learned this many years ago building High pressure pump stators for The Polaris factory race team. Anyway its working well for you and thats Awesome sauce. Keep rocking !!! 

[rsterne]

Actually, the "area don't matter" is straight from any Physics text for a flat block sliding on a flat surface with gravity being the force pushing them together.... The area is not part of the equation, only the weight and the friction coefficient.... It doesn't apply if the surfaces are not smooth, however.... like a rubber tire on pavement.... If the hammer is a tight fit in the tube, then area would certainly make a difference because of the interference fit.... but they need to slide under their own weight.... Offcenter forces could mess it all up, too....

For most hammers, the bottom, where the sear contacts, can't have a flute.... and if there is a cocking pin on the top, there could be a reason not to have a flute there.... I see no reason not to cut away the sides, though, with one exception.... If you have a side handle on the hammer to cock it directly, the offset weight will cause it to skew in the tube on firing (and worse, on cocking).... so flutes should not be in the plane of the handle, or opposite it, IMO.... I say leave the surfaces alone where you know there will be contact points, and the rest.... cut 'em away if you don't need the weight.... Same goes for milling off the sides (ala MRod hammer), or turning a waist (like a 22XX Disco), if the trigger sear doesn't need a flat bottom on the hammer....

However, as you say, whatever works for you....

Bob

[aceflier]

Looks like you've got a couple of powerhouse .25's there Bob. Hope your accuracy is as good. I'm sure they will be hammers. keep us updated on the progress.