Large Ports in an MRod Valve

[rsterne]

The great thing about this Forum is how ideas are shared, and then create new ideas from that sharing.... Tim (shorty) recently posted in the Engineering Gate the idea of using a square exhaust port in an MRod valve to allow larger port area without hitting the seat.... In case you don't know, the seat is so close to the front of the exhaust port in a stock MRod valve, that if you drill it out much larger than 3/16" you run the risk of breaking through into the back of the valve, and ruining it.... About the largest I have heard of anyone doing, straight down, is 13/64" (0.203"), and IMO even that is risky.... Scott (Motorhead) angles the port back, away from the seat, but that makes the air turn more than 90* to get into the exhaust port.... Although I wasn't the first to do it, my solution in my BRods was to move the valve seat forwards, which allows not only a larger port, but you can angle it better for improved flow.... However, that is difficult to do, and the new seat I installed leaked between the insert and the valve body, and I had to redo it, and seal it with an O-ring.... more hassle.... That remains the best solution for really large ports (1/4" and larger), however, and is why the WAR and Cothran valve have the seat further forward....

For the average home machinist, with moderate skills and access to a Mill, but who wants to max. out the ports for a .25 cal, or build a .30 cal MRod, I have come up with an easier solution.... Here is a drawing, top view....



Magenta is the drilled out valve throat and stem, which are 17/64" and 1/8" respectively.... That is the largest you can go using an MRod poppet and expect it to live at 3000 psi.... any larger, and it will extrude into the throat of the valve.... You drill the throat out first....

Blue is the new exhaust port.... It is in the same fore-and-aft location as the original, and is made by plunging a 3/16" end mill down into the throat, then move it sideways 0.030" left and right (up and down in the drawing) to enlarge the width of the port to 1/4"....

Red is the new barrel port, for a .25 cal barrel, at the bore line.... It is also done with a 3/16" end mill.... The back of the port is centered 0.015" behind the original port location.... The front of the port is milled on an angle, sloping forward, so that where it intersects the bore it is 1/4" long x 3/16" wide.... At the TP face, you use a countersink, centered on the original location, to make the port round and 0.222" diameter (the dotted black line).... and do a little blending with a Dremel (from the red line to the dotted black line), within the thickness of the barrel wall....

Black is the new transfer port.... It is 5/16" OD x just over 7/32" ID (you can use a #2 drill).... Yes, this port OD requires you to enlarge the TP flat in the valve and barrel, and drill out the tube and the receiver, to 5/16", and use a transfer port made of Teflon or Delrin a few thou long, so that it crushes to seal.... You will need to do a small amount of work with a countersink on the lower end of the TP to mate up with the sides of the oval valve port, just to smooth the flow and eliminate any steps.... Just a touch with a countersink will suffice....

The advantage to the above porting should be obvious.... The area of the oval valve and barrel ports is equivalent to the 0.222" transfer port.... If you slim the bolt probe down to 7/64", in a .25 cal, the area around the probe will also match that.... You could even slim the probe to 3/32" if you wish.... The 0.266" throat, with the 1/8" valve stem, is the equivalent of a 0.235" hole, so the throat has about 12% more area, just about perfect.... If you want to do a little smoothing/blending with a small spherical burr in a Dremel, where the throat meets the exhaust port, it can't hurt, just make sure you don't damage the valve seat....

These changes will allow the modded MRod valve to flow all the air you can use in a .25 cal unless you use a retractable bolt, and eliminate the probe completely.... You can also use this valve in a .30 cal conversion, and the barrel port can be a simple drilled hole of 0.222" (a #2 or even #1 drill).... I hope the home machinists among you can make use of this idea....

Bob

[ackuric]

Nice schematics and write-up. I personally have safely drilled (by hand) up to .2 on the marauder valve (gen2) and don't plan on going further...it's scary close to the valve seat! I thought about going oval to get even more which heck one day I may. I must have hands of a surgeon to be able to pull of .2 ayy?

The stock tp size limits really how far you can go with port size when going oval, without changing that I wouldn't go over .22/.20 oval which should flow equally to a .21 port I think. You certainly have the right idea by enlarging it.

My valve throat starts at .240" and tapers down to .22 half way through the Exit port where the rest is .203, which almost matches the valve exit port of .2" With the .055" stem I flow plenty.

I would love to send a valve to you one day and let you work this magic on it though! I am no machinist just a handy man with some crazy ideas

Doesn't drilling the throat out bigger than .240 change the valve throat angle (angle behind the seat)? I believe seat angle is very important in valve design and I am pretty sure the stock valve tapers like so \  / for a reason, but that's just imho and I could be wrong, hence why I ask!

[DougGuy]

I went through 3 or so valves trying to gain plenum volume and none of them ever really delivered because of the limitations you noted Bob.

My attempt at moving the seat forward was using a 1/2" end mill and boring the ID of the valve all the way back to where the stem entered the valve from the back.  I used a piece of .035" wall thickness 3/8" seamless stainless tubing which had a .300" ID, and I made another poppet out of some PEEK and some 1/8" drill rod.  I made a pilot to hold the tubing centered with the valve stem bore so the valve would be squared with it, coated all of it except the outside with a thin coating of floor wax, and I set the stainless seat into some JB weld and let it harden.  I had a 1/4" port, I had milled my air tube .375" (centered on the factory port with a pin gage in the mill) and the design worked but I never got the valve to hold air reliably.  It made hella loud whump when it fired a heavy pellet, but it just turned out to be too much work and too much time invested for too little of a return.

Enter the Cothran valve.  This valve came with a .375" fly cut, which aligned with the .375" hole I had milled in my air tube PERFECTLY!  It had a .250" t port, and it fit dimensionally into the design and machine work I had already done, it was like we were both reading the same drawing so it indeed was a true drop in!  I will get some chrony results eventually.  The gun is working now, it groups well, it hits HARD, and it holds air.

I know this doesn't really add anything to your post about getting the most out of it using a Crosman valve, my one goal was to have a plenum with bore area the whole way up from the valve seat to the port in the barrel which my brass tport tapers from .250" ID at the valve to a .187" x .309" slot at the barrel.  I just couldn't do it well enough without compromising something one way or another with the Crosman valve.

[rsterne]

Matt, I have used many valves with no taper inside the seat, that is just part of the "raised seat" design that Crosman use.... IMO no tapering of the ID is necessary, it just increases the seat diameter, and hence the opening force, for a given flow....

Bob

[ackuric]

Okay, I was thinking maybe the angle after the seat may slightly aid in some way other than raising the seat as that taper is not necessary for a 'raised seat', you can maintain the raised seat without a taper so wasn't sure why that was part of the design as it seems harder to manufacturer something that tapers from .240" down to .203" rather than something that goes straight .240". At least that was my train of thought.

Increasing the throat without increasing seat diameter doesn't impact the opening force correct?

You could do a .235" throat through and through which is under the stock diameter of the seat is at the smallest ID and a stem thinned down to only .08" (2mm) which is plenty of meat imo for up to 3k and not require more hammer energy with the .222 equivalent oval porting, if you planned on going over 3k psi I wouldn't thin the valve stem.

[rsterne]

You would have to ask Crosman why they use a raised seat design on their valves.... I suspect it may have to do with ease of sealing without lapping required....

If you narrow the actual seat, it will sink into the poppet head further, increasing the energy required to open the poppet, although not the opening force.... That is because the poppet has to lift further to clear the seat.... If when you open up the throat to 0.266" it creates a knife edge to the seat, touching it with an end mill to create at least a 0.020" seat width would be a good idea, of course.... It would be a good idea to run a 1/2" end mill down inside the MRod valve to open up the ID for increased flow past the OD of the poppet if you are going to this extent in modding a valve anyways.... You can expect this valve to be harder to open than a stock MRod valve, a stronger hammer spring will be required, that goes hand in hand with flowing large amounts of air at high pressures, without a balanced valve like the Cothran.... It simply takes a lot more air to create 80-100 FPE in .25 cal (or 100-130 FPE in .30 cal, which I have done with similar port sizes) than it does to create 50 FPE....

I don't want to get into the idea of thinning the poppet stem in this thread, it is much more difficult to do properly than simply drilling out the throat, and if you don't drill the throat to at least 1/4", you can't make the exhaust port that wide anyways.... If someone wants to use a 1/4" throat (to match the width of the wider exhaust port), they could reduce the poppet stem to 0.100" and still achieve sufficient throat area.... but I would not go smaller at 3000 psi.... Your ideas are covered well in your own thread, Matt....

Bob

[oldpro]

 Do you still have pics Bob of the rear cored Mrod valve? It takes more work but allows you to change internals and not worry about T-port at all. Good write up as usual My friend on how to make the stock valve purr.

[rsterne]

No, I don't.... As you say, trying to help guys mod a stock valve without a total rebuild of the "guts"....

Bob

[ackuric]

You would have to ask Crosman why they use a raised seat design on their valves.... I suspect it may have to do with ease of sealing without lapping required....

If you narrow the actual seat, it will sink into the poppet head further, increasing the energy required to open the poppet, although not the opening force.... That is because the poppet has to lift further to clear the seat.... If when you open up the throat to 0.266" it creates a knife edge to the seat, touching it with an end mill to create at least a 0.020" seat width would be a good idea, of course.... It would be a good idea to run a 1/2" end mill down inside the MRod valve to open up the ID for increased flow past the OD of the poppet if you are going to this extent in modding a valve anyways.... You can expect this valve to be harder to open than a stock MRod valve, a stronger hammer spring will be required, that goes hand in hand with flowing large amounts of air at high pressures, without a balanced valve like the Cothran.... It simply takes a lot more air to create 80-100 FPE in .25 cal (or 100-130 FPE in .30 cal, which I have done with similar port sizes) than it does to create 50 FPE....

I don't want to get into the idea of thinning the poppet stem in this thread, it is much more difficult to do properly than simply drilling out the throat, and if you don't drill the throat to at least 1/4", you can't make the exhaust port that wide anyways.... If someone wants to use a 1/4" throat (to match the width of the wider exhaust port), they could reduce the poppet stem to 0.100" and still achieve sufficient throat area.... but I would not go smaller at 3000 psi.... Your ideas are covered well in your own thread, Matt....

Bob

I would have to call Crossman and ask why their machining included a taper right after the seat, but you and I know that response you gave me was very passive aggressive and uncalled for...a call to Crossman will yield no answers....I am not concerned about the raised seat in my previous questions, I was concerned about the taper right after the seat...but I will certainly take your word that its of no importance and just how Crossman did it...even though it seems more difficult to do than a straight non-tapered port which mildly confuses me, nonetheless, thanks for the information Bob.

[shorty]

Yes, sharing is fun. Good write up Bob and I wish I had a mill. Just can't talk the boss into it yet. Since I really liked your idea of a "D" shaped port, I had to try it.

This is what you get when you drill a .19" hole and push a .25" HSS boring tool ground flat on one side to have a .1875" distance from flat to round.

I guess this may be plan B ?

[rsterne]

Add a slight radius to where the flat joins the circle, just to get a smoother corner.... and then figure out how to hold it straight while pushing it, of course.... How to you clean up the inside of the throat afterwards?.... easy or

Bob

[shorty]

Re-drill the throat after punching.

Still real hard to pull the punch out. It's not that it's hard to pull the punch but, I scar up the aluminum while prying off the "valve/ test piece" in the vise.

[ackuric]

Yes, sharing is fun. Good write up Bob and I wish I had a mill. Just can't talk the boss into it yet. Since I really liked your idea of a "D" shaped port, I had to try it.

This is what you get when you drill a .19" hole and push a .25" HSS boring tool ground flat on one side to have a .1875" distance from flat to round.

I guess this may be plan B ?

I am curious, as to what Bob said earlier with the square porting not flowing along the sides/corners...wouldn't the same apply to the flat+corners on the D? Either way looks promising!

On the note of port configuration, I certainly like how a d shaped one looks over the mixed and matched porting of two eclipses that aren't aligned and require a transfer passage to blend, just my personal preference.

[rsterne]

For those who have never done an oblong barrel port, here is a drawing of the .25 cal barrel port that would mate up to a round 0.222" transfer port, as used above.... The barrel port is 3/16" wide and 1/4" long at the boreline.... but 0.222" diameter, and round, at the transfer port face.... The transition from one to the other is done within the thickness of the barrel wall itself....



To machine this, first set up square to the bore, and centered on the existing barrel port.... Mill a 5/16" diameter spot face for the top of the transfer port to seal against.... Only go down until the sides of the spot face JUST touch the barrel OD.... Using a 3/16" end mill, enlarge the existing barrel port to 3/16", through to the bore.... Then using a countersink, taper the barrel port until the diameter at the top of the taper is 0.222", to match the transfer port ID.... This gives you a visible witness mark 0.222" in diameter to stay within when milling the oblong port.... Using the 3/16" end mill again, move it back about 0.016", until the back of the mill is level with the back of the ID of the transfer port, and mill straight down to create the back of the barrel port.... Next, angle the mill about 20 deg. and mill the front edge of the barrel port, so that it meets the front of the ID of the transfer port, but at the boreline it is 1/4" long.... The more you angle the mill, the longer the port will be at the boreline, of course.... The final step is to use a small burr in a Dremel to taper the sides of the port, to blend the circle at the transfer port (cut by the countersink) into the oblong barrel port.... You don't need to remove much metal, and be careful not to make the barrel port wider at the boreline, as that can damage the pellet on loading.... Don't forget to carefully deburr the barrel port where it meets the bore.... and check that your bolt probe is long enough to load the skirt of the pellet flush or a few thou ahead of the front of the (now longer) port.... You may need a longer bolt probe, make it 3/32-7/64" in diameter for a .25 cal....

I have done lots of these, and it takes almost as long to tell you how to do it as it takes to machine it, once you have done a few.... There is no magic involved, the longest part of the job is setting the barrel up square and centered in the Mill....

Bob

[triggertreat]

Do you have photos of a finished barrel you've done?  Can this be done successfully without a mill using a tungsten cutting bit in a Dremel tool?

[PakProtector]

I think that the next valve I build will be using the extended seat. Cut out the old one with an endmill, and glue in a replacement. That will net the angled porting, and larger throat area. Likely throat will be .275, and PEEK for the head will be required. Or make the replacement seat out of PEEK and the stem/valve head from O1. The more I think of PEEK seat, the more I like it... and an O1 valve poppet is not so hard to make.
cheers,
Douglas

[oldpro]

I think that the next valve I build will be using the extended seat. Cut out the old one with an endmill, and glue in a replacement. That will net the angled porting, and larger throat area. Likely throat will be .275, and PEEK for the head will be required. Or make the replacement seat out of PEEK and the stem/valve head from O1. The more I think of PEEK seat, the more I like it... and an O1 valve poppet is not so hard to make.
cheers,
Douglas

Hey Douglas long time no hear!!! Glad your still at it.

[PakProtector]

I think that the next valve I build will be using the extended seat. Cut out the old one with an endmill, and glue in a replacement. That will net the angled porting, and larger throat area. Likely throat will be .275, and PEEK for the head will be required. Or make the replacement seat out of PEEK and the stem/valve head from O1. The more I think of PEEK seat, the more I like it... and an O1 valve poppet is not so hard to make.
cheers,
Douglas

Hey Douglas long time no hear!!! Glad your still at it.

Was just out with my Flex in the breeze at 100 yards. Using the trees as wind flags is less than ideal. It is a fine shooter...
cheers,
Douglas

[rsterne]

Douglas.... LTNS!.... I made a valve with a glued in seat, and the glue eventually cracked and it leaked.... I adapted Travis' solution of boring the entire back of the valve out to 1/2" ID, making an insert which contained the seat and exhaust port, and tow O-rings to seal it (each side of the TP).... I used longer valve screws, and drilled and tapped the insert, to retain it.... It's a LOT or work, but once the glue cracked (JB Weld) I had no choice....

Bob

[oldpro]

 If you have the tools you can affordabley make a Mrod valve into a very high power valve but not easy like Bobs original post that's for sure. But I think it's worth sharing again. Like Bob said you bore the value strait threw .500 and then make your push in seat any size you want. My version above has one oring to seal from HP side as my Tport has a oring cut into new seat( not shown ) In a Marauder you'd simply put a oring before and after the T port. But you need the proper tools and this is not nearly as easy to make as Bobs first post.