Author Topic: MRod .25 cal Valve Mods (Read 23422 times))

rsterne · « **on:** March 14, 2013, 06:37:40 PM »

Well, let me start by saying I don't have a Marauder.... I did, however, purchase a .25 cal valve a couple of years ago out of curiousity.... Today I decided to see how big I could hog it out.... and here are the results....

The first thing I did was remove the velocity adjusting screw and glue in a brass 8-32 screw tightened fully (partly into the port) and cut and grind it off flush on the outside.... I figured if I was going to enlarge the ports, there was no point in having it adjustable.... I did a lot of measuring, and decided that I could enlarge the exhaust port to 3/16", which is 75% of a .25 cal barrel, and should be plenty big enough to make big power.... I needed the valve throat at least that much area, so I drilled it out to 15/64" (0.234").... which when you subtract the 0.125" stem diameter is the equivalent of a 0.198" hole, about 11% more area than the 3/16" exhaust port.... That should be plenty, and doesn't enlarge the seat, so the valve shouldn't be any harder to crack open.... I then ran a 7/16" end mill down the inside, followed by a 1/2", to 1.95" from the end, just shy of the valve seat.... I then ground the end of a 1/2" drill to fit over the seat area and used it to finish the 1/2" ID hole down to when it JUST removed the anodizing on the botom of the valve around the seat area.... I then milled out the exhaust port, first to 5/32" to remove the end of the brass screw, then to 3/16", and enlarged the transfer port recess to 5/16" so that a 5/16" OD Teflon transfer port can be used.... Here are the photos....

As you can see above, I also modified the inlet plug to allow more air into the valve.... I milled two 1/4" slots in the sides at 90* to the screwdriver slot, just removing the threads.... I marked the end of the valve where those flats tightened up, and plunged a 1/4" end mill down to remove the thread as far down as I could reach with the mill (but staying clear of the 4-40 breech screw holes), tightened the end cap in, and then countersunk the end of the holes and smoothed them over with a Dremel.... Here is a photo of the assembled valve....

I also drilled the middle hole in the end plug to 17/64", and shortened the shoulder around the spring seat to 0.120" and reduced the OD inboard of the threads to 7/16", to unshroud the bottom of the new outer slots.... The total inlet area is now more than double what it was originally, and more than 3X the exit area.... In addition, the much larger ID virtually completely unshrouds the head of the poppet, removing any previous resistance to flow through the relatively narrow gap there.... The new ports are 78% larger than stock, and of course the barrel port would have to be enlarged to 3/16" as well, and a 3/16" ID transfer port fitted.... The bolt probe might also need some work....

There are a couple of areas where Crosman left VERY little material, and when you are hogging out an MRod valve, if you go too far, you will ruin it.... The two 4-40 screws that hold down the breech are 0.58" apart to the inside of the screws, so 0.50" is the largest you can go with the ID of the valve and still leave enough material around them.... The transfer port is VERY close to the back of the inside of the valve (thanks, Rescue35?), and if you enlarge the exhaust port too much, you will cut through behind the raised valve seat.... That will also happen if you cut too deep when boring the valve out to 1/2" ID.... If you try and radius the front edge of the exhaust port, where it goes around the corner into the valve throat, you can easily destroy the seat.... All I did was smooth the sides where the 3/16" vertical port meets the 15/64" horizontal throat, and JUST broke the edge on the forward side.... Even so, there is little material remaining on the top edge of the seat....

Why did I go to all this trouble when I don't own an MRod?.... Good question.... I did it so that I can understand what you guys are going through trying to free up the flow on an MRod.... Crosman left so little material in the areas mentioned, that you can actually get bigger ports in a Disco valve.... Having said that, the ports I have achieved could be done reliably and if a guy set up for it, at relatively low cost.... I see no reason why this shouldn't allow an MRod to produce enough FPE to use 50 gr. cast bullets like those from the RCBS mold for the .25 auto, or the 50 gr. Mr. HollowPoints.... I checked both of those, and they both fit in the .25 cal MRod magazine and it still rotates freely.... The 54 gr. Mr. HollowPoint will fit inside but drags, and the 58 gr. is simply too long.... I'm thinking that given what Lloyd and I achieved with a .25 cal Disco, it should be possible to get very close to 100 FPE at 3000 psi with 50 gr. bullets (OK, maybe 90ish FPE with a 20" barrel) and a valve like this in an MRod.... Who's going to be first to do it (or has it already been done)?....

Bob

ezman604 · « **Reply #1 on:** March 14, 2013, 06:46:33 PM »

Ship it, I'll try it.

Thanks for the experimentation Bob!!!!
SAFE & Happy Shooting!!!!
Dave

Rescue912 · « **Reply #2 on:** March 14, 2013, 07:02:27 PM »

^^^ What he said !

rsterne · « **Reply #3 on:** March 14, 2013, 07:11:52 PM »

Very funny, guys!....

Bob

gene_sc · « **Reply #4 on:** March 14, 2013, 07:12:47 PM »

U better just hook up ur 4500 psi tank directly to your rifle..

U might get 5 shots with your stock air tube...

Gene

rsterne · « **Reply #5 on:** March 14, 2013, 07:19:03 PM »

I got 6 shots within a 5% ES from 3000 psi down to 1900, averaging ~95 FPE, with those bullets in my Disco.... should be nearly 10 shots on an MRod at similar power.... just based on the reservoir volume.... I could be wrong, of course....

Bob

rsterne · « **Reply #6 on:** March 15, 2013, 12:44:16 PM »

I snuck a little information into this thread about using 50 gr. cast bullets in an MRod magazine.... I'm surprised nobody jumped on that discussion....

Bob

ezman604 · « **Reply #7 on:** March 15, 2013, 01:15:44 PM »

Nope, missed that. My eyes were glazed over and watering from the 95fpe number.

rsterne · « **Reply #8 on:** March 15, 2013, 02:08:28 PM »

Well that's part of the trick to get that kind of FPE.... heavy bullets.... I was surprised when I found that the 50 gr. (actually 53 gr.) RCBS, the 46 gr. hollowppint version, and the 50 gr. Mr. Hollowpoints would all function in an MRod magazine.... I don't think I've ever read of anyone trying that before.... Here is what happened in my Disco with different weights....

Note that the FPE was still rising at 65 gr.... I eventually pushed a 50 gr. bullet to 1002 fps (112 FPE) at 3000 psi during testing.... The ports were only slightly larger than what I was able to get in the MRod valve, and the Disco has a 24" barrrel, not 20".... but I'm thinking somewhere in the 90's for FPE should be possible (maxed out) with this valve....

Of course the power would have to be backed down a bit to get a usuable shot string.... but it would sure be interesting to see what you could get for an 8-shot (1 clip) string with a 3000 psi (maybe even 3200?) fill.... Makes me wish I had an MRod to try it in....

BTW, I see Hunter's Supply sell a 53 gr. round nose.... it looks like the RCBS.... $28.00 per hundred, $93.00 for 500.... You can cast for less, but that would be a good alternative for those than don't.... http://www.hunters-supply.com/shop/index.php?cPath=27

Bob

rsterne · « **Reply #9 on:** March 15, 2013, 03:46:02 PM »

If you have knowledge of a valve change in the new Marauder, please share that.... They may have moved the trigger, but unless they took that opportunity to redesign the valve (which would surprise me) then nothing would change.... If you were OK with moving the port back in the valve, main tube, breech, and barrel (not sure how you'd do that) then you could use a bigger port.... The parts you showed are more than likely the result of somebody boring the exhaust port out and ruining a valve.... That would certainly be a way to fix that problem, however.... The seat could also be made from metal (OD still sealed with an O-ring) and use a stock MRod poppet, just machine some of the back of the valve (there is enough material, I checked into this idea already) to get the valve travel back.... That would also increase the hammer travel, which is not a bad thing....

So, it is possible to increase the porting larger than I did, but only if you replace the seat in the valve, moving it forward a bit.... or move the ports back.... You have to ask yourself if that is really necessary when the heaviest bullets that will fit the magazine are about 50 gr., though....

Bob

Rescue35 · « **Reply #10 on:** March 15, 2013, 04:29:26 PM »

Just tagging to read later. Looks great!

PakProtector · « **Reply #11 on:** March 15, 2013, 05:13:04 PM »

hey Bob,
Now go and cut down the poppet head diameter a wee bit closer to the seat diameter. That goes in direction of boring the ID of the main valve body out, and since you can't go over .500, might as well take the poppet diameter down...

One of Jim Gaska's magazine covers, perhaps taken further than he does usually would lengthen the allowable bullet selection. I may try this; I have a 24-1/2" BSA barrel for my 2563..

and a 500cc tank. I am a wee bit tired of hearing about, 'get a sumatra iff you want big power, mrods can't do it'...lol
cheers,
Douglas

rsterne · « **Reply #12 on:** March 15, 2013, 05:35:00 PM »

Quote

might as well take the poppet diameter down

Not quite sure why?.... The area around the poppet is already about 4 times the throat area with the 0.5" valve ID.... I guess you might benefit from a slight streamlining, but since the spring is the same OD as the stock poppet, do you really think there is much to be gained?.... The choke point is still the 3/16" exhaust, transfer, and barrel ports....

If you figure you can get over 100 FPE from a quarterbore MRod, then you might need to go heavier than 50 gr.... otherwise, why bother?....

Bob

badredbird · « **Reply #13 on:** March 15, 2013, 07:39:17 PM »

i shot the rcbcs 50 grain casts out of my mrod was curious had the trajectory of a lobbed softball lol slowpitch leaugue it would be mice to get some more oooomph considering how accurate they are

PakProtector · « **Reply #14 on:** March 16, 2013, 10:02:21 AM »

Quote from: rsterne on March 15, 2013, 05:35:00 PM

Quote
might as well take the poppet diameter down
Not quite sure why?.... The area around the poppet is already about 4 times the throat area with the 0.5" valve ID.... I guess you might benefit from a slight streamlining, but since the spring is the same OD as the stock poppet, do you really think there is much to be gained?.... The choke point is still the 3/16" exhaust, transfer, and barrel ports....

If you figure you can get over 100 FPE from a quarterbore MRod, then you might need to go heavier than 50 gr.... otherwise, why bother?....

Bob

Well Bob, how about cutting the poppet down instead of the slightly more difficult opening the valve...if yours has gotten 4x the area, do you really need all that...can you get 2.5x by just cutting the head?
cheers,
Douglas

nevermind, I'll just get a big one...

rsterne · « **Reply #15 on:** March 16, 2013, 01:51:21 PM »

Quote

how about cutting the poppet down instead of the slightly more difficult opening the valve

The OD of the head has virtually nothing to do with the force closing the valve (that's mostly the stem diam.) and nothing to do with the force to crack the valve.... which is the sealing diameter....Changing the head OD or the valve ID is just about area for flow, and IF the area is relatively small, the associated drag through that space, IMO....

As I said, the spring is the same OD as the poppet head anyway, the best you could do is a slight tapering towards the throat....

Bob

PakProtector · « **Reply #16 on:** March 16, 2013, 11:57:06 PM »

Thanks for thinking it through with me Bob...going to have some fun modifying drills...

cheers,
Douglas

rsterne · « **Reply #17 on:** March 20, 2013, 02:55:24 PM »

I just ran the 53 gr. RN Lyman through the Border Barrels Twist Calculator.... With an 18" twist, the Stability Factor is 1.6 at Mach 1, and 2.6 at 500 fps.... The 46 gr. Hollowpoint version works out to 1.7 at Mach 1 and 2.1 at 500 fps.... The 50 gr. Hr. HollowPoint is 1.5 to 1.9 at subsonic velocities.... These are the three cast bullets that will fit in an MRod magazine, so it looks like conventional airgun twists should be perfect for them.... A stability factor of 1.5 - 2.0 is "optimum" and anything over about 4.0 is "overstablized", while 1.0 or less is unstable....

Bob

lloyd-ss · « **Reply #18 on:** March 26, 2013, 01:36:26 PM »

Bob, I just saw this thread 'cause it was going on when I was sick and moving (yes, both at the same time, yuck.)

I ran into the same situation when trying to boost the power for the .308 M-rod bottle gun I just did. Like you, and everyone else, it seems, when I first saw that big, fat M-rod valve, I thought, wow, plenty of room to open it up. But with no extra metal behind the valve seat, and the two 4-40 breech screws crowding the hole thru the valve, and the restriction of the spring retainer, I finally gave up on modifying the valve and made my own from scratch. It wouldn't be necessary for smaller calibers, including maybe the .308, but I wanted to do it anyway.
What you have done is good for over 120 fpe, but of course, I needed more power, LOL.

I didn't take a lot of pictures, but I ended up moving the valve seat forward to get a .437 O.D. transfer port, sealed with o-rings and with a .301 hole. I opened the front end of the valve up as big as I could and made a spider to capture the front end of the valve spring. The main restriction in the system is the nose of the flow-thru bolt. The breech is now attached to the tube with four diagonal 4-40 SHCS that are threaded into the air tube, not the valve.

I didn't have the gun long enough to really play with the tune, but hopefully, I will soon.
Lloyd-ss

.328 valve throat, .312 exhaust port

The front of the spider isn't too pretty with all the hand work done late at night.

.308 M-rod bolt. The .238 hole thru the tip of the bolt will be the choke point, and I don't know if it will be a problem or not.

rsterne · « **Reply #19 on:** March 26, 2013, 08:50:23 PM »

Looks good, Lloyd.... I know you can make a bigger custom valve, I just wanted to see what the limits are on modding a stock one.... If it will reach 100 FPE in .30 cal that would be great!....

Bob

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MRod .25 cal Valve Mods

Author Topic: MRod .25 cal Valve Mods (Read 23422 times))