GTA
Airguns by Make and Model => Crosman Airguns => Topic started by: Rivers3Plinker on September 11, 2014, 12:01:21 PM
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So my initial thoughts when shooting this gun lead me to believe piston stuffing by itself will make a big difference in power and it's the first thing I plan to do because I can do it quickly and easily without having to tear the gun completely apart. Knowing it makes a difference is key though. I have a Caldwell Balistic precision in my Amazon cart and will probably order it today. Been needing one for a long time anyway.
That said, the reason I think stuffing the piston will make a big difference is:
- Crosman advertised FPS is significantly higher than what users are reporting. Perhaps they calculated this value or it's the old alloy pellet rating trickery that seems pervasive for all ratings these days. I can't say for sure but I will check mine before and after
- After about 8 pumps I can feel pushback or "spring" from the pump arm similar to what I've felt with a mis-adjusted (needed to crank the piston out farther ) FT piston in my 2289 guns. I believe there is significant head space between the pump cup and the valve face because the piston is deflecting or compressing at around 8 pumps
That said, I have tried ( only once ) pumping this gun to 15 pumps and it dumps it all, although I have my doubts how much I was really adding past about 10 to 12 anyway. Don't like doing this without strengthening some things.
I took a tape measure to piston and it measures about 4 inches OAL. I have no experience with the 2100 or any other guns with which this might share components so it anyone has a guess, feel free to share where Crosman sourced this piston.
I'm going to use either 1/8" brass rod or plain steel depending on what my LHS has in stock.
Finally I noticed the pump cup is gray on this gun and different looking than the ones that came with my 2289g. Not sure what's up but I imagine it's a newer material, more durable perhaps or reduced friction skirt or something.
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well it is not a 2100 piston like it looked like it might be in the evp it looks to have its own piston, the 2100 piston is ~6" and the 760/66/13xx etc all share a piston that is ~ 2.5" long.
I think the extra length is just to be able to use a longer pump arm with the 3.5" stroke it shares with the 760/13xx etc.
Longer lever = reduced effort thru better leverage.
That feeds a valve that looks to be larger than the 2100 valve but smaller than the 13xx valve so it is meant to build pressure quick.
My 18.7" barrel 1377 tops out about 750 fps with a .13ci valve so I am guessing these with stock valve volume will top out around 680-720 fps or so with 7.9g pellets after mods.
stuffing the piston will not do quite as much until the valve and pump cup match of course.
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This post seems to have as much or more technical stuff on the MK than the rest so I figured I'd add my findings here since I just got mine today...
I like it, feels good, less awkward than some people make it out to be... pump with left hand while holding pistol grip, butt pad in your right armpit if it makes it easier, cock with left hand and push the clip over to the right (opposite of normal) for the first 4 shots, reach over on the 5th shot and pull it to the last detent, fairly natural and reasonable balance.
Now for technical information, I didn't try the chrony but might this weekend. I lined up 4 Crosman pumpers with the pumps open side by side, the piston is about 1/2" shorter than a 2100 piston, but the pump linkage is the same as a 1377... No I didn't disassemble it to look close or take calipers to it but form the looks of things it's the same as the 13XX/760 platform stroke wise. I have the MK, 2100, Recruit, and 1377 all side by side and that's what I noticed.
If I get motivated to finish my garage as good as I can this weekend I might finish my M4-177 then might modify this MK...
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well it is not a 2100 piston like it looked like it might be in the evp it looks to have its own piston, the 2100 piston is ~6" and the 760/66/13xx etc all share a piston that is ~ 2.5" long.
I think the extra length is just to be able to use a longer pump arm with the 3.5" stroke it shares with the 760/13xx etc.
Longer lever = reduced effort thru better leverage.
That feeds a valve that looks to be larger than the 2100 valve but smaller than the 13xx valve so it is meant to build pressure quick.
My 18.7" barrel 1377 tops out about 750 fps with a .13ci valve so I am guessing these with stock valve volume will top out around 680-720 fps or so with 7.9g pellets after mods.
stuffing the piston will not do quite as much until the valve and pump cup match of course.
Interesting - I wonder why Crosman would use such a large valve then on a 1377 with such a short barrel as opposed to a gun like this or the 2100? Perhaps a manufacturing / production compromise so the valve works OK with both calibers of the 1377 and 1322? A little too much volume for a 1377 but adequate for a 1322?
I put a calipers on the piston last night and carefully cut and sized 4 sections of 1/8" steel rod for stuffing. I'll also make some for the short little top section of the piston and hopefully have a chance to install this weekend. I promise pics!
I debated using brass instead but I plan to epoxy them into the piston and don't see any reason to worry here.
Ordering chronny today.
My plan is to just stuff it and then shoot it. Get some objective results until the chronny arrives and then some real numbers. I'll shim the cup after that and see how goes. After that, my plan is mild mods to the valve, primarily softer spring and only minor work to the exhaust port for better flow. I'd like it to have the numbers for pellets you've posted. I'm not sure it's even 600fps at this point.
Another thought when modding this thing. Looking at the EVP it looks like the transfer port assembly that goes on the end of the barrel is the ONLY thing supporting the valve in the tube. There's no grip screw opposite of the detent in the tube like on the 13xx. I am considering pinning the valve depending on what I find when I finally get it apart this far.
One other idea I had was to mail the piston and valve off to AC Customs and have him make me a nice aluminum flat top and face the valve for me. With the additional leverage here and a stronger pivot pin, I think this may be the path to more impressive numbers out of this gun.
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This post seems to have as much or more technical stuff on the MK than the rest so I figured I'd add my findings here since I just got mine today...
I like it, feels good, less awkward than some people make it out to be... pump with left hand while holding pistol grip, butt pad in your right armpit if it makes it easier, cock with left hand and push the clip over to the right (opposite of normal) for the first 4 shots, reach over on the 5th shot and pull it to the last detent, fairly natural and reasonable balance.
Now for technical information, I didn't try the chrony but might this weekend. I lined up 4 Crosman pumpers with the pumps open side by side, the piston is about 1/2" shorter than a 2100 piston, but the pump linkage is the same as a 1377... No I didn't disassemble it to look close or take calipers to it but form the looks of things it's the same as the 13XX/760 platform stroke wise. I have the MK, 2100, Recruit, and 1377 all side by side and that's what I noticed.
If I get motivated to finish my garage as good as I can this weekend I might finish my M4-177 then might modify this MK...
I agree I like the action of the gun and the way it feels. It's really fun to shoot and the slight awkwardness of the bolt on the left for me is actually kind of fun. It's just different in a good way.
BTW, here's something to try that really made it easier to pump for me. I noticed the sides of the pump arm rub on the forearm of the gun where it overlaps. There's small catches molded into the forearm to hold it too. I sprayed this area of the forearm on both sides with some food grade silicone spray and WOW. It immediately made it easier to pump it. You could use almost anything like furniture polish or automotive detail spray here too. Just need some clear, clean, plastic safe lubricant. Just try not to get it on the outside of the pump arm obviously.
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I just took the tip of my pocket knife and scraped the catches down a little, it's a little less harsh now, not a big deal
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Found this thread it has links to fps some numbers.
http://rimfirecentral.com/forums/showthread.php?t=510608 (http://rimfirecentral.com/forums/showthread.php?t=510608)
the Thing about the clip feed is how do you get the pellet to seat just past the port? Imo worth about 15-25 fps...
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Steve; measure the pump tube OD please. The EVP's O-ring callout makes me think they used a larger pump and valve to get the increased velocity with the same stroke.
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On another note, thanks for the tip on the Amazon deal Steve, they're apparently out of them now for the price we got, they're at $50 shipped now instead of $40...
I dug out a pair of calipers but I don't feel like taking the thing that far apart at this point in time, maybe in a few hours if Steve doesn't chime in, several parts have to come off before you can measure the outside of the tube, from a rough guestimate it looks like it might be a little bigger... If it's bigger, that might lead to another source for upgrade parts for certain platforms :), might have to make a parts order...
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Reading the EVP, the outer valve O-ring is 3/4 OD, so the tube may be 7/8x.058-.065. ;D Would be interesting to upgrade a 2289 with a MK tube. 8)
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By my rough guesstimate I think you are correct on it being 7/8" OD as I got about .2" deeper measuring down through the slot, and the slot is about .040" narrower on the MK, which equates to about .125" difference in diameter if you take into account that all my measurements were rough :)
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Reading the EVP, the outer valve O-ring is 3/4 OD, so the tube may be 7/8x.058-.065. ;D Would be interesting to upgrade a 2289 with a MK tube. 8)
Bingo we have a winner!
It's odd because the valve is bigger, but it's mostly aluminum and looks to be small volume. Staring down into the back half at the poppet, it's pretty obvious. A lot of work could be done with this valve if adding volume was a goal.
Couple of key things I found.
The EVP shows most of this but.....
Indeed the pump assembly is much larger diameter than the 13xx.
Beyond that, the gun is essentially two plastic halves screwed together with a large number of small screws. Support for the pump tube consists of holes at the rear of the tube which go over posts molded into the halves of the gun frame back by the trigger assembly. Not the strongest looking arrangement for going all out, crazy town pumper but there's other reasons I don't believe that's much of an option with this gun. The front of the pump tube is held with the flash hider section and the pivot pin. The barrel band is part of this section and it holds the barrel very well.
The interesting thing ( to me ) is the transfer port seal and breech or receiver setup. A flat o-ring type gasket shoved in a counter bored pocket at the exhaust port. The barrel butts into a plastic breech like part which has tube off the bottom of it and this basically just sits on this seal. This breech part is held in position by posts on either side of it which go into holes molded into the frame of the gun right above the barrel. There's all sorts of wiggle room here. It holds it down OK when assembled but I think there's room for improvement here for better seal.
When I took mine out, I noted the seal was only contacting the TP tube from this breech piece about 70% and I'm sure air was blasting past it each shot. I suspect the valve was not accurately indexed 180 degrees from the bottom in the tube for a perfectly flush seal. The seal had a deep impression in the area it was contacted by the breech ( crooked ). I flipped the seal over before putting it back together and made sure to get the valve perfectly straight up.
Mods I made were:
1. Put a small o-ring from a 1322 bolt over the TP on the bottom of the breech. I'm not sure if this is helping much but there seemed to be all sorts of slop here and the o-ring took up a good deal of it.
2. Disassembled the valve and cut down the stem a bit behind the head on the poppet.
3. Angle ported the exhaust with dremel and smoothed with some fine emery cloth strips.
4. Cut 1/8 steel rod supports for the piston and super glued them into place.
The piston looked a bit bent to me. Not much I could do about that.
The cup seal cannot be easily shimmed because of how it attaches to the piston and I elected to leave it as-is.
One could pull it off and maybe silicone a washer under it. I tried putting one in it but fought with getting the seal fully seated and didn't feel like it would stay without glue. Left it alone.
I successfully reassembled the trigger and safety assembly after some fiddling and then finally the whole gun.
First impressions - again without a chronny is..... the gun lostpower!
I was disappointed obviously. It will however dump 15 pumps and that works really well. It feels like it's able to pump more air into the valve due to the reinforced piston but I'm anxious to measure it with chronny to see.
Sorry for the lame pics. Also no pics of the modded parts but I was rather anxious to reassemble and shoot it.
Opened up:
(http://i575.photobucket.com/albums/ss198/braumeisterwurf/20140912_135922.jpg) (http://s575.photobucket.com/user/braumeisterwurf/media/20140912_135922.jpg.html)
Piston is around 5.345 long or so with seal.
(http://i575.photobucket.com/albums/ss198/braumeisterwurf/20140912_141642.jpg) (http://s575.photobucket.com/user/braumeisterwurf/media/20140912_141642.jpg.html)
Port:
(http://i575.photobucket.com/albums/ss198/braumeisterwurf/20140912_141008.jpg) (http://s575.photobucket.com/user/braumeisterwurf/media/20140912_141008.jpg.html)
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Support for the pump tube consists of holes at the rear of the tube which go over posts molded into the halves of the gun frame back by the trigger assembly. Not the strongest looking arrangement for going all out, crazy town pumper but there's other reasons I don't believe that's much of an option with this gun. The front of the pump tube is held with the flash hider section and the pivot pin. The barrel band is part of this section and it holds the barrel very well.
The interesting thing ( to me ) is the transfer port seal and breech or receiver setup. A flat o-ring type gasket shoved in a counter bored pocket at the exhaust port. The barrel butts into a plastic breech like part which has tube off the bottom of it and this basically just sits on this seal. This breech part is held in position by posts on either side of it which go into holes molded into the frame of the gun right above the barrel. There's all sorts of wiggle room here. It holds it down OK when assembled but I think there's room for improvement here for better seal.
When I took mine out, I noted the seal was only contacting the TP tube from this breech piece about 70% and I'm sure air was blasting past it each shot. I suspect the valve was not accurately indexed 180 degrees from the bottom in the tube for a perfectly flush seal. The seal had a deep impression in the area it was contacted by the breech ( crooked ). I flipped the seal over before putting it back together and made sure to get the valve perfectly straight up.
Piston is around 5.345 long or so with seal.
(http://i575.photobucket.com/albums/ss198/braumeisterwurf/20140912_141642.jpg) (http://s575.photobucket.com/user/braumeisterwurf/media/20140912_141642.jpg.html)
The pump tube design goes all the way back to the 60's, all of the clamshell guns are that way. The T-Port seal/breech design goes back to the '77 variant 760, and is used in all clamshells now. The best way I've found to seal these is a delrin or teflon piece, made with a step to fit into the valve, and over the breech nipple. My hot-rodded 766 and the 2100 and 2200PCP's use this system.
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Excellent - I'm still learning obviously so thanks Bart.
I found I damaged the poppet. The gun seemed weak, and when I pumped it I noticed if I let it sit a few minutes the valve lost all pressure.
Thankfully one from a 1322 valve works fine and is almost identical. I say almost because the one in the MK valve had a black head on it instead of white, and was about .010 smaller diameter. Everything else was the same. I wasn't sure if this was critical or a MFG tolerance difference but it fixed it and it shoots fine now.
I do not recommend repeated disassembly and reassembly of this gun because no matter how careful you are, you will eventually wear out or break or strip one or more of the many screws holding the halves of this thing together. Same goes for the pump arm which also has three screws and must be removed each time to take down the gun.
I'm leaving it alone for awhile and will chronny when I get one
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did you get any valve measurements?
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Sorry guys I missed the posts requesting the valve dims! I'll at least measure the tube for you tomorrow and post.
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It's a .75 tube like Bart said.
Spent an hour sighting it today. Frustrating. POI would be consistent and I'd get the red dot set, sneaking up on perfectly sighted, then it would move like over an inch and just stay there. Resight to new POI then it would move again. This is at about 10 yards.
Started getting a blister on my thumb from the pump arm and called it a day. I'm going to try redoing the crown, improving the seal between valve and barrel, and making sure the front barrel band has no slop with the screw tightened. I thing something is moving and/or not sealing properly.
Bart any tips or ideas for that Delrin seal you made? Wonder if I could make something from nylon spacer from the hardware store.
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I waited a few days to order that chronny and it went down in price by almost the same cost as the MK177 recently went up.
I decided that justifies ordering another MK177!
My Dad shot the MK this weekend and liked it. Has more power than his POS Winchester pumper with smoothbore barrel no less, and he liked the clip for the pellets. He also really liked the red dot which makes sense since he's shooting through one of those tiny little Tasco $12 scopes.
Had him shoot my modified 2289 with 1701p trigger and he kept shooting it early by accident calling it a "hair trigger". He also felt the .22 caliber and additional power was over the top for shooting pest birds. All this conversation was not contextualized around his upcoming birthday ( he has no idea ) so the feedback was excellent information.
Based on his obvious preference, I decided an MK177 is probably a better route for him than a custom 2289g.
Having it shipped to my work so my wife won't see it. ::)
Going to hang on to the 1701p and barrel parts, build myself yet another long barrel 2289g and some day giving one to each of my twin boys for a birthday in a few years most likely.
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Ordered a new valve, left side pump arm cover, and piston from Crosman yesterday.
I decided I wanted to replace the cover I damaged as it only has three screws and needs all the support you can give it.
Was only $4.00.
I'll take pics and provide dimensions for the valve.
I plan to work over the piston I reinforced that's in the gun now, and do some shimming of the cup.
I bought a replacement just in case I screw it up.
I'll chronny it before shimming and after. I'm curious to see what the exhaust port work has done so far comparing numbers to both published by Crosman and those from others who have chronnied the thing out of the box.
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Very interested but very busy/worn out till about Sunday...
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OK preliminary chronno numbers are:
10 pumps avg 598 fps
15 pumps 649 fps
For kicks I tried 20 pumps a few shots through the chronny and got 650fps or so each time. So.....that was disappointing! It held air too.
I will try shimming the piston cup and eventually throw a poppet with a reduced shaft in the valve too. Exhaust port work doesn't seem to have accomplished much and same goes for Reenforcing the piston.
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Ok, I'll try to chrony mine tomorrow if I get a chance...
I might just have to order a spare valve for this guy (and tube just in case things work out... for future projects, I have a 1322 and 2289 that are unmolested...) and make a flat top piston and such, see if I can duplicate or exceed the results from my Recruit modifications, if I could get the same results on this as I did on the Recruit I'd be very pleased... two 750ish fps shots from around 18 pumps is nice, would be even better if it held air for more than 2 hours unlike the Recruit though :)
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Hey S&S (Steve and Scott ;) )
I did a little math and it makes me wish I had more $$ had to spend more than a bit on my daughters( they are worth it college students
need help) just so I could be the first to turn the powerplant into a 1377/22/25WT... (WT=wide tube)
The pump has just a touch more swept volume than a 2100 :o
So what that means is if you can get about .12-.13ci out of the valve and solve the leaks and get the piston good enough you should be able
to match my 1322XLTs performance -(~5" of barrel) but being it .177 not .22 not as big of a deal
point is things are pointing to being able to get close to or beating the 800 fps advertised with 7.9s @ 11-12 pumps depending on valve
volume, valve spring and striker spring...
It just may be the Best platform to get a large valve volume as a 13xx/MK177/xx40 hybrid but that depends on how much the valve can be bored and how far the punched ledge in the pump tube that acts as a stop for the valve is from the back of the tube.
:D :D :D
I wonder if the Benji valves are threaded the same as these...
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Received my extra valve from Crosman but it's the wrong one. Not sure if it's my fault or Crosman parts yet but the one they sent me is not the larger diameter, it's smaller size same as a 1377. They did send me the correct piston so I'll have a backup in case I screw up what came with the gun.
Haven't had any time to play with this gun lately but next time I take it apart I'll get some measurements from the tube and post. Seems like a good candidate for a super pumper.
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Tore it down and setup the piston with a .060 thick washer under the cup for optimal compression. There was a lot of lash in this gun. Stock piston seems a little bent to boot. Tried sveral thinner washers before settling on this thickness making valve contact about 1/2 inch before fully closed. I fixed it in place by siliconing the cup and washer. I epoxied all the steel reinforcement rods that I'd installed in piston before because several had fallen out. Cyanoacrylate glue doesn't hold well here.
I reduced the poppet stem in the valve per typical mods for flow. No other mods other than the previous exhaust port angle porting. Put everything back together and took it apart and back together a few times. Had some problems but its working.
No chronny numbers yet but its much harder to pump at 8 pumps and beyond. Seems louder and maybe a bit stronger but I will confirm with numbers soon and post.
Not expecting to hit the advertise 800fps anytime soon and frankly I'm done with this thing. Moving on to another 2289 project.
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More information:
Per Bart's knowledge of this gun's 760 based power plant, the check in the valve is way up in the nose and no way to make a flat top with it. Best bet would be a custom machined, flat faced nose piece based off the stock part. This short section is threaded on the valve body. Making a custom flat faced valve and keeping as much pumping volume as possible would require positioning the check farther back in the nose and reduce volume in this section of the valve. To make up for that, you would ideally need to enlarge the back half through machining. At least I would think so anyway. There's a lot of "meat" in the back half of the valve and the cavity has very thick walls. One could easily turn this out or drill it out on a lathe. It would also be a good idea to pin this valve if you were going this route. Both of these mods would work well in the quest for a MK tube based super pumper 2289 IMO.
Since I've shimmed the piston significantly, in retrospect, I should have pinned my valve. There's ample room to do so forward of the exhaust port about 1/2 inch or so. Maybe in the future after I chrony it and see how it's shooting.
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I'm just learning how to make this chrony work and of course I shot the thing right in the display panel! Fortunately it missed the LCD and the thin factory plastic shield slowed the shot enough to save the sensor behind it. I made a new, supplemental shield out of some 1/8 polycarbonate I had handy for future stupidity. That should stop anything I can throw at it in my garage although I plan to test that soon just for fun ( not at the chrony ).
Looks like the mods pay off at the higher pumps as expected.
CPHP pellets
10 pump string ( avg 593 fps) :
12 pump string ( avg 630 fps) :
15 pump string ( avg 670 fps) :
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It looks like I was close on where it would top out with out enlarging valve volume.
did you measure the transfer port/breech size I usually open to .140 on .177s I think stock the are usually .125 or so
Hey I forgot to ask are they straw barrels or 7/16" barrels?
from your numbers it looks like the stock valve volume is about the same as a 2100...
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It looks like I was close on where it would top out with out enlarging valve volume.
did you measure the transfer port/breech size I usually open to .140 on .177s I think stock the are usually .125 or so
Hey I forgot to ask are they straw barrels or 7/16" barrels?
from your numbers it looks like the stock valve volume is about the same as a 2100...
I did not measure the TP but it definitely looks larger than .125 and I'm almost positive it is. I think it's same as stock 1377.
Crosman is using 7/16 barrels in this gun.
This gun also has a shifting POI issue now and I'm not sure why yet.
Sight it in, shoots great groups and then suddenly starts shooting almost an inch or more higher. Seems to happen when I repeatedly pump to 15 pumps and shoot for awhile. Is not gradual at all, so that leads me to believe the barrel is moving somehow, or something is shifting around inside. Maybe it's heat related and the pump tube and valve are heating up or something.
Also I tore the factory seal on the TP to the barrel adapter when I removed the valve last time. I made a replacement out of a short section of 1/4 poly tubing heated with a lighter, shoved on the barrel adapter and back filled with hot glue. Sanded down to the correct height and seems to be OK but I may take this thing apart again and rep that off. I have another factory seal for it now.
Still a fun gun to shoot but the shifting POI drives me nuts. It was doing it before to modded it too.
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My 66/664 was doing the same thing very accurate and then bang new poi. messed with a few fixes and it helped some but the plastic receiver just liked to flex.
I got tired of it so it became my 1322MLT...
stock t port on the 1377/22 is about .135"
on my 760, 664,and 2100 it was about .125"
Been putting thought into how to turn an mk 177 into a 13xx wide tube
I have an extra linkage with backpacker pump grip and grip frame and plastic breech...
so I looked at just buying parts and estimated $
$ part & notes
3.00 piston & cup ?
7.00 pump tube going by 66 and 2100 cost
18.00 barrel but can choose .177 or .22
5.00 valve 13xx price
7.00 front barrel band/sight read it
4.00 Shipping
44.00
walmart tan Mk 177 currently $43.40 and will still have extra linkage but will be .177 and about 16" barrel after cutting and re chambering...
other parts possibly needed
2240 striker mk 177 may be to long for 13xx conversion like 66 and 2100 will work but limits power...
2240 end cap
2200 striker spring ?? do not know about mk 177 or 2240 springs
lighter valve spring I accidently have a stock one very stiff
What may stop the switch or make a larger valve possible;
the hole in the pump tube for the transfer port needs to be at least 3" back edge of hole to end of pump tube (13xx distance) if longer the tube can be cut or long plug used or best can make larger valve...
hmm am I forgetting anything..?
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well I hope you are proud of your self Steve I now have an early(slightly its next month) Bday present coming... just to see what I can do with it...
It will be here next week...
Enabler... ;)
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Glad I could be of help to you sir.
I have completed my third 2289g build last week myself, and have a 2nd MK177 new in a box as well.
Help......me......I cannot stop! They are actually gifts for later this year though.
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Well mine should get here today ;D
I also found this on ebay and copied the picks nevermind the ebay price...
http://www.ebay.com/itm/like/321555355165?lpid=82 (http://www.ebay.com/itm/like/321555355165?lpid=82)
what matters is barring something weird it does look from some rough estimates that it will be pretty easy to hog the valve out to get about twice the valve volume of a stock 13xx valve or more.
This just may be the pump tube I have been looking for... to get the .2 to .23 ci I have been wanting for a .25
if I can hog just an inch of it out to .55 I.D. pie are squared... ;) (I should probably leave more wall thickness maybe .125-.140 but make up for it with length)
3.14x .2752x1 = .2374 ci. (wall thickness .095)
3.14x .252x1.2= .2355 ci (wall thicknes .12) (same as hogging 13xx to .4 has not exploded valve yet)
etc.
So if we got the pumps bore and stroke right..
.75x 3.5
swept volume is 1.545 C.I. ( the 2100 at .62" by 5" has a swept vol of 1.508 C.I.)
so
with a valve size of and atmospheric pressure of 14.3 (and 60% or 70% pump eff.)
.07 = 315.7 psi per stroke, x10 x .6=1894 psi, x10 x .7=2210 psi
.13 = 170 1020 psi 1190 psi (my 1322XLT 700 fps at 11 pumps or 1122 to 1309 psi)
.2 = 110 663 psi 773 psi
so the large valve will take 20 pumps to get 1546 psi at 70% efficiency
but
it should really get a .177 zinging maybe even 900+ with 7.4gr
800 + for 14.3g .22
and 725+ for 20g .25 ( crossed fingers and even higher hopes of 775+) ;)
it all depends on what can be achived for valve volume... a 13xx valve is about the same length and .13 slimmer and I get ~.13C.I. from them...
???
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well mine got here and I think the sights not so good... at 10 yards the front sight need screwed out far enough that it has a bit of movement so at 20 yard it just might be solid but its raining and son came home so can not shoot in the house.
The flip sight on mine is drilled cockeyed bad enough that using it moves the impact point to the side 1.5" @ 30' can you say useless...
Fps test and further impressions later...
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Well Steve the piston on mine is straight and here are its #s out of the box...
it is the avg of three shots for me it is not comfortable to pump I do not mind high effort but the corners are not enjoyable.
it does seem that I got one that the fit is pretty good and the piston is not bent I do not know if getting some rod in there will improve things like the piston did on the 2100 or stuffing did for yours.
pumps 7.9g 7.4g
6 520 541
8 559 590
10 612 642
12 632 672
14 675 692
with the same pump vol as a 2100 if the valve vol is the same it should be within 30 fps or so (mk has about 3" shorter barrel)
2100 pre metal piston with 7.9g, 7.4g was about + 30 fps
6 548
8 576
10 605
12 600
post metal piston same pells
6 610
8 664
10 710
12 737
15 762
I think that they will shoot fater even without enlarging the valve(will know in a while when I open it up)
I think that porting might be holding it back also...
I think I will see if a 2100 striker spring will fit, below are some screen shots of shots and it looks like hammer bounce is not allowing it to dump cleanly.
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Very interesting. Yes I know what you mean about comfortable to pump - works the skin on my thumb a bit after lots of shots. Gloves are recommended.
The bounce is interesting too - what are you using to determine that? Looks like a sound recorder on a PC with a mic by the gun perhaps?
I have both new, straight, MK177 piston assembly and a 2100 hammer spring. I'm curious what the spring might do, especially considering it dumps all the air even at 15 pumps. I have to go past that for it to hold air and don't see increased FPS of course.
Currently the slightly warped piston which has been stuffed also has a pretty thick washer under the cup and it's making contact with the valve face with about 3/4 inch of clearance before the pump arm is fully closed. Not sure I would gain much replacing it with new.
Looking forward to your findings and opinions on this gun. My priority with it now is to fix that shifting POI which caused me to completely miss a chipmunk yesterday ( again ). It's shooting low now instead of high. Gotta take it apart and think some good ways to solidify the barrel and pump tube together and in one place inside this clamshell.
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The screen shots are from Soft Chrono before I downloaded it I used Audacity and then did the math for fps...
A 2100 hammer spring will not help the mk177 spring is about .2 longer and otherwise a 2100 spring.
I added .3 of preload by slipping a collar over the spring guide portion of the end plug, made no difference it is still bouncing.
The problem is the valve spring it is a monster way to strong about twice as long as a 13xx spring( I in my last order accidently got one instead of a valve nose).
The .177 is really helped by that initial blast of high speed air so if it is not dumping cleanly it can be costing acceleration by letting the velocity of that initial pulse drop. so I think it needs a lighter valve spring...
The transfer port/barrel support... You are right it is .140" and so is the exit hole in the exhaust passage.
I only tore down far enough to try the striker spring preload so I still do not know the id of the exhaust throat or valve volume...
Sorry I did not get back to you yesterday.
October 23 1991 there was a full moon and the below math major was born about the time Lady Luna Stepped in front of King Sol yesterday on her Golden Birthday. I told her it was the universes way of letting her know just how special she is... ;)
yeh I know sappy but my saving grace is the eclipse was watched thru a welding helmet...
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Ok back on topic (just had to let you know I had important things to do yesterday)...
Fixing the wandering point of aim can be a pain on these plastic clamshell rifles (me I do not want my pellet rifles to remind me of a military weapon so it will become a 13xx and will get fixed that way)
I also do not like what having the sights 2.6" above the bore does to a short range pellet rifle.
Point of impact and a .5 inch killzone 610fps 7.9g CPHP 20 yard zero... sights at 2.6" and .5"...
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now the good if you can make a valve extension you have up to an inch of length you can add that means plenty of valve volume and maybe another option I will bring up again after I tear it down again this time for valve/valve spring mods.
when I turn this one into a 13xx I will get another barrel this one is already set to go right in my 66/760 to replace the straw barrel who knows it just may cure the wandering point of aim...
But back to the pump tube
as a tube for a 13xx you can move the breech back up to an inch or shorten the tube to stick with a stock MK177 valve...
point is it should get enough valve volume for a .25...
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Ok I found what I was looking for:
http://www.network54.com/Forum/581291/thread/1210733453/Discovery+tube+size+and+2260+tube+size- (http://www.network54.com/Forum/581291/thread/1210733453/Discovery+tube+size+and+2260+tube+size-)
I think this means that this valve and piston and cup can be used with a 2100 lever assembly in a Disco tube...
another slight possibility is a 66/664 lever assembly in this tube will check when I tear down this afternoon...
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When I tore mine all the way down it did in fact fit very well the valve and piston just barely make contact it will only need about a .03 shim I think...
I stuffed my piston with some large nails...
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" another slight possibility is a 66/664 lever assembly in this tube will check when I tear down this afternoon..."
it is a no go I forgot about the stupid hole they put in the side of the pump tube on these clamshell guns...
Its to bad would have been an easy route to another .7" of pump stroke :(