Mk 177 mods ? 800+ fps with 7.9g SUCCESS 831 fps @ 16 pumps now 1024 fps @ 20

[K.O.]

 The Mk 177 has a Barrel length of  16.75".  I have my 1377 hitting 755 fps on 20 pumps  with a 18.75 barrel and ~.12 ci for valve volume.

 If I put a  that .177 barrel on my 2100/13xx hybrid I am pretty sure with its ~.13 ci valve it would be hitting about 760-790 fps with 7.9g CPHP @ 11 pumps and 800+ @15...  Based on 750 fps @ 15 pumps with 14.3g CPHP... it does have a 22" barrel but I feel it is not as important going down to .177)


Ok so stock here is what my MK was getting

pumps    7.9g     7.4g

6            520      541
8            559      590
10          612      642
12          632      672
14          675      692

I did do a bit of preload on the striker spring and retested and no change  I even did one 20 pump shot but only got 630 fps...

I measured the transfer port size and  it was .140", so I knew that was not holding it back... 

I had looked at the sound of the shot and saw striker bounce and had a MK  valve spring  well I put 2+2 together and figured it was a part of the problem...

In my mind we were dealing with a valve size of about  .07- .1 (stock13xx, Stock 2100) so I thought the valve not dumping cleanly was really costing... 

I figured just get the right spring in the valve and maybe add a bit of volume(if possible)  and all the other mods and should be able to get about 800 fps.




Well,

 I finally took the Mk's valve apart...

WOW I found the valve that I have been hoping for stock minus the poppets(but not spring) it is about  .17 ci and with all out work (reduced poppet size and bore rear portion of exhaust side out to .5 from .4 (about a .63 section)) it looks like a ~.25 ci valve is possible... and that is without making an extension...  and at .74+ for an O.D. it looks like it would fit in a Discovery tube...

Since it is staying a MK for a while before becoming a hybrid and it will stay a .177 I took up some space with an extra  .5 size intake poppet
to get a valve volume of about  .15-.16 (less pumping than .17)

this also allowed me to use a 13xx sear spring to replace the MK's over done valve spring (stronger & longer than 13xx striker spring,to limit power?)

next I looked at the valves exhaust throat it was .2" and the exhaust poppet shaft .12" since they determine flow out of the valve...

not good...

you have a .8 throat feeding a  .14 transfer port  you would be better off with a small transfer port to keep charge velocity  high... and with a large valve it makes no sense other than keeping velocity down...

The large valve can afford to eat air and still be a faster charge velocity than the choked setup that is stock IMO...

So,

Bored the exhaust throat to .220" and turned down the poppet shaft to .95" so now I have .125" feeding .140"

I then rounded the corner to help flow...

Last time I had it apart I pulled the magnetic bolt probe out until it seated a CPHP just past the breech port and it still clears the clips...

it IMO is way to fat at .11 in a .177 bore it is blocking almost 60% of the bore...
as soon as I get done typing I will be replacing it   with a thinner steel probe of about  .06-.07...

almost forgot  I stuffed the piston with some large nails  I forgot what size...   20d..?

So I have reassembly and testing ahead of me... 10 min to do the job it is as simple as the 760... But testing will have to wait till late today or some time tomorrow.


Any one  want to venture any guesses as to how it does  at  6,10,14, 18, 20 pumps.... with 7.9 CPHP

14.3 for atmospheric pressure

pumps       psi @70 eff      @60 eff          My optimistic fps

6                  580               497               610
8                  773               663               670
10                967               829               730
14               1354             1160              790
18               1740             1492              825
20               1934             1658              850

wish me luck

[K.O.]

well had a little time for testing but had to stop with only one shot at each and did not get 8 & 20 pumps


pumps             fps

6                   577
10?               623        (might have miscounted)
12                 708
14                 752
16                 776
18                 798

I am still getting hammer BOunce  (below pic at 16 pumps)

so I will be taking it down one more time,  I will remove the collar that I put on the spring guide portion of the end plug for spring preload and  smash one or two Jsb 25g Kings in the back of the  striker for preload and weight...

Maybe it will help..?  Or do I just need 5 more inches of barrel?

Or both

[K.O.]

Just hit a soup can bottom almost dead center at 30 yards 14 pumps with 7.4 points and it was with my 2nd shot first trying to hit the can...

because the sights are 2.6" above the bore and the fact that I was zeroed for 10 yards with 7.9g CPHP (pre mods)I knew my 30 yard shot would be high

so I used my first shot as a ranging shot and it hit dead center about 6" high dropped my aim point and bam almost right on...

Long time since I used the m16 style peep and post but I guess I still can...

[N6CRV]

KO, sounds like you are having all kinds of fun today

[K.O.]

yep I enjoy figuring them out as much as shooting with them once I get them shooting as close to their best as I can...

[K.O.]

I forgot to shim the pump cup and wonder just how much it is costing at the higher pumps I am pretty sure it is compressing enough to cause a bit of headspace

I also did not sleeve the intake passage but with a big valve well it might be .06% or just a few fps.

I think it is not to bad but have a feeling that the above and striker bounce just may be costing me 20fps  or more...

it goes from gaining 42fps to 22 fps per pump to 11fps per pump rather quick.

I will test some more tomorrow...

and then most likely try the fixes  if the above # are confirmed.

I was going to wait to get the 2240 parts to turn it into a 1377wt  but my curiosity  gets the best of me...

[K.O.]

an oops I made pointed out by Gippeto at Capof.

---------------------
Just a small point...you're not actually comparing port area.


.200" throat and .120" stem have an effective area of .0201 sq.in. 

.140" transfer port has an effective area of .0154 sq.in.

.220" throat and .095" stem have an effective area of .0309 sq.in.

Keep after it.

Al


-----------------------
The embarrassing part is I am the one talking about,  pi are squared...

I do suffer from concentration probs and it shows sometimes even on the basic stuff...

I do still think opening the exhaust throat to .22 from .2 will help it only cost about 4 psi over the .2 throat and will feed the transfer port better IMO.

I figured the above with a .15 valve and 1500 psi  (about 18-20 pumps).

[K.O.]

shimmed the pump cup .035 with a guitar pick and smashed  two JSB 25g pellets into the  back of the striker(necessitating shortening the spring guide portion of the  end plug .25")

pump effort after ten pums is a bit stiff and past 14 may be a bit hard on the gun I did not go past 16 pumps.

more complete numbers to follow later but

at 16 pumps  7.4g pointed  859 fps, 7.9g CPHP 831 fps,  Winchester 9.75g pointed 738 fps.

just a little headspace makes a big difference...

[K.O.]

There are 2 mods that I forgot to tell about.  I zip tied the barrel to the pump tube close to the barrel support/breech and made a solid pump pin.

I forgot to  do a few drops of oil so did in the middle of testing... so a few are a bit low plus I think the cup seated to the tube better with a bit of use. (I think)...

pumps       7.9g CPHP          7.4 points      9.75g points

4               492
5               540
6               579                    617               
7               601             
8               643,666             694              608
9               686,702             
10             692, 722            753              669
11             743,744           
12             755,783,779      782               692
13             787,785             
14             795,798             826              719
15             815
16             831,827             859              738

It has a bit of space taken up by an extra 2/3 size check poppet that I should have left out it looks like... 20fps ?

For an all out .177 or as a .22 and .25 there is a .6" or so section that is .40" that could easily be opened  up to .5"  I am guessing 20+ fpe  as a .22 or .25...

I really want to go all out with a disco tube, this valve(with an extension) and piston and a 2100 linkage to see what I could get...

Does any one know how much a Disco air tube runs?

it will have to wait till next year tho...

I need to think about the pro and con  of 66 linkage vs 2100 linkage also...

I was hoping the MK had it in it... when  I opened the valve  I was like  OH YEAH...

[K.O.]

I feel the plastic shell of this thing can hold up to 10-12 pumps for a good while...

I will be getting the 2240 striker and end cap to turn this into a  1322WT shortly but for now I am all   

how much does a 2240 striker weigh? the MK striker came in at 45 grams pre pellets in the back...

[Rivers3Plinker]

There are 2 mods that I forgot to tell about.  I zip tied the barrel to the pump tube close to the barrel support/breech and made a solid pump pin.

I forgot to  do a few drops of oil so did in the middle of testing... so a few are a bit low plus I think the cup seated to the tube better with a bit of use. (I think)...

pumps       7.9g CPHP          7.4 points      9.75g points

4               492
5               540
6               579                    617               
7               601             
8               643,666             694              608
9               686,702             
10             692, 722            753              669
11             743,744           
12             755,783,779      782               692
13             787,785             
14             795,798             826              719
15             815
16             831,827             859              738

It has a bit of space taken up by an extra 2/3 size check poppet that I should have left out it looks like... 20fps ?

For an all out .177 or as a .22 and .25 there is a .6" or so section that is .40" that could easily be opened  up to .5"  I am guessing 20+ fpe  as a .22 or .25...

I really want to go all out with a disco tube, this valve(with an extension) and piston and a 2100 linkage to see what I could get...

Does any one know how much a Disco air tube runs?

it will have to wait till next year tho...

I need to think about the pro and con  of 66 linkage vs 2100 linkage also...

I was hoping the MK had it in it... when  I opened the valve  I was like  OH YEAH...

Nice numbers!
Would love to see some pics of the work you did.
I'm curious and don't quite understand the changes you made to the valve. Did you open up the exhaust to 5/32 or so?

I want to mod mine based on some of your changes as I have it all apart again. Would love to have the numbers you're making at 12 pumps and be done.

My pump cup came off the piston so I tore it down to swap it out. I am making a new seal for the barrel adapter to the valve exhaust port too. Just ordered some goodies from McMaster that I hope will help.

Didn't the zip tie interfere with clamshell housing at the pump tube? Or did you notch the plastic for clearance?

[Rivers3Plinker]

Also how did you go about making a new probe tip for the bolt and installing that?
Pics!

[K.O.]

well I did not take any pics and other than the bolt probe it looks like a stock MK 177  but I offer the below as proof of life...

the bolt probe

first with the MK still together I  marked the plastic part of the probe at the point it enters the breech (scratched with a razor knife)

when I had it apart I then took the barrel and the bolt and  pulled the probe out until it looked like it would seat the pellet past the port (looking though the port)

I then checked with a pellet and had gone  about .1 to far so I push it back in that bit and checked again... perfect...

so I then measured how far it extended out of the plastic portion...   about .175  I also made sure it would clear the clip at this time...

I then found a nail that was just a bit larger around than the stock magnet/probe and chucked it up in my Dremel and turned it down to the same

diameter as the stock probe  I then cut it to length with a cutting disk.   Then I turned down the part that is not seated in the bolt.

[K.O.]

On your pump cup coming loose  my 1322 came stock drilled and pinned.

Steve the biggest thing is getting the pump cup shimmed just right basically you want it to just make contact at fully closed and then shim it about .035- .4past  that. works about the same as the  .5-.75 from closing thing...

If you have to shim the cup much they will need supported around the outside. this one I did not have to because of  how it fit stock.

On the valve I just drilled out the exhaust passage to 15/64  ~.23 I did not open up where the transfer port sits stock it is .140 but I did round the inside corner to help flow.

 
I  used a 1377 sear spring but it was just a bit short so I took up a bit of space and it was just long enough. 

If you get the right valve spring and do not take up that space your lower pump # will be a little lower than mine but will be above mine by 5-6 pumps  or less probably

Maybe even 20-40 fps at the very top end?

I think if I went for max volume and 20-25 pumps  950 fps with  7.4-7.9 pellets is not crazy

 

[K.O.]

"Didn't the zip tie interfere with clamshell housing at the pump tube? Or did you notch the plastic for clearance?"

nope there is plenty of room....

[K.O.]

Here is its first post mods groups with the stock sights at 20 yards and 10 pumps...

I did it to illustrate my problem with the MK I did the group by the quarter first it was with 7.4 points and the aim point was post at the bottom of bottom  target

next was the CPHPs and top target, to bring poi down is two turns up which makes the front post loose and so it wobbles quite a bit as I pump, you can see it does not stop at the same place twice...

at the bottom is no further adjustment but using the Winnie 9.7g points...

it wants to be accurate especially for open sights...  just ~2.25" high at 20 yards...

[Rivers3Plinker]

On your pump cup coming loose  my 1322 came stock drilled and pinned.

Steve the biggest thing is getting the pump cup shimmed just right basically you want it to just make contact at fully closed and then shim it about .035- .4past  that. works about the same as the  .5-.75 from closing thing...

If you have to shim the cup much they will need supported around the outside. this one I did not have to because of  how it fit stock.

On the valve I just drilled out the exhaust passage to 15/64  ~.23 I did not open up where the transfer port sits stock it is .140 but I did round the inside corner to help flow.

 
I  used a 1377 sear spring but it was just a bit short so I took up a bit of space and it was just long enough. 

If you get the right valve spring and do not take up that space your lower pump # will be a little lower than mine but will be above mine by 5-6 pumps  or less probably

Maybe even 20-40 fps at the very top end?

I think if I went for max volume and 20-25 pumps  950 fps with  7.4-7.9 pellets is not crazy

The exhaust passage you drilled - is this the area behind the head of the poppet? A straight shot back to the transfer port? How did you drill it?

Thanks for your other info and replies. I have some springs for the valve that should work too. The seal cup came off because the RTV wasn't applied properly and I did not pin it. I have many ideas now. Very cool stuff KO. Thanks again.

[K.O.]

"The exhaust passage you drilled - is this the area behind the head of the poppet? A straight shot back to the transfer port?"

Yep

I drilled this one with my drill press but the last one I did with a hand drill stuck in place and then pulled the trigger, have to be very careful that way though, it happens quick and then hit the reverse switch to smooth and then pull the bit out.

I do not think it made much diff...  It could have helped or hurt... is probably a wash if you stay with .177...

I did it because I plan on chopping an inch off the back of the tube, mounting a 1322 breech,  pistol grip and a 22" .22 barrel and boring the valve out. Hopefully to about .25 ci and I hope to get it to 21- 24 fpe

since I will be using at least a .160 transfer port I decided to go ahead and open it up especially when I did the math and found it would only cost me 4psi at 1500 psi

One of the above posts gave the # for effective area that a  .22 passage gives.If you do the math  on a .160 transfer port its area is  .02 square in. and a .180 port has an area of .0254  and so they are getting closer to matching...

now imagine you are 1500 psi of air rushing to equalize back to atmosphere  and as soon as you get going bamm right into a corner and anything that causes turbulence can also be thought of as a narrowing of the passage. I feel going bigger and rounding the corner lessen that effect...

the cool thing so far though is if you put a 22" inch barrel on this thing as is I bet it would be breaking 900 fps with 7.4s... and that is with reduced valve volume...   

[K.O.]

have been shooting a bit and fixed the wobbly front sight with painters tape in the post portion(a little goes a long ways)

It  was shooting great and I stuck with 10 pumps none past since testing.  well after about 20 shots todays it started spraying in about a 4" radius...

I do not know why yet but this thing is plastic...?

[K.O.]

finally got the parts ordered...

When I get the chance  while waiting I think I will mod the valve and see what I get out of enlarging volume...