Disassembly instructions for IZM Baikal 46M to change the piston seal

[subscriber]

In order to enable rated velocity, I wanted to adjust (reduce) the residual compression volume on my 46M. 

In looking for guidance on how to take apart the piston and cocking linkage assembly , I found instructions for how to change the piston seal. 

I wanted to attach them in their original "slide show" file format, but GTA won't accept such large files; nor .exe files.  So, copied, compressed and attached the images below...


I took my pistol apart and put it back together, so now feel armed to provide further advice on the topic

Do yourself a favor; take the front sight off.  Leaving it in place forces contortions of the piston to get it in and out.  I was not prepared to risk any damage to the seal, so I took the front sight off.  Not that the end of the compression cylinder is sharp, but as a matter of general principle; why make something difficult when it does not have to be.

At first, I could not get the left hinge support guide, shown in "photo 8" to come out of the compression cylinder.  I was not going to brute force it.  Then it occurred to me that the "barrel band" had slightly collapsed the compression cylinder, such that the hinge support guide was being trapped (due to the fact that the cylinder has a groove cut at the front).  Removing the barrel band freed up the "stuck" part.  However, I would advise against completely removing the barrel band, unless you have no other choice: 

The most difficult aspect in taking apart and reassembly of this pistol was reinstalling the barrel band.  I battled to get the screw to start in its nut.  Later figured out the nut was canting over in its recess.  Thus, my suggestion to not completely unscrew the barrel band retaining screw; if you can help it.  I believe that just taking the load off the compression cylinder by breaking the tension on the barrel band screw should be enough...

To remove the valve actuating plate on top of the barrel, cock the valve by flipping up the TP lever by hand; without charging the pistol with air.  The rear of the piston needs to be forward of the the little window in the top of the cylinder that the valve cocking plate hooks into.  The rest of this action is explained in the attachment, and it is as simple as it looks.

Once you have the pistol stripped this far, it becomes apparent that it really is beautifully simple in design and construction; and quite easy to work on.


Had to attach individual images due to single file size limitations.  Continued below...

[subscriber]

Continued from above...

[Stinger177]

Peter - Thanks for going to all that work to post those pics. I have never seen the internals of my 46M.

I also saw your thread about adjusting the piston stroke - https://www.gatewaytoairguns.org/GTA/index.php?topic=153027.new;topicseen#new

Something else I didn't know about.

I'm thinking that the latest 46M's came with better instructions than when I bought mine 14 yrs. ago. Mine didn't even come with extra seals.

[Manning]

Thank you!

[theanalogman]

Thanks! I wonder if I will ever get this advanced....

[Stinger177]

Thanks! I wonder if I will ever get this advanced....

I'm packaging and labeling your seals this very moment. Will get them off to you tomorrow morning.

[Yogi]

In order to enable rated velocity, I wanted to adjust (reduce) the residual compression volume on my 46M.

You lost me.   Do you mean to say that you wish to reduce the length of the piston stroke?   Does the shape of the face of the piston seal make that big a difference?

-Y

[subscriber]

Yogi,

Short answer to the first question, is "no".   Reducing the stroke would increase the residual volume.  I reduced the residual volume, without changing the stroke length.


The long answer is this:

There is the swept volume and the residual volume.  I wanted to decrease the residual volume.  Changing the swept volume can't be done without using different link lengths, and/or moving the vent hole in the (top of my) cylinder.

The swept volume depends on the bore and piston stroke.  I did not change that.

The compression (and expansion) ratio depends on the swept volume / residual volume.  To increase the working pressure (and thus the power), I decreased the residual volume:

The residual volume (at the end of the full stoke) depends on the volume of the valve, plus the space between the end of the piston and the "cylinder head".  I decreased the residual volume, by screwing the piston a 0.5 mm half a turn closer to the cylinder head; away from the connecting rod pivot pin; by relative motion at the thread visible in the piston's backside ID.

Any other questions

[subscriber]

The shape of the piston face matters.  It has a deep wide groove in it, as part of the "parachute seal" design.  The groove is part of the residual volume. 

Air trapped in the groove would be slow to flow out, and thus makes the system less efficient.  Filling the groove with thick grease that stays in place would reduce the gap, while still allowing the seal to function normally.  This would boost performance noticeably, based on my recent exploits.  However, if you filled in that groove completely with grease (you could partially fill it), you might need to back off the piston adjustment (opposite of what I did), or the cocking force may be so high that the linkages can't handle it...

[subscriber]

If you came here to try and understand this pistol's mechanism, you might find this thread helpful:  https://www.gatewaytoairguns.org/GTA/index.php?topic=153475.msg155688851#msg155688851

[rws45user]

 I have had my izh  since it first  came to the USA I think it was in 98 or 99 ? I can't remember  but I got it for 275 dollars  new .  My power dropped down to around 300 from 500  so I took it apart . Like your photo shows above the old  seal  had just collapsed. It wasn't worn at all . It still had the 2.5  thickness  it has just caved it . The gap size is 1mm and the depth is about 4mm . I used a  1mm rubber band and cut it to about 3mm depth. I used a few smaller  screwdrivers to pry open the cap and feed the RB into the slot . Once I got close to the  end I cut the RB so the 2 ends would touch and I used a tiny amount of super glue to hold the RB in place  so it wouldn't work its way out . With the RB in place the diameter is now the same as  a new seal again . I popped it back in the gun and it jumped it  up to 455 fps . I did a tissue test  and  found the  trigger cocking o-ring  was blowing the  tissue out . I added a layer of yellow electric tape I cut in a washer shape  behind the o-ring's put the o-rings back in . This raised up the o-rings and gave it a nice tight seal in front and behind the o-ring .  I would like to add that the  o-rings  are flat on the back side and the  rings are very hard and air can leak out of the back side even if they are sealed good on the front side . I would recommend adding a layer of electrical tape behind  both  o-rings . This will up your fps because air is going to leak from behind .  This caused the fps to just from 455 up to 500 fps . So I'm using the same  seal and o-rings I have had for over 20 years  and I'm getting 500 fps with a 7.0 grain pellet . Here are a few photos of the RB in the slot and of the  flair back in the seal, that was  straight up and down  before the  RB .

[subscriber]

Thanks.  The rubber band in the piston face groove seems like a nice way to increase overall air efficiency.

[Offhand]

Thanks Subscriber and rws45user for the information!

RWS45user, did you put tape behind each of the breech o-rings? 

thanks again

Offhand

[rws45user]

Yes after I saw the jump with the first one  it was at  450 and it jumped up to something like  470  so  I added  it to the other and it got me up to 500 . The o-rings on the back side are flat and  I think to hard  to get a good leak proof seal  between the metal and the  hard  seal surface.  At first  I was trying out  the teflon tape  you use on gas pipe the yellow  color  pipe threads tape  . I noticed  it was pushing it up and was trying to come out the sides  so I know  air was not being  stopped by the hard seal . I went with the thicker electrical tape . Its not the good electrical tape   but the cheaper  colored tape   that's thinner . When I cut that tape  and put the seal  over it  the fps really jumped up . I wish they made the seal out of a softer  rubber type seal   the one that came on my gun is really hard  and I don't think its  rubber at all instead its a plastic type seal .  The pressure the  locking system  without the  tape behind it  is just not enough pressure to force a air tight seal   on the back side . It seems to be good on the  front end but not on tr back and thats where air is being leaked out . If you have the gun   go out a buy the multi color cheap tape  here is a link  https://www.lowes.com/pd/Utilitech-6-Pack-20-ft/3659364?cm_mmc=shp-_-c-_-prd-_-elc-_-google-_-lia-_-106-_-electricalaccessories-_-3659364-_-0&placeholder=null&ds_rl=1286981&ds_a_cid=112741100&gclid=CjwKCAiA9vOABhBfEiwATCi7GHlrwLBoaLSOcwo3KGJpGvC75a-pPPWAy1-NvN6ms1PgP8zFGDk6ihoC6CcQAvD_BwE&gclsrc=aw.dsnk to the tape I use .   If you have a chrony shoot your gun  first then  add one layer  to each seal  and shoot it again  .  I will guarantee it will jump up the fps if below 500 .

[subscriber]

I like the 46M.  Its pump mechanism is clever; but the breech sealing seem almost silly by comparison.  The principle that needs to be satisfied is this:

If you want a leak proof seal for a gas conduit or vessel to contain 1000 PSI, then the contact pressure between compliant seal and hard surfaces must be at least 1000 PSI.  This contact must form an unbroken line around the perimeter of the parts being sealed. 

While the 46M's seal contact area is small I seriously doubt that with this breech mechanism and finger pressure, the seals generate anywhere near 1000 PSI contact pressure.  This breech lacks mechanical advantage to actually compress the seals even close to the breech in a spring air rifle.   Yes, springers generate much higher pressures, and leaks would cause the superheated air to cut the seal very rapidly; so zero leakage is a must for springers.

Yes, this assumes the 46M has a 1000 PSI operating pressure.  It may actually be 800; but I am trying to make a point in principle.  As a data point, the average pressure is less than 200 PSI, based on the 4 FPE pellet energy delivered - accounting for 1/2 lb average pellet to bore friction.

So, perhaps the breech seals are not seals, but "leak restrictors".  Similar to the flashgap found at every revolver cylinder to barrel gap (except for 1895 Nagant).

I would think that a round section O-ring seal would be better than a flat face seal, if actual sealing were the goal, rather than something closer to a labyrinth seal (zero contact force, leak flow limiting glands used in gas turbines).  The line contact with an O-ring would have very little contact area, until you have squashed it significantly.  The force required to generate useful contact pressure would be low, until the deformation resulted in much larger contact area...

If someone can come up with an aftermarket breech seal for the 46M that boosts the FPE by lets say 10+%, and does not make the breech hard to close or open, that would be a great seller at $25...

[Offhand]

RWS45user,
     Thanks for the clarification!

[subscriber]

The current breech seals on the Baikal / IZZY 46M are soft, closed cell foam:


https://www.pyramydair.com/product/izh-46m-breech-seal-kit?a=2090






Much cheaper when ordered directly from Russia:

https://www.ebay.com/itm/Six-O-rings-number-19-Baikal-Izh-46-MP-46M-MP-532-AV-46M/283718363983?hash=item420eeeab4f:g:VqgAAOSwkiVd~j5p




https://www.ebay.com/itm/O-rings-Baikal-Izh-46-MP-46M-MP-532-AV-46M/282978613971?hash=item41e2d6fad3:g:bPsAAOSwVgtZ3FgU




Compared to this:  https://www.pyramydair.com/product/izh-46m-seal-o-ring-replacements?a=592