.22 transfer port sizes?

[mcoulter]

I finally took the time to measure the ports on my Kral Bigmax .22.  Here is what I found...

Transfer ports sizes:

valve = 3.7 mm
power wheel = 3.46 mm (0.136 in)
block = 4.05 mm (0.159 in)
barrel = 3.8 mm

I've explored getting a looooong, flat shot string at typical fpe, now I'd like to open it up and let it breath a bit.  In it's current configuration it's shooting about 31 fpe (880 fps with jsb 18s).   My goal is to "up" this to about 36-37 fpe (950 fps). 

Does it seem reasonable to open ports to the largest current opening (4.05 mm)?   Does anyone have any guestimates on what power change this would give me?

I have read Bob's article here:
https://hardairmagazine.com/ham-columns/the-effect-of-port-sizes-on-pcp-airgun-power/

His Beeman Chief achieved my target fpe at 0.203 port size but that was at much lower operating pressure than the Kral. 

Any suggestions would be great, thanks!

Oh, almost forgot the pics and yeah, the barrel port IS really close to the o-ring...

Valve:


Power wheel:


Power wheel OD:


Block (I used a drill bit to gauge the diameter):





Barrel:

[Dairyboy]

On .22 I open to 11/64 or roughly .172. You should easily reach your power goals that way. But in your case easiest would probably be the .159 then you don't need to drill the block out. So I say start there and test. You should still be able to reach your goals I would guess. Only issue I see is that barrel port seems awfully close to the oring groove...

[rsterne]

I would agree, open all the ports up to the 4.05 mm that is in the hardest part to modify and see what you get.... As important as the size, is the alignment, of course, it doesn't do a lot of good to match the diameters and not have the holes line up.... The good news on that barrel port is that the air pressure is pushing the O-ring away from the port, so even if you touch the groove it should not matter.... Better not to, of course....

Bob

[Motorhead]

We found the Puncher Breaker to suffer these same transfer path inconsistencies.
Evening up the sizes and getting better alignment indeed helps the power and a adds a bit of consistency.

[mcoulter]

Thanks for your input guys.  After pondering the alignment issue, I am now tempted to open up the port that goes through the block just a little bit.  Instead of going with the current 4.05 mm port size, I may go with my next drill size which I think is 4.10 (0.161 in) or 4.15 mm (0.163 in).

I'm assuming one way to ensure that the port alignment between the valve and the block is correct is to install the valve body (with its guts removed), fasten it together into the block using the two grub screws (letting them find their natural alignment) and then drill down through through the barrel grub screw hole (from the top of block) and continue carefully into and through the wall of the valve body.

I am less confident in how I may create or verify proper alignment between the block port and the barrel port.  I just need to decide if I'm going to visit a friend with a drill press (and vice) or if I'm going to finally get one of my own.   

[Rallyshark]

I usually set my absolute limit for .22 at .1875(3/16), but that is the upper limit that I'll use on the barrel port(60+fpe).  Any larger and you can encounter loading issues with some pellets(sometimes that can happen at 3/16).  11/64 would be a safer bet, and will easily get you to the power level you're shooting for(assuming you can get the alignment right).  11/64 can take you up to 50+fpe, and I think you'd easily get your power level at the .159-.163 you mentioned too. 

[rsterne]

I never go wider than 80% of bore on a barrel port, which for a .22 cal would be 11/64".... If I need more flow than that (basically only for bullets), I mill the port to an oblong shape, keeping the width at 80% to avoid loading problems.... but I may go as long as 1.2 calibers to get equivalent area to the bore.... Virtually all of my .22 cal pellet shooters are 0.166" porting or equivalent all the way through.... That will deliver 37 FPE on just 1600 psi (18.1 gr. @ 960 fps)....

Bob

[mcoulter]

Thanks again for your input on this guys.  This is exactly the info I was hoping to receive.  This will be my first time modifying a PCP like this.  I still have not had time to get a drill press (or visit my neighbor who has one).  But here are a couple more pics.  First is how the valve body lines up (or should I say does NOT line up...) when fully seated and lined up with the grub screws that hold it in.  So it does look like I'll end up with an oblong port here:



And in this photo I have inserted the power wheel adjuster:

[MJP]

Align all the ports and you may just achieve your goal with that.
All the steps cause turbulence and hinder performance.
Openin it all up to 4.1mm should do the trick, aligning the ports and adding extra flow capacity.

Marko

[mcoulter]

Align all the ports and you may just achieve your goal with that.
All the steps cause turbulence and hinder performance.
Opening it all up to 4.1mm should do the trick, aligning the ports and adding extra flow capacity.

Marko

Hi Marko,

I wish fixing the port alignment in the pix above was possible.  Unfortunately, the valve is already pushed as far into the block as it can go and that is the direction it would need to be moved for correct alignment.  I think drilling it out more will be my only option. 

Does anyone have any tips or suggestions for testing or verifying alignment between the block and barrel ports?  I have eyeballed the position of the barrel transfer port in relation to the grub screw marks on the barrel.  FWIW, they do seem to be on exact opposite sides of the barrel.

[rsterne]

The valve will move back a surprising amount when there is air pressure against it.... I would allow for that (ie check it) before making any mods....

Bob

[mcoulter]

The valve will move back a surprising amount when there is air pressure against it.... I would allow for that (ie check it) before making any mods....

Bob

Hi Bob, I'm not sure that I understand what you said above.  Can you clarify a bit? 

Here's a wider photo showing how the valve block mates with the main receiver block:



And this is the valve body unscrewed from what I call the valve block:




And valve body screwed into the valve block:

[Motorhead]

The valve will move back a surprising amount when there is air pressure against it.... I would allow for that (ie check it) before making any mods....

Bob

Hi Bob, I'm not sure that I understand what you said above.  Can you clarify a bit? 

When there is pressure bearing against the valves "BODY" ... there is a trend for the valve to be firmly set back against the retention bolt/s holding it into position.  This "Can" create a misalignment that was not present during a NO PRESSURE alignment check.

IMO ... the KRAL guns like the EVANIX they loosely copy, are not really prone to this simply because the valve is retained within it's own housing where the bottle also attaches and simply rests within the receiver held by a single screw that holds the position aligned.

[mcoulter]

Thank you Scott.  I follow what Bob was saying now  :-)

[Rallyshark]

I never go wider than 80% of bore on a barrel port, which for a .22 cal would be 11/64".... If I need more flow than that (basically only for bullets), I mill the port to an oblong shape, keeping the width at 80% to avoid loading problems.... but I may go as long as 1.2 calibers to get equivalent area to the bore.... Virtually all of my .22 cal pellet shooters are 0.166" porting or equivalent all the way through.... That will deliver 37 FPE on just 1600 psi (18.1 gr. @ 960 fps)....

Bob

I don't disagree with that at all  

Thanks again for your input on this guys.  This is exactly the info I was hoping to receive.  This will be my first time modifying a PCP like this.  I still have not had time to get a drill press (or visit my neighbor who has one).  But here are a couple more pics.  First is how the valve body lines up (or should I say does NOT line up...) when fully seated and lined up with the grub screws that hold it in.  So it does look like I'll end up with an oblong port here:



And in this photo I have inserted the power wheel adjuster:

Yep, just getting it aligned would allow you to hit the goal probably.  I'm not very familiar with the Krals, but I don't think making the port in the valve oval shaped or oblong to get the alignment will hurt anything.  Not that I can see, at least.  I think doing that, and working that adjuster some too will get you to your goal.  You obviously have flow issues the way that stuff is lining up.

If you are good with your hands, and are careful, you don't have to have a drill press to do this stuff.  Actually, you could use a dremel and open the valve/power adjuster a little at a time, until you get the holes to line up better.  Every bit of tuning I do is with hand tools, and a dremel is a huge help too.  You just have to be careful and take off a little at a time.  In cases such as this, the dremel is a must have for me.  If you're good at doing things by eye, you should be able to make a rather large improvement on that big 'ol Kral

[rsterne]

Sorry, I was talking in generalities.... This particular gun design may not have that issue....

Bob

[azran1@hotmail.com]

Hi All,

There are many people that advise me that in order have good alignment between the breach and the power adjuster is for the power adjuster to be remain in the breach during the drilling process. This is so that the breach hole and the power adjuster hole will be perfectly align as the outcome. Some has advise me against it. I'm confuse.

Ozzie

[Wayne52]

11/64's is what I've been using on my .22 pcp's as well for the .25 I've been going with .187.

[mcoulter]

I nailed it!

I finally got the time (thanks to an ice storm) to do the port modification on my Kral Bigmax .22.  I decided to go with a .164 drill size for all the ports.  The power adjuster wheel was the biggest challenge.  I did drill it to that size while locked in the vice, but as it was inserted into the receiver block, the overall port opening was considerably smaller due to alignment issues.  I ended up running the wheel port manually up and down on the drill bit to knock down the offending inside edges of the power wheel port.  And no, I was not brave enough to drill through the receiver with the wheel installed.  I suspect that unicorns are more common than replacement Kral parts like this...

Anyhow, the resulting velocity is EXACTLY where I had hoped it would be.  JSB 18s are shooting between 950 and 960 fps. 

I only had time to shoot one magazine, so I can't say what the overall string is like now, but the initial power is just where I wanted it to be which is 37 fpe with the JSB heavies.  Oh, I have the hammer spring turned out (CCW) 1 turn from the point where the gun won't cock.  This leaves me about 1.5 more turns of CCW adjustment out if I want it.

Thanks everyone for your input and advice on this small project!!!

:-)

[Rallyshark]

Nice work!  I think it is time to test it with the Barracuda Match and Monster Redesigned now