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Author Topic: Factory Breech Hole Size on QB78S in 177 ?  (Read 389 times))

Offline wll2506

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Factory Breech Hole Size on QB78S in 177 ?
« on: February 20, 2021, 07:05:27 PM »
What is the factory size of the plastic breach hole (ID) compared to the Crosman #136-030 ? Is it that much smaller or is it just that the factory seals are often crushed ?

I don't want to replace if it is a small difference, but if it is substantial I will.


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Offline wll2506

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Re: Factory Breech Hole Size on QB78S in 177 ?
« Reply #1 on: February 21, 2021, 09:51:21 PM »
Anyone know :-)

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Offline Doug Wall

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Re: Factory Breech Hole Size on QB78S in 177 ?
« Reply #2 on: February 21, 2021, 10:44:46 PM »
What exactly do you mean by "plastic breach hole"? It's not a technical term that I'm familiar with.
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Offline Ribbonstone

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Re: Factory Breech Hole Size on QB78S in 177 ?
« Reply #3 on: February 22, 2021, 01:42:59 AM »
Will....not real sure either.

Could be the transfer port....which would be the only plastic part. 

Anyway...getting all ticky about it..while we call that the transfer port, it's not. The holes in metal leading from the valve stem to the barrel would be the port (or ports)....the plastic part on the QB would be more like a port gasket as there is no metal part between the tube and the breech.

If that's it...it's at least use to be a somewhat "squishy" rubbery type, that would read bigger on hole dimeter outside the rifle, but would often close up when the breech was tightened down.  Most folks just made one from ice-maker tube, but there were/are  lots of more complicated or creative ways.
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Offline wll2506

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Re: Factory Breech Hole Size on QB78S in 177 ?
« Reply #4 on: February 22, 2021, 10:21:28 AM »
Will....not real sure either.

Could be the transfer port....which would be the only plastic part. 

Anyway...getting all ticky about it..while we call that the transfer port, it's not. The holes in metal leading from the valve stem to the barrel would be the port (or ports)....the plastic part on the QB would be more like a port gasket as there is no metal part between the tube and the breech.

If that's it...it's at least use to be a somewhat "squishy" rubbery type, that would read bigger on hole dimeter outside the rifle, but would often close up when the breech was tightened down.  Most folks just made one from ice-maker tube, but there were/are  lots of more complicated or creative ways.

Yes Ribbonstone, the rubber seal/ice maker tube piece, what is the factory ID of that section ?

Thank you,

wll
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Offline nervoustrigger

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Re: Factory Breech Hole Size on QB78S in 177 ?
« Reply #5 on: February 22, 2021, 10:43:28 AM »
Icemaker tubing (0.250” OD, 0.170” ID) is a loose fit…the recess is closer to 0.275” (7mm).  The tubing is a time-tested solution and so is the Crosman transfer port seal. 
 
The OEM seal has a very restrictive ID by comparison.
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Offline Ribbonstone

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Re: Factory Breech Hole Size on QB78S in 177 ?
« Reply #6 on: February 22, 2021, 12:51:12 PM »

The material the factory used was soft enough that it squished a bit when the rifle was assembled.  So while the hole measured one size out of the rifle, it was actually smaller once squished together.  Doesn’t help that there really isn’t a good seat on the bottom of the receiver.

Usually it ended up being the smallest hole from the valve to the barrel, so we just make a new one with a bigger hole.

Lot of us just used icemaker tubing….but the trick is getting it just high enough to seal, but not so high it buckled,  Certainly works once you get just the right fit…. also demonstrates whey the factory used a soft/squishy “fit everything” version to crank them out.  Also expalins why so many QB shooters report way different pout the box speeds (some just squish more than others).

Actually….most icemakers have enough slack in the connecting line that you could disconnect and cut off more than enough for making dozens of them.

-------

OK...pulled some parts out of the parts box.

Hole in the valve runs a pretty generous .185”

Hole in a .177 barrel runs about .125”

If the transfer port (gasket) passage is less than .125”...you’d gain speed over factory size by making it  .125”.  If it’s not a squishy material, likely it would stay whatever size you made it once reassembled.

If you made the transfer port (gasket) larger than .125”, would still run into that sized restriction just above it.

Making the transfer port (gasket) hole larger that the .125” restriction just above it wouldn’t hurt anything, but wouldn’t gain much either.


NOT talking about all the ways to make for even better gas flow….just the simplest way to make the existing system work to it’s base-potential.   Past the transfer port, are gas flow mods that you can’t easily reverse.

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Offline wll2506

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Re: Factory Breech Hole Size on QB78S in 177 ?
« Reply #7 on: February 22, 2021, 02:00:38 PM »

The material the factory used was soft enough that it squished a bit when the rifle was assembled.  So while the hole measured one size out of the rifle, it was actually smaller once squished together.  Doesn’t help that there really isn’t a good seat on the bottom of the receiver.

Usually it ended up being the smallest hole from the valve to the barrel, so we just make a new one with a bigger hole.

Lot of us just used icemaker tubing….but the trick is getting it just high enough to seal, but not so high it buckled,  Certainly works once you get just the right fit…. also demonstrates whey the factory used a soft/squishy “fit everything” version to crank them out.  Also expalins why so many QB shooters report way different pout the box speeds (some just squish more than others).

Actually….most icemakers have enough slack in the connecting line that you could disconnect and cut off more than enough for making dozens of them.

-------

OK...pulled some parts out of the parts box.

Hole in the valve runs a pretty generous .185”

Hole in a .177 barrel runs about .125”

If the transfer port (gasket) passage is less than .125”...you’d gain speed over factory size by making it  .125”.  If it’s not a squishy material, likely it would stay whatever size you made it once reassembled.

If you made the transfer port (gasket) larger than .125”, would still run into that sized restriction just above it.

Making the transfer port (gasket) hole larger that the .125” restriction just above it wouldn’t hurt anything, but wouldn’t gain much either.


NOT talking about all the ways to make for even better gas flow….just the simplest way to make the existing system work to it’s base-potential.   Past the transfer port, are gas flow mods that you can’t easily reverse.


Icemaker tubing (0.250” OD, 0.170” ID) is a loose fit…the recess is closer to 0.275” (7mm).  The tubing is a time-tested solution and so is the Crosman transfer port seal. 
 
The OEM seal has a very restrictive ID by comparison.

Great info guys, thank you very much for your help.

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Offline Ribbonstone

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Re: Factory Breech Hole Size on QB78S in 177 ?
« Reply #8 on: February 22, 2021, 06:52:44 PM »
Here is an exceptionally bad example of a squishy transfer port/gasket...certainly didn’t design it to do this.


DSCN2835 by Robert Dean, on Flickr

Other times, would take ta QB rapart and the issue part looked fine (although small...likely more like .1” once reassembled tightly.





Looking though an aux. Hard drive from 2 computers ago, did come up with a few numbers where I changed ONE think at a time.  Whole lot of taking apart/putting together to see what a change in isolation would do rather than (like most of us) change several things so long as we have it apart.

Just arranging for a 1/8” transfer port/gaske hole (no other changes) boosted velocity up by about 12%.  That’s basically 7.9gr.@650 fps 7.9 fpe to 727fps 9.3 fpe.
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Offline wll2506

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Re: Factory Breech Hole Size on QB78S in 177 ?
« Reply #9 on: February 22, 2021, 07:05:20 PM »
Great, great info.

I'm putting on my big boy pants tomorrow and going for it :-)

I'll keep you posted.

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Offline wll2506

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Re: Factory Breech Hole Size on QB78S in 177 ?
« Reply #10 on: February 23, 2021, 12:05:58 PM »
Put my big boy pants on and took gun apart before work at around 4:30 this am ....... The swearing you heard was the sound of me trying to put the top and bottom sections together ... what a pain !!

Anyway, the port in the barrel was about .130, (I did not take out the bolt and check the hole size in the bolt probe) I brought a couple of drills up to that the .125 size so no drilling was needed. I used a piece of water tubing instead of the Crosman 136-030 seal. A pic of both seals are next to the seal that I pulled from this gun. The ID of the seal I pulled was .100ish, the water line ID is about .170 or so, the Crosman was about .160. I'm hoping the bigger diameter gives me another 40-50 fps --- we will see later today ! Getting the velocity up to around 615+fps would be great as that gives me ~6fpe at 35 yards which is what I would really like for this gun and my use --- anymore is a plus :-  )




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Offline Ribbonstone

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Re: Factory Breech Hole Size on QB78S in 177 ?
« Reply #11 on: February 23, 2021, 01:18:08 PM »
Oh yeah...getting both pins to seat to the bolt and work right and yet still not have the replacement transfer port get out of place....likely you had a couple of "almost right" assemblies.

Test your new one for leaks....I don't mind messy, so I often dust all around with flour and see what gets blown off...if it does at all...if it does, then I know where it leaked and  test with a finger to see how powerful that stray leak is.

Considering how squishy the orginal...and how tightly it fit in the tube recess...very likely it compressed into something smaller once it was reassembled.
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Offline Kevinbear

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Re: Factory Breech Hole Size on QB78S in 177 ?
« Reply #12 on: February 23, 2021, 04:00:07 PM »
This is a little off topic but bear with me a moment, I bought a 177 barrel and bolt from Archer to replace the 22 barrel, it eats up bolt seals like crazy even though I've polished everything. Archer charges way to much for them, does anyone know where they can be bought in bulk?
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Offline wll2506

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Re: Factory Breech Hole Size on QB78S in 177 ?
« Reply #13 on: February 23, 2021, 04:13:45 PM »
This is a little off topic but bear with me a moment, I bought a 177 barrel and bolt from Archer to replace the 22 barrel, it eats up bolt seals like crazy even though I've polished everything. Archer charges way to much for them, does anyone know where they can be bought in bulk?

Yes, I get mine from "The "O"Ring Store" for .22 I Think the size is 006 ----- 177 is 003 I do believe ---- I'll let someone else chime in to be exact !!

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Offline rsterne

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Re: Factory Breech Hole Size on QB78S in 177 ?
« Reply #14 on: February 23, 2021, 04:27:47 PM »
I have always used the Crosman TP seal.... It curves to fit the valve and receiver....

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Offline nervoustrigger

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Re: Factory Breech Hole Size on QB78S in 177 ?
« Reply #15 on: February 23, 2021, 05:12:33 PM »
Kevin, the OEM size for the .177 bolt O-ring is M1 x 2.5 (represents 2.5mm ID and a 1mm cross-section, so 4.5mm OD).
 
An -003 has a much thicker cross-section…it thins out a little when stretched into place but may not fit unless you either enlarge the groove or open up the leade a smidge.  However here in the states you’ll probably be able to put your hands on the dash size quicker than the metric so it’s worth a try.
 
Otherwise here are a couple of sources for the metric
https://www.mcmaster.com/9262K511
https://www.amazon.com/gp/product/B00595IU1M
 
Having said all that, be advised that a common problem with the QB bolts is that the body is frequently bent/deformed very slightly in the region where the material was hogged out for the hammer pin.  What happens is the bolt is biased to one side, causing the O-ring to get pinched or abraded preferentially on one side as it moves into its home position.
 
Might want to roll it on a piece of glass or granite and see if you can detect any slight banana shape.  Also look for any evidence that the tip of the bolt orbits a bit…the thin portion alone could be bent.  Either way, it may take a little persuasion with a hammer and hardwood block to straighten it.  BTW if it’s bad enough, you may be able to detect it by removing the O-ring and watching carefully as the tip of the bolt approaches the leade.
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Offline Ribbonstone

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Re: Factory Breech Hole Size on QB78S in 177 ?
« Reply #16 on: February 23, 2021, 07:15:10 PM »
Some of the bolts are crooked, sometimes the pellet seating is rough..either one tends to chew up the o-ring.

Have had some hard bolt probes that will snap when attempting to straighten (they at least use to be hollow stems rather than probes).

Usually smoothed the barrel’s bolt probe o-ring seating area by hand (bamboo skewers with tofts of steel wool) but not enlarge it…..just getting the rough spots off, not really polishing bright.

For whatever reason, the Crosman TP seal only workd for me about ½ the time...the other half always showed a bit of a leak.  Just gun to gun variation I guess.
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Offline wll2506

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Re: Factory Breech Hole Size on QB78S in 177 ?
« Reply #17 on: February 23, 2021, 08:17:27 PM »
Tested the gun after I changed the Co2 seal --- well to say I was happy would be an understatement ---- I got 20 powerful shots starting with the second shot after piercing the Co2 at 624fps next shot 629fps, and all the rest between 636 and 643fps, the 20th shot was 629fps and I stopped shooting. My average for 20 shots was 637fps :-)

My average velocity before the gasket change was 575fps, so now I'm averaging 637fps, that is 62fps faster and now the gun is really a great small pest gun for my use. My house was 69° inside.

This velocity gives me 6fpe at 40 yards, and 5.8fpe at 45 YARDS :-) 98% of my shots are between 25 to 35 yards --- 40 yards is a long shot for this gun and caliber. MAN ALIVE I'M HAPPY :-) With this energy at the 40ish yard range I feel good about Pigeon size pest with a good shot. This gun has the accuracy for sure.

I can hardly wait to try this gun out this weekend :-)

Thank you every one for your input and guidance as always ;- )

Till tomorrow,

wll

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Offline rsterne

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Re: Factory Breech Hole Size on QB78S in 177 ?
« Reply #18 on: February 23, 2021, 08:29:38 PM »
I have heard there are two different thicknesses of Crosman TP seals.... but I neither know if that is fact, or what model uses which....

Bob
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Offline Kevinbear

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Re: Factory Breech Hole Size on QB78S in 177 ?
« Reply #19 on: February 23, 2021, 08:33:52 PM »
Thanks, never considered the bolt being bent.
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