QB79 tank block improvement

[rsterne]

Is that tapped into both pieces while assembled?.... If so, looks like a good solution....

Bob

[ped]

that's how I do mine

[wll2506]

Is that tapped into both pieces while assembled?.... If so, looks like a good solution....

Bob

Yes, side screws were in place with a small amount of  air pressure and then drilled and tapped. I have looked and thought of everything, but this was simple enough for me to do, and screw size was way bigger than what most use so I figured staying at 1350psi outgoing and using a burst disk I was in a good safety margin using this big allen screw maybe not up to using two 10x32 screws or a dowel pin, but still a margin of safety well beyond factory.

Thank you Bob for all your input !

wll

[Buldawg76]

Will
I would say that is plenty of surface area to prevent any unintentional parts flying out of the tube under the 1350 psi you are running and is an nice clean method of doing it since it is hidden under the barrel when assembled.

My 78 is apart right now getting the mods performed to mount the tank block and other ones talked about on the two threads for modding the QBs. So far have my 177 probe drilled out to .125" in the nose bore and .140" for the breech port interface and barrel port at .140" as well. Getting ready to do the valve throat and angle the port wall out of the valve and then work on the poppet and valve nose and select the poppet spring I think will be the best tension.

Then it will be on to the cock on close mod and RVA mod.

I have one question about the RVA mod since the retaining screw that holds the trigger, tube end cap and breech end cap runs up thru the middle of the tube end cap and threads into the breech cap. To install the screw for the RVA does the hole get drilled off center of the end cap so it pass by the rear retaining screw to push on the rear spring guide or does the tube end cap get tapped for a bigger thread and two screws installed from the trigger side and top of breech side with hole on the top of the breech being drilled to allow for the top retaining screw to thread into the tube end cap like crosman gun are designed.

My concern with drilling the RVA screw hole off center of the retaining screw hole to pass by that screw in the end cap is binding of the rear spring guide in the tube since it would be pushing off center of the rear of the guide or does the guide fit snug enough and have enough surface contact on the tube walls that it is a non issue and work just fine without binding of the rear guide.

Any thoughts.

Mike

[sr1sws]

Re: RVA - I had no problems with the RVA screw being off-center... No binding.

[wll2506]

Will
I would say that is plenty of surface area to prevent any unintentional parts flying out of the tube under the 1350 psi you are running and is an nice clean method of doing it since it is hidden under the barrel when assembled.

My 78 is apart right now getting the mods performed to mount the tank block and other ones talked about on the two threads for modding the QBs. So far have my 177 probe drilled out to .125" in the nose bore and .140" for the breech port interface and barrel port at .140" as well. Getting ready to do the valve throat and angle the port wall out of the valve and then work on the poppet and valve nose and select the poppet spring I think will be the best tension.

Then it will be on to the cock on close mod and RVA mod.

I have one question about the RVA mod since the retaining screw that holds the trigger, tube end cap and breech end cap runs up thru the middle of the tube end cap and threads into the breech cap. To install the screw for the RVA does the hole get drilled off center of the end cap so it pass by the rear retaining screw to push on the rear spring guide or does the tube end cap get tapped for a bigger thread and two screws installed from the trigger side and top of breech side with hole on the top of the breech being drilled to allow for the top retaining screw to thread into the tube end cap like crosman gun are designed.

My concern with drilling the RVA screw hole off center of the retaining screw hole to pass by that screw in the end cap is binding of the rear spring guide in the tube since it would be pushing off center of the rear of the guide or does the guide fit snug enough and have enough surface contact on the tube walls that it is a non issue and work just fine without binding of the rear guide.

Any thoughts.

Mike

Mike I wish I could help on the RVA but my QB's are all cock on close ... I just like that/ cocking method, even though the other is far easier to adjust.

I set my QB 78 in .22 cal up with a 22ci tank, a 1350 outgoing and a 1/4 inch shim under the striker spring to open the poppet with this much pressure. I'm in the 850's using 18.1 heavies with the '78 and 808fps using heavies with the '79 and the same outgoing ... the '79 is using a 17ci tank . My QB78 in 177 has a outgoing of about 1200+, uses a 22ci tank and is shooting Kodiaks around 880ish last I checked

wll

[ped]

I did make a hammer spring adjuster for cock on close qb's ,you had to remove the rear tube cap though
basically a new cocking block with the guide threaded so it can be adjusted can be 

[Austringer]

Yup. The rva screw hole is drilled off center. The spring guide fits pretty snug in the tube so binding is not normally a problem. I worried about the same thing. I've done two with no issues. It's a great mod. Improves tuneability and versitity if the qb. Troy

[Buldawg76]

To all who responded on the RVA and cock on open versus cock on close thanks for the input and I will be going the off center RVA route as I want to have external adjustments capable of being changed easily without disassenbly at all. I want the ability to turn it up or down within certain limits with just the screw and prefer cock on open as well.

Mike

[SpiralGroove]

Hey Mike,
The RVA with cock on open are very easy modifications to do, even though cutting your breach the first time will make you think twice .  Again, the hardest part for me is RVA drilling.  For a $500 drill press, mine is really a POS --> too much spindle run-out

[Buldawg76]

Hey Mike,
The RVA with cock on open are very easy modifications to do, even though cutting your breach the first time will make you think twice .  Again, the hardest part for me is RVA drilling.  For a $500 drill press, mine is really a POS --> too much spindle run-out

Kirk
Yea I am going very slow and thinking out every step closely so I measure 4 times and cut or drill once. LOL My 35 year old craftsman 8 inch bench top drill press is actually still quite tight in the spindle bearing despite have one spindle bent from milling my striker block on the 60C off by .050" and the end mill hung in the steel and stopped the drill press cold. It bent the spindle and the end mill but 35 bucks later with a new spindle and its back good as new.  I have more issues with my Chinese cummins tool sale drill press vise not holding the position its set it at while drilling and the gibs and ways are as tight as possible and still be capable of moving without a cheater bar on the handles.

Just making do with what I have at home until it has to be precise then its over to the buddies for his south bend lathe and huge Yakima mill to get it perfect when needed.

Still working on finding room for that 10x 48 south bend in my garage maybe one day in the future.

Mike 

[wll2506]

Close on open is the way to go for ease of changing power setting that is for sure even though I don't do it. Because I have a cock to close my fine tuning is very much a pain so I will live with being close. For me, drilling out the breach side for a bolt slot is not an option

I can tell you guys that having a RVA on my Disco is a real pleasure and dialing in the power setting I wanted with the pellet I wanted to use was no problem.... now if someone can figure out a way on a QB to have the bolt stay back when cocked (without a breach modification) so it does not get in the way of pellet feeding ... then I would re-consider ;- )

BTW: On a cock on close is the only thing you do is remove that one bolt (#800) ?

wll

[SpiralGroove]

Hey Wll,
I believe it's part 1000, the pin that fits in part 800 (spring guide).

Not that I'm any expert (as I still have a hard time drilling a straight hole), but I did my first rear cocking mod. with a hand drill and cleaned up with a diamond bit, Dremmel tool.  I usually really clean up the whole cocking slot too, so the bolt action is very smooth.

A couple of years ago, I noticed Bob always (automatically) included the RVA on any gun he was working on.  I thought that was odd and was a lot of extra work   But after the mod. on my first QB, I had a "Oh my God" moment, and now try to incorporate it whenever possible.

Sometimes I need to be hit over the head a few time before it sinks in - Just ask Bob  .

[wll2506]

Hey Wll,
I believe it's part 1000, the pin that fits in part 800 (spring guide).

Not that I'm any expert (as I still have a hard time drilling a straight hole), but I did my first rear cocking mod. with a hand drill and cleaned up with a diamond bit, Dremmel tool.  I usually really clean up the whole cocking slot too, so the bolt action is very smooth.

A couple of years ago, I noticed Bob always (automatically) included the RVA on any gun he was working on.  I thought that was odd and was a lot of extra work   But after the mod. on my first QB, I had a "Oh my God" moment, and now try to incorporate it whenever possible.

Sometimes I need to be hit over the head a few time before it sinks in - Just ask Bob  .

Yes you are correct, it is #1000, and ya I can often use a good smack too ;- )

I'm looking at the RVA but I don't want to mess with the breech and the rear screw in the back is just ugly I'll need to find something good looking.

wll

[SpiralGroove]

Use a polished brass screw .
Not as durable, but will add some bling

[mackeral5]

For cock on opening, cutting your receiver isn't necessary.  Moving the cocking pin rearward on the bolt is all you have to do.  Drilling a new cocking pin location in the bolt isn't too difficult--- if you grind through the hardened surface most any drill bit will do the rest.

For RVA's, I prefer the crosman type.  this does require drilling/countersinking a hole in the breech for the upper screw.  However if you start with a crosman end cap($6 via Alchemy), only a hand drill and a tap is required for the rva adjustment hole.  This drilling operation is very easy as the hole is aleady centered, and you only have to drill about a 3/16 deep hole.  The breech and trigger screw holes are already drilled/tapped in the crosman end cap.  If you make an SSG like the one in my K.I.S.S. SSG thread, NO TAPPING is required.

[SpiralGroove]

Hey mackeral5,
Where can I find this Alchemy?  For only $6 it could be worth it .

[mackeral5]

Alchemy airgun works ebay store.  Looks like they've gone up a bit since my last purchase.  Today 2 cost me $20 and change after tax/shipping.


http://m.ebay.com/itm/290810725554?_trksid=p2057872.m2749.l2649&_mwBanner=1

[rsterne]

Here is a simple solution for pinning a QB79 tank block.... http://www.mcmaster.com/#90692a725/=11lg1pi

This zinc plated steel spring pin is rated at 4,100 lbs. total force to shear in two places (ie into three parts).... By itself, that would provide a safety margin at 1800 psi of 4100 / (.439 x 1800) = 4100 / 790 = 5.2:1 in shear.... It would also provide a safety margin, by itself, in compression (bearing load in the tube wall) of 1.7:1 at 1800 psi, or 2:1 at 1500 psi.... The location would have to be such that the inner half of the pin (the load side in the tank block) was bearing full width between the O-ring grooves, supported to the full diameter of the tank block stub.... When used in conjunction with the original screws, with the 3/16" hole drilled while under a low pressure load against those to insure some support from the original screws.... I see no reason why this would not be an excellent, and simple solution to retaining the tank block when used with CO2 at temperatures to 120*F, or HPA with a 1.8K burst disc installed.... IMO....

Bob

[SpiralGroove]

Thanks Bob,
We'll have to label this as THE Tank Block Fix





Looking at the two tank block pictures above, we can see that this needs to be a pretty precise hole.
And this needs to be drilled with the block in the gun under slight pressure.  So a good fix, but this needs to be a well planned drill job .  Not too far right, left or low - maybe right on center or a whisker high.