weird qb78 question

[nervoustrigger]

Bob, I suggest getting both some medium and some fine polishing compound.  The medium for removing material to form the bevel, and fine to polish it.  You can find it at most auto stores. 

Valve grinding compound will do as a medium.  Usually in a paste form sold in a tube. 

Something like rottenstone works for fine polishing, or a fine metal polish like Fitz or J-B Bore paste.  Some have said certain kinds of toothpaste can be used but I have not tried that.

You can also use the abrasives sold in stick form at tool centers...the stuff used with bench polishing wheels.  Just have to scrape a little off with a blade and put in on your workpiece.

As far as doing a complete cut and recrown, there are lots of ways to do it.  It usually comes down to what tools you have access to. A lathe is preferable of course.  Next would be a special crowning tool...pricey unless you're doing a lot of barrels to justify the expense.  A link to my DIY method with a drill press is toward the bottom of my previous post.  Or you can do it by hand with a great deal of patience but don't expect a perfectly perpendicular result.

[ScarletFire186304]

Thanks Jason,

you're always so good at helping us. it's appreciated. 

bob

[brapshwing]

Way to bring back up an old thread Troy!

Now I gotta read through it and get lots of really good information by one of GTA's finest (Jason). Ugh, the humanity!

Doing my part to keep the addiction alive!

[Ribbonstone]

The only real problem in building a QB PCP is getting  or making a 3K air tube.  There is enough striker weight,  internal striker spring length, air flow, and over all strength to make a 3,000psi PCP.

Way back in the day were two choices, Jack Haley occasionally made tubes,  was an on-off availability as he preferred to use the tubes in his custom rifles.  When I wanted one, he had none.  Cross that source out as Jack passed away awhile back.

Comasseco (a defunct retailer) did have some made and were marketing  a QB PCP.  Was only marketed for a very shot time as (1) the price as Disco-ish and (2) they had no clue what it took to make a decent PCP.

Ordered the Compasseco .177 PCPs.  Testing found it pretty much of a “$)((#” but the air tube seemed healthy.  So I put a barrled breech (.25 LW barrel) on the tube and fixed the short cuts. 

(Was a complete standard valve including piercing pin.  Fiber filter shredded and destroyed the stock valve stem.  Striker spring was 1 1/2 QB springs (they literally cut a 1/s a spring and added it to the spring guide along with a full length spring). Same mushy/soft, restrictive transfer port seal as in the co2 models.  Way-way too fast in .177. Basically, it was ‘carp’.)

Reworked valve, new Delrin valve stem poppet,  new one-piece spring,, and a bit of tuning got it working right as a .25.

Hobbiest have made tubes from scratch.  Some have made tubes from Discovery tubes, although if going that route I’d not stress it to 3K (maybe 2.2-2.5K).  Probably with starting with a Disco tube is that once you cut off the superfluous holes/cuts you can’t use and machine in cuts/holes you can use, the tube ends up short (small volume). 

Are others that just make the sign of the cross, ask the Saints to look over them, maybe sacrifice a rooster at the crossroads under a full moon, and use the issue tube.    I’d really-really-really rather not.

Haven’t read many reviews of the Polish company (Wolf?) offering PCP tubes and PCP conversion kits for QB’s.  The ones I’ve read haven’t been “glowing endorsements”. What I suspect is that they can be MADE to work, but a direct drop in fit is not what they are (but I’ve not tried one).


Haven’t had much problem in getting it to go fast.  Are some limits, and the air volume isn’t large, so shot count hasn’t been great but the foot pounds can certainly get up there.

[rsterne]

If you are going to limit your pressure to under 1800 psi, which can be achieved by CO2 on a really hot day, in theory at least you can use a stock QB tube.... The problem is that it doesn't hold much air, about 55cc, less than a 2260, and only about 40% of what a Disco holds.... Ribbonstone outlined the availability problems, and the best solution would be to use a tube of good quality with the same dimensions as a Disco tube, 7/8" OD x 0.065" wall.... 1021 Steel is good to 3000 psi with a 3:1 safety margin, and 4130 CrMoly will be 3.7:1, and those figures are to yield, not burst, providing the valve is located properly, of course.... If you are looking to get one made, I would suggest you contact stalwart.... He does a lot of work on the XS-60c, which is somewhat similar, and has the necessary tubing and the equipment to make HPA tubes, including thread the end for a Disco fill fitting....

Bob

[YEMX]

Perhaps a strange question- but why is a disco rated to only 2k psi, and the Prod to 3k?  Is the main tube different material?  I do see that the fill plug is different between the two, is this the reason?  Theoretically, you can have a tube made, with all the QB holes and slots, but the front will be threaded for either a disco or Prod fill plug, right?  I wonder how the valve would be pinned?  Is there enough material to pin it?

[rsterne]

The tube in the Disco isn't the problem, it's the valve mounting screws.... The Disco was designed to be pumped by hand, hence the lower fill pressure.... The QB valve is almost identical to the one in the XS-60c, and they both use a steel backing block that the valve sits against (or should).... That backing block can be fastened to the tube with three 10-32 screws arranged identically to a Disco, and that will look after the 3000 psi fill, providing the tube is strong enough.... I suggest you look at the "Grizzly" HPA conversion I did on the XS-60c to see how I fastened the valve.... The Disco fill fitting is not a problem at 3000 psi, provided the O-ring sealing it to the tube is on the full 0.065" tube wall the way it is designed to be used....

The QB valve and hammer may be a few thou too large to fit inside a Disco sized tube.... It is 0.003" smaller ID nominally.... It just depends on where the tolerances fall if they will work or not.... but skimming down the valve isn't an issue anyway....

Bob

[YEMX]

Cool beans- I forgot about that steel block the QB valve rests against...  I was thinking like the Disco valve, where the retention screws thread into the valve body.  Thanks for clearing that up for me Bob!! 

[lillysdad621]

Bob... what is the capacity of the disco tube? also i built a 2240 with a hpa bottle and wanted to run at about 600 fps. I achieve that but i adjusted the regulator to run at about only 750 fps. with the hammer spring i have it seems to "fart" on each shot and im getting only about 62 shots (very consistent) from 2500 to the reg limit... how would i go about making it more efficient yet retaining the power level i desire?

[rsterne]

A Disco tube holds 135 cc of air....

You likely have too much hammer strike for the pressure you are running, and getting hammer bounce.... Try backing off the preload on the hammer spring, or a lighter spring, until the velocity starts to drop.... My guess is that won't be immediately.... Once your gun is tuned to just below the maximum velocity for the pressure, your hammer bounce, and the brraaappppp sound that goes with it, should go away, and your shot count will increase.... If by the time you tune it that way your velocity is a bit too low, increase the pressure 100 psi and retune it....

Bob

[lillysdad621]

I increased the regulator pressure to 900 psi and replaced the hammer spring with a stiffer spring. velocity has reached 700 fps  but the burping has stopped. I will refill to 2500 psi and will sit in front of the chrony till the velocity drops. lets see if my shot count increased. (by the way, if i took the string from say 2500 to 2000, and multiplied by 3, would that give me a somewhat accurate shot count as i stopped right above the regulator limit?) just trying to see if its necessary to do an entire string or i can use this aproximation...

[UCChris]

I increased the regulator pressure to 900 psi and replaced the hammer spring with a stiffer spring. velocity has reached 700 fps  but the burping has stopped. I will refill to 2500 psi and will sit in front of the chrony till the velocity drops. lets see if my shot count increased. (by the way, if i took the string from say 2500 to 2000, and multiplied by 3, would that give me a somewhat accurate shot count as i stopped right above the regulator limit?) just trying to see if its necessary to do an entire string or i can use this aproximation...

The string of a regulated gun should be linear down to the setpoint.

[lillysdad621]

just wondering about shot count... if i get 30 shots between 2500 and 2000 psi, would i get 90 from 2500 down to 1000?

[rsterne]

You should, and 120 from 3000 down to 1000, assuming the setpoint is 1000 or less.... You REALLY need to experiment with the hammer spring preload and see how it affects the velocity.... You should get something like this....



More preload than you need to reach the velocity plateau just wastes air.... and can waste a LOT of air.... plus the gun is prone to hammer bounce as well.... You want to operate on the "knee" of the curve, which on that graph is 4-5 turns out from coil bind.... The tune I chose was 4.5 turns out....

Bob